AIM 2/20/25
from A TC. The NAS has long been in need of a means of delivering terminal weather information to the cockpit
more efficiently in terms of both speed and accuracy to enhance pilot awareness of weather hazards and reduce
air traffic controller workload. With the TWIP capability, terminal weather information, both alphanumerically
and graphically, is now available directly to the cockpit for 46 airports in the U.S. NAS. (See FIG 7−1−21.)
FIG 7−1−21
TWIP Image of Convective Weather at MCO International
(b) TWIP products are generated using weather data from the TDWR or the Integrated Terminal W eather
System (ITWS). These products can then be accessed by pilots using the Aircraft Communications Addressing
and Reporting System (ACARS) data link services. Airline dispatchers can also access this database and send
messages to specific aircraft whenever wind shear activity begins or ends at an airport.
(c) TWIP products include descriptions and character graphics of microburst alerts, wind shear alerts,
significant precipitation, convective activity within 30 NM surrounding the terminal area, and expected weather
that will impact airport operations. During inclement weather, i.e., whenever a predetermined level of
precipitation or wind shear is detected within 15 miles of the terminal area, TWIP products are updated once each
minute for text messages and once every five minutes for character graphic messages. During good weather
(below the predetermined precipitation or wind shear parameters) each message is updated every 10 minutes.
These products are intended to improve the situational awareness of the pilot/flight crew, and to aid in flight
planning prior to arriving or departing the terminal area. It is important to understand that, in the context of TWIP,
the predetermined levels for inclement versus good weather has nothing to do with the criteria for
VFR/MVFR/IFR/LIFR; it only deals with precipitation, wind shears and microbursts.
7−1−60 Meteorology
AIM2/20/257/9/26 AIM
TBL 7−1−12
TWIP−Equipped Airports
Airport Identifier
Andrews AFB, MD KADW
Hartsfield−Jackson Atlanta Intl Airport KATL
Nashville Intl Airport KBNA
Logan Intl Airport KBOS
Baltimore/Washington Intl Airport KBWI
Hopkins Intl Airport KCLE
Charlotte/Douglas Intl Airport KCLT
Port Columbus Intl Airport KCMH
Cincinnati/Northern Kentucky Intl Airport KCVG
Dallas Love Field Airport KDAL
James M. Cox Intl Airport KDAY
Ronald Reagan Washington National Air-
port
KDCA
Denver Intl Airport KDEN
Dallas−Fort Worth Intl Airport KDFW
Detroit Metro Wayne County Airport KDTW
Newark Liberty Intl Airport KEWR
Fort Lauderdale−Hollywood Intl Airport KFLL
William P. Hobby Airport KHOU
Washington Dulles Intl Airport KIAD
George Bush Intercontinental Airport KIAH
Wichita Mid−Continent Airport KICT
Indianapolis Intl Airport KIND
John F. Kennedy Intl Airport KJFK
Airport Identifier
Harry Reid Intl Airport KLAS
LaGuardia Airport KLGA
Kansas City Intl Airport KMCI
Orlando Intl Airport KMCO
Midway Intl Airport KMDW
Memphis Intl Airport KMEM
Miami Intl Airport KMIA
General Mitchell Intl Airport KMKE
Minneapolis St. Paul Intl Airport KMSP
Louis Armstrong New Orleans Intl Air-
port
KMSY
Will Rogers World Airport KOKC
O’Hare Intl Airport KORD
President Donald J. Trump Intl KDJT
Philadelphia Intl Airport KPHL
Phoenix Sky Harbor Intl Airport KPHX
Pittsburgh Intl Airport KPIT
Raleigh−Durham Intl Airport KRDU
Louisville Intl Airport KSDF
Salt Lake City Intl Airport KSLC
Lambert−St. Louis Intl Airport KSTL
Tampa Intl Airport KTPA
Tulsa Intl Airport KTUL
Luis Munoz Marin Intl Airport TJSJ
7−1−25. PIREPs Relating to Volcanic Ash Activity
a. V olcanic eruptions which send ash into the upper atmosphere occur somewhere around the world several
times each year. Flying into a volcanic ash cloud can be extremely dangerous. At least two B747s have lost all
power in all four engines after such an encounter. Regardless of the type aircraft, some damage is almost certain
to ensue after an encounter with a volcanic ash cloud. Additionally, studies have shown that volcanic eruptions
are the only significant source of large quantities of sulphur dioxide (SO2) gas at jet-cruising altitudes. Therefore,
the detection and subsequent reporting of SO 2 is of significant importance. Although SO 2 is colorless, its
presence in the atmosphere should be suspected when a sulphur-like or rotten egg odor is present throughout the
cabin.
b. While some volcanoes in the U.S. are monitored, many in remote areas are not. These unmonitored
volcanoes may erupt without prior warning to the aviation community. A pilot observing a volcanic eruption who
has not had previous notification of it may be the only witness to the eruption. Pilots are strongly encouraged
to transmit a PIREP regarding volcanic eruptions and any observed volcanic ash clouds or detection of sulphur
dioxide (SO2) gas associated with volcanic activity.
Meteorology 7−1−61
AIM 2/20/25
c. Pilots should submit PIREPs regarding volcanic activity using the V olcanic Activity Reporting (V AR) form
as illustrated in Appendix 2. If a V AR form is not immediately available, relay enough information to identify
the position and type of volcanic activity.
d. Pilots should verbally transmit the data required in items 1 through 8 of the V AR as soon as possible. The
data required in items 9 through 16 of the V AR should be relayed after landing if possible.
7−1−26. Thunderstorms
a. Turbulence, hail, rain, snow, lightning, sustained updrafts and downdrafts, icing conditions−all are present
in thunderstorms. While there is some evidence that maximum turbulence exists at the middle level of a
thunderstorm, recent studies show little variation of turbulence intensity with altitude.
b. There is no useful correlation between the external visual appearance of thunderstorms and the severity or
amount of turbulence or hail within them. The visible thunderstorm cloud is only a portion of a turbulent system
whose updrafts and downdrafts often extend far beyond the visible storm cloud. Severe turbulence can be
expected up to 20 miles from severe thunderstorms. This distance decreases to about 10 miles in less severe
storms.
c. Weather radar, airborne or ground based, will normally reflect the areas of moderate to heavy precipitation
(radar does not detect turbulence). The frequency and severity of turbulence generally increases with the radar
reflectivity which is closely associated with the areas of highest liquid water content of the storm. NO FLIGHT
PATH THROUGH AN AREA OF STRONG OR VERY STRONG RADAR ECHOES SEPARATED BY 20−30
MILES OR LESS MAY BE CONSIDERED FREE OF SEVERE TURBULENCE.
d. Turbulence beneath a thunderstorm should not be minimized. This is especially true when the relative
humidity is low in any layer between the surface and 15,000 feet. Then the lower altitudes may be characterized
by strong out flowing winds and severe turbulence.
e. The probability of lightning strikes occurring to aircraft is greatest when operating at altitudes where
temperatures are between minus 5 degrees Celsius and plus 5 degrees Celsius. Lightning can strike aircraft flying
in the clear in the vicinity of a thunderstorm.
f. METAR reports do not include a descriptor for severe thunderstorms. However, by understanding severe
thunderstorm criteria, i.e., 50 knot winds or 3/4 inch hail, the information is available in the report to know that
one is occurring.
g. Current weather radar systems are able to objectively determine precipitation intensity. These precipitation
intensity areas are described as “light,” “moderate,” “heavy,” and “extreme.”
REFERENCE−
Pilot/Controller Glossary− Precipitation Radar Weather Descriptions
EXAMPLE−
1. Alert provided by an ATC facility to an aircraft:
(aircraft identification) EXTREME precipitation between ten o’clock and two o’clock, one five miles. Precipitation area is
two five miles in diameter.
2. Alert provided by an FSS:
(aircraft identification) EXTREME precipitation two zero miles west of Atlanta V−O−R, two five miles wide, moving east
at two zero knots, tops flight level three niner zero.
7−1−27. Thunderstorm Flying
a. Thunderstorm Avoidance. Never regard any thunderstorm lightly, even when radar echoes are of light
intensity. Avoiding thunderstorms is the best policy. Following are some Do’s and Don’ts of thunderstorm
avoidance:
1. Don’t land or takeoff in the face of an approaching thunderstorm. A sudden gust front of low level
turbulence could cause loss of control.
7−1−62 Meteorology
2/20/25 AIM
2. Don’t attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and
wind shear under the storm could be hazardous.
3. Don’t attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme clear
air turbulence.
4. Don’t fly without airborne radar into a cloud mass containing scattered embedded thunderstorms.
Scattered thunderstorms not embedded usually can be visually circumnavigated.
5. Don’t trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.
6. Don’t assume that ATC will offer radar navigation guidance or deviations around thunderstorms.
7. Don’t use data-linked weather next generation weather radar (NEXRAD) mosaic imagery as the sole
means for negotiating a path through a thunderstorm area (tactical maneuvering).
8. Do remember that the data−linked NEXRAD mosaic imagery shows where the weather was, not where
the weather is. The weather conditions depicted may be 15 to 20 minutes older than indicated on the display.
9. Do listen to chatter on the A TC frequency for Pilot Weather Reports (PIREP) and other aircraft requesting
to deviate or divert.
10. Do ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed.
11. Do use data-linked weather NEXRAD mosaic imagery (for example, Flight Information
Service-Broadcast (FIS-B)) for route selection to avoid thunderstorms entirely (strategic maneuvering).
12. Do advise ATC, when switched to another controller, that you are deviating for thunderstorms before
accepting to rejoin the original route.
13. Do ensure that after an authorized weather deviation, before accepting to rejoin the original route, that
the route of flight is clear of thunderstorms.
14. Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This
is especially true under the anvil of a large cumulonimbus.
15. Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage.
16. Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm.
17. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is
visually sighted or determined by radar.
18. Do give a PIREP for the flight conditions.
19. Do divert and wait out the thunderstorms on the ground if unable to navigate around an area of
thunderstorms.
20. Do contact Flight Service for assistance in avoiding thunderstorms. Flight Service specialists have
NEXRAD mosaic radar imagery and NEXRAD single site radar with unique features such as base and composite
reflectivity, echo tops, and V AD wind profiles.
b. If you cannot avoid penetrating a thunderstorm, following are some Do’s before entering the storm:
1. Tighten your safety belt, put on your shoulder harness (if installed), if and secure all loose objects.
2. Plan and hold the course to take the aircraft through the storm in a minimum time.
3. To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level
of -15ºC.
4. Verify that pitot heat is on and turn on carburetor heat or jet engine anti-ice. Icing can be rapid at any
altitude and cause almost instantaneous power failure and/or loss of airspeed indication.
5. Establish power settings for turbulence penetration airspeed recommended in the aircraft manual.
Meteorology 7−1−63
AIM 2/20/25
6. Turn up cockpit lights to highest intensity to lessen temporary blindness from lightning.
7. If using automatic pilot, disengage Altitude Hold Mode and Speed Hold Mode. The automatic altitude
and speed controls will increase maneuvers of the aircraft thus increasing structural stress.
8. If using airborne radar, tilt the antenna up and down occasionally. This will permit the detection of other
thunderstorm activity at altitudes other than the one being flown.
c. Following are some Do’s and Don’ts during the thunderstorm penetration:
1. Do keep your eyes on your instruments. Looking outside the cockpit can increase danger of temporary
blindness from lightning.
2. Don’t change power settings; maintain settings for the recommended turbulence penetration airspeed.
3. Do maintain constant attitude. Allow the altitude and airspeed to fluctuate.
4. Don’t turn back once you are in the thunderstorm. A straight course through the storm most likely will
get the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft.
7−1−64 Meteorology
2/20/25 AIM
7−1−28. Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR)
FIG 7−1−22
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Front)
Key to Aerodrome Forecast (TAF) and Aviation
Routine Weather Report (METAR) (Front)
TAF KPIT 091730Z 0918/1024 15005KT 5SM HZ FEW020 WS010/31022KT
FM091930 30015G25KT 3SM SHRA OVC015
TEMPO 0920/0922 1/2SM +TSRA OVC008CB
FM100100 27008KT 5SM SHRA BKN020 OVC040
PROB30 1004/1007 1SM -RA BR
FM101015 18005KT 6SM -SHRA OVC020
BECMG 1013/1015 P6SM NSW SKC
NOTE: Users are cautioned to confirm DATE and TIME of the TAF. For example FM100000 is
0000Z on the 10th. Do not confuse with 1000Z!
METAR KPIT 091955Z COR 22015G25KT 3/4SM R28L/2600FT TSRA OVC010CB 18/16 A2992 RMK
SLP045 T01820159
Forecast Explanation Report
TAF Message type: TAF-routine or TAF AMD-amended forecast, METAR-
hourly, SPECI-special or TESTM-non-commissioned ASOS report
METAR
KPIT ICAO location indicator KPIT
091730Z Issuance time: ALL times in UTC “Z ”, 2-digit date, 4-digit time 091955Z
0918/1024 Valid period, either 24 hours or 30 hours. The first two digits of EACH
four digit number indicate the date of the valid period, the final two di
gits indicate the time (valid from 18Z on the 9th to 24Z on the 10th).
In U.S. METAR: CORrected ob; or AUTOmated ob for automated re
port with no human intervention; omitted when observer logs on.
COR
15005KT Wind: 3 digit true-north direction, nearest 10 degrees (or VaRiaBle);
next 2-3 digits for speed and unit, KT (KMH or MPS); as needed, Gust
and maximum speed; 00000KT for calm; for METAR, if direction varies
60 degrees or more, Variability appended, e.g., 180V260
22015G25KT
5SM Prevailing visibility; in U.S., Statute Miles & fractions; above 6 miles in
TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as re
quired, lowest value with direction)
¾SM
Runway Visual Range: R; 2-digit runway designator Left, Center, or
Right as needed; “/”, Minus or Plus in U.S., 4-digit value, FeeT in U.S.,
(usually meters elsewhere); 4-digit value Variability 4-digit value (and
tendency Down, Up or No change)
R28L/2600FT
HZ Significant present, forecast and recent weather: see table (on back) TSRA
FEW020 Cloud amount, height and type: Sky Clear 0/8, FEW >0/8-2/8, ScaTtered
3/8-4/8, BroKeN 5/8-7/8, OverCast 8/8; 3-digit height in hundreds of ft;
Towering Cumulus or CumulonimBus in METAR; in TAF, only CB.
Vertical Visibility for obscured sky and height “VV004”. More than 1
layer may be reported or forecast. In automated METAR reports only,
CleaR for “clear below 12,000 feet”
OVC 010CB
Temperature: degrees Celsius; first 2 digits, temperature “/” last 2 digits,
dew-point temperature; Minus for below zero, e.g., M06
18/16
Altimeter setting: indicator and 4 digits; in U.S., A-inches and hun
dredths; (Q-hectoPascals, e.g., Q1013)
A2992
WS010/31022KT In U.S. TAF, non-convective low-level (≤2,000 ft) Wind Shear; 3-digit
height (hundreds of ft); “/”; 3-digit wind direction and 2-3 digit wind
speed above the indicated height, and unit, KT
Meteorology 7−1−65
AIM 2/20/25
FIG 7−1−23
Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Back)
Key to Aerodrome Forecast (TAF) and Aviation
Routine Weather Report (METAR) (Back)
In METAR, ReMarK indicator & remarks. For example: Sea- Level
Pressure in hectoPascals & tenths, as shown: 1004.5 hPa; Temp/dew-
point in tenths °C, as shown: temp. 18.2°C, dew-point 15.9°C
RMK SLP045
T01820159
FM091930 FroM: changes are expected at: 2-digit date, 2-digit hour, and 2-digit
minute beginning time: indicates significant change. Each FM starts on a
new line, indented 5 spaces
TEMPO
0920/0922
TEMPOrary: changes expected for <1 hour and in total, < half of the
period between the 2-digit date and 2-digit hour beginning, and 2-digit
date and 2-digit hour ending time
PROB30
1004/1007
PROBability and 2-digit percent (30 or 40): probable condition in the
period between the 2-digit date & 2-digit hour beginning time, and the
2-digit date and 2-digit hour ending time
BECMG
1013/1015
BECoMinG: change expected in the period between the 2-digit date and
2-digit hour beginning time, and the 2-digit date and 2-digit hour ending
time
Table of Significant Present, Forecast and Recent Weather - Grouped in categories and
used in the order listed below; or as needed in TAF, No Significant Weather.
Qualifiers
Intensity or Proximity
“-” = Light No sign = Moderate “+” = Heavy
“VC” = Vicinity, but not at aerodrome. In the US METAR, 5 to 10 SM from the point of observation. In the US
TAF, 5 to 10 SM from the center of the runway complex. Elsewhere, within 8000m.
Descriptor
BC – Patches BL – Blowing DR – Drifting FZ – Freezing
MI – Shallow PR – Partial SH – Showers TS – Thunderstorm
Weather Phenomena
Precipitation
DZ – Drizzle GR – Hail GS – Small Hail/Snow Pellets
IC – Ice Crystals PL – Ice Pellets RA – Rain SG – Snow Grains
SN – Snow UP – Unknown Precipitation in automated observations
Obscuration
BR – Mist (≥5/8SM) DU – Widespread Dust FG – Fog (<5/8SM) FU – Smoke
HZ – Haze PY – Spray SA – Sand VA – Volcanic Ash
Other
DS – Dust Storm FC – Funnel Cloud +FC – Tornado or Waterspout
PO – Well developed dust or sand whirls SQ – Squall SS – Sandstorm
- Explanations in parentheses “()” indicate different worldwide practices.
- Ceiling is not specified; defined as the lowest broken or overcast layer, or the vertical visibility.
- NWS TAFs exclude BECMG groups and temperature forecasts, NWS TAFS do not use PROB in the first 9
hours of a TAF; NWS METARs exclude trend forecasts. US Military TAFs include Turbulence and Icing groups.
7−1−66 Meteorology
2/20/25 AIM
7−1−29. International Civil Aviation Organization (ICAO) Weather Formats
The U.S. uses the ICAO world standard for aviation weather reporting and forecasting. The World
Meteorological Organization’s (WMO) publication No. 782 “Aerodrome Reports and Forecasts” contains the
base METAR and TAF code as adopted by the WMO member countries.
a. Although the METAR code is adopted worldwide, each country is allowed to make modifications or
exceptions to the code for use in their particular country , e.g., the U.S. will continue to use statute miles for
visibility, feet for RVR values, knots for wind speed, and inches of mercury for altimetry. However, temperature
and dew point will be reported in degrees Celsius. The U.S reports prevailing visibility rather than lowest sector
visibility. The elements in the body of a METAR report are separated with a space. The only exceptions are RVR,
temperature, and dew point which are separated with a solidus (/). When an element does not occur, or cannot
be observed, the preceding space and that element are omitted from that particular report. A METAR report
contains the following sequence of elements in the following order:
1. Type of report.
2. ICAO Station Identifier.
3. Date and time of report.
4. Modifier (as required).
5. Wind.
6. Visibility.
7. Runway Visual Range (RVR).
8. Weather phenomena.
9. Sky conditions.
10. Temperature/dew point group.
11. Altimeter.
12. Remarks (RMK).
b. The following paragraphs describe the elements in a METAR report.
1. Type of report. There are two types of report:
(a) Aviation Routine Weather Report (METAR); and
(b) Nonroutine (Special) Aviation Weather Report (SPECI).
The type of report (METAR or SPECI) will always appear as the lead element of the report.
2. ICAO Station Identifier. The METAR code uses ICAO 4−letter station identifiers. In the contiguous
48 States, the 3−letter domestic station identifier is prefixed with a “K;” i.e., the domestic identifier for Seattle
is SEA while the ICAO identifier is KSEA. Elsewhere, the first two letters of the ICAO identifier indicate what
region of the world and country (or state) the station is in. For Alaska, all station identifiers start with “PA;” for
Hawaii, all station identifiers start with “PH.” Canadian station identifiers start with “CU,” “CW,” “CY ,” and
“CZ.” Mexican station identifiers start with “MM.” The identifier for the western Caribbean is “M” followed
by the individual country’s letter; i.e., Cuba is “MU;” Dominican Republic “MD;” the Bahamas “MY .” The
identifier for the eastern Caribbean is “T” followed by the individual country’s letter; i.e., Puerto Rico is “TJ.”
For a complete worldwide listing see ICAO Document 7910, Location Indicators.
3. Date and Time of Report. The date and time the observation is taken are transmitted as a six −digit
date/time group appended with Z to denote Coordinated Universal Time (UTC). The first two digits are the date
followed with two digits for hour and two digits for minutes.
EXAMPLE−
172345Z (the 17th day of the month at 2345Z)
Meteorology 7−1−67
AIM 2/20/25
4. Modifier (As Required). “AUTO” identifies a METAR/SPECI report as an automated weather report
with no human intervention. If “AUTO” is shown in the body of the report, the type of sensor equipment used
at the station will be encoded in the remarks section of the report. The absence of “AUTO” indicates that a report
was made manually by an observer or that an automated report had human augmentation/backup. The modifier
“COR” indicates a corrected report that is sent out to replace an earlier report with an error.
NOTE−
There are two types of automated stations, AO1 for au tomated weather reporting stations without a precipitation
discriminator , and AO2 for automated stations with a precipitation discriminator. (A precipitation discriminator can
determine the difference between liquid and frozen/freezing precipitation). This information appears in the remarks section
of an automated report.
5. Wind. The wind is reported as a five digit group (six digits if speed is over 99 knots). The first three digits
are the direction the wind is blowing from, in tens of degrees referenced to true north, or “VRB” if the direction
is variable. The next two digits is the wind speed in knots, or if over 99 knots, the next three digits. If the wind
is gusty, it is reported as a “G” after the speed followed by the highest gust reported. The abbreviation “KT” is
appended to denote the use of knots for wind speed.
EXAMPLE−
13008KT − wind from 130 degrees at 8 knots
08032G45KT − wind from 080 degrees at 32 knots with gusts to 45 knots
VRB04KT − wind variable in direction at 4 knots
00000KT − wind calm
210103G130KT − wind from 210 degrees at 103 knots with gusts to 130 knots
If the wind direction is variable by 60 degrees or more and the speed is greater than 6 knots, a variable group consisting
of the extremes of the wind direction separated by a “v” will follow the prevailing wind group.
32012G22KT 280V350
(a) Peak Wind. Whenever the peak wind exceeds 25 knots “PK WND” will be included in Remarks,
e.g., PK WND 28045/1955 “Peak wind two eight zero at four five occurred at one niner five five.” If the hour
can be inferred from the report time, only the minutes will be appended, e.g., PK WND 34050/38 “Peak wind
three four zero at five zero occurred at three eight past the hour.”
(b) Wind shift. Whenever a wind shift occurs, “WSHFT” will be included in remarks followed by the
time the wind shift began, e.g., WSHFT 30 FROPA “Wind shift at three zero due to frontal passage.”
6. Visibility. Prevailing visibility is reported in statute miles with “SM” appended to it.
EXAMPLE−
7SM − seven statute miles
15SM − fifteen statute miles
1/2SM − one−half statute mile
(a) Tower/surface visibility. If either visibility (tower or surface) is below four statute miles, the lesser
of the two will be reported in the body of the report; the greater will be reported in remarks.
(b) Automated visibility. ASOS/AWOS visibility stations will show visibility 10 or greater than 10
miles as “10SM.” AWOS visibility stations will show visibility less than 1/4 statute mile as “M 1/4SM” and
visibility 10 or greater than 10 miles as “10SM.”
NOTE−
Automated sites that are augmented by human observer to meet service level requirements can report 0, 1/16 SM, and 1/8
SM visibility increments.
(c) Variable visibility. Variable visibility is shown in remarks (when rapid increase or decrease by 1/2
statute mile or more and the average prevailing visibility is less than three miles) e.g., VIS 1V2 “visibility variable
between one and two.”
(d) Sector visibility. Sector visibility is shown in remarks when it differs from the prevailing visibility,
and either the prevailing or sector visibility is less than three miles.
7−1−68 Meteorology
2/20/25 AIM
EXAMPLE−
VIS N2 − visibility north two
7. Runway Visual Range (When Reported). “R” identifies the group followed by the runway heading
(and parallel runway designator, if needed) “/” and the visual range in feet (meters in other countries) followed
with “FT” (feet is not spoken).
(a) Variability Values. When RVR varies (by more than on reportable value), the lowest and highest
values are shown with “V” between them.
(b) Maximum/Minimum Range. “P” indicates an observed RVR is above the maximum value for this
system (spoken as “more than”). “M” indicates an observed RVR is below the minimum value which can be
determined by the system (spoken as “less than”).
EXAMPLE−
R32L/1200FT − runway three two left R−V−R one thousand two hundred.
R27R/M1000V4000FT − runway two seven right R−V−R variable from less than one thousand to four thousand.
8. Weather Phenomena. The weather as reported in the METAR code represents a significant change in
the way weather is currently reported. In METAR, weather is reported in the format:
Intensity/Proximity/Descriptor/Precipitation/Obstruction to visibility/Other
NOTE−
The “/” above and in the following descriptions (except as the separator between the temperature and dew point) are for
separation purposes in this publication and do not appear in the actual METARs.
(a) Intensity applies only to the first type of precipitation reported. A “ −” denotes light, no symbol
denotes moderate, and a “+” denotes heavy.
(b) Proximity applies to and reported only for weather occurring in the vicinity of the airport (between
5 and 10 miles of the point(s) of observation). It is denoted by the letters “VC.” (Intensity and “VC” will not
appear together in the weather group).
(c) Descriptor. These eight descriptors apply to the precipitation or obstructions to visibility:
TS ........... thunderstorm
DR ........... low drifting
SH ........... showers
MI ........... shallow
FZ ........... freezing
BC ........... patches
BL ........... blowing
PR ........... partial
NOTE−
Although “TS” and “SH” are used with precipitation and may be preceded with an intensity symbol, the intensity still applies
to the precipitation, not the descriptor.
(d) Precipitation. There are nine types of precipitation in the METAR code:
RA .......... rain
DZ .......... drizzle
SN .......... snow
GR .......... hail (1/4” or greater)
GS .......... small hail/snow pellets
PL .......... ice pellets
SG .......... snow grains
IC ........... ice crystals (diamond dust)
UP .......... unknown precipitation (automated stations only)
Meteorology 7−1−69
AIM 2/20/25
(e) Obstructions to visibility. There are eight types of obscuration phenomena in the METAR code
(obscurations are any phenomena in the atmosphere, other than precipitation, that reduce horizontal visibility):
FG .......... fog (vsby less than 5/8 mile)
HZ .......... haze
FU .......... smoke
PY .......... spray
BR .......... mist (vsby 5/8 − 6 miles)
SA .......... sand
DU .......... dust
VA .......... volcanic ash
NOTE−
Fog (FG) is observed or forecast only when the visibility is less than five−eighths of mile, otherwise mist (BR) is observed
or forecast.
(f) Other. There are five categories of other weather phenomena which are reported when they occur:
SQ ........... squall
SS ........... sandstorm
DS ........... duststorm
PO .......... dust/sand whirls
FC ........... funnel cloud
+FC ......... tornado/waterspout
Examples:
TSRA ......... thunderstorm with moderate rain
+SN .......... heavy snow
−RA FG ....... light rain and fog
BRHZ ........ mist and haze (visibility 5/8 mile or greater)
FZDZ ......... freezing drizzle
VCSH ........ rain shower in the vicinity
+SHRASNPL .. heavy rain showers, snow, ice pellets (intensity indicator refers to the predominant rain)
9. Sky Condition. The sky condition as reported in METAR represents a significant change from the way
sky condition is currently reported. In METAR, sky condition is reported in the format:
Amount/Height/(Type) or Indefinite Ceiling/Height
(a) Amount. The amount of sky cover is reported in eighths of sky cover, using the contractions:
SKC ......... clear (no clouds)
FEW ........ >0 to 2/8
SCT ......... scattered (3/8s to 4/8s of clouds)
BKN ......... broken (5/8s to 7/8s of clouds)
OVC ......... overcast (8/8s clouds)
CB .......... Cumulonimbus when present
TCU ......... Towering cumulus when present
NOTE−
1. “SKC” will be reported at manual stations. “CLR” will be used at automated stations when no clouds below 12,000 feet
are reported.
2. A ceiling layer is not designated in the METAR code. For aviation purposes, the ceiling is the lowest broken or overcast
layer , or vertical visibility into an obscuration. Also there is no provision for reporting thin layers in the METAR code. When
clouds are thin, that layer must be reported as if it were opaque.
(b) Height. Cloud bases are reported with three digits in hundreds of feet above ground level (AGL).
(Clouds above 12,000 feet cannot be reported by an automated station).
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(c) (Type). If Towering Cumulus Clouds (TCU) or Cumulonimbus Clouds (CB) are present, they are
reported after the height which represents their base.
EXAMPLE−
(Reported as) SCT025TCU BKN080 BKN250 (spoken as) “TWO THOUSAND FIVE HUNDRED SCATTERED
TOWERING CUMULUS, CEILING EIGHT THOUSAND BROKEN, TWO FIVE THOUSAND BROKEN.”
(Reported as) SCT008 OVC012CB (spoken as) “EIGHT HUNDRED SCATTERED CEILING ONE THOUSAND TWO
HUNDRED OVERCAST CUMULONIMBUS CLOUDS.”
(d) Vertical Visibility (indefinite ceiling height). The height into an indefinite ceiling is preceded by
“VV” and followed by three digits indicating the vertical visibility in hundreds of feet. This layer indicates total
obscuration.
EXAMPLE−
1/8 SM FG VV006 − visibility one eighth, fog, indefinite ceiling six hundred.
(e) Obscurations are reported when the sky is partially obscured by a ground−based phenomena by
indicating the amount of obscuration as FEW, SCT, BKN followed by three zeros (000). In remarks, the
obscuring phenomenon precedes the amount of obscuration and three zeros.
EXAMPLE−
BKN000 (in body) ........ “sky partially obscured”
FU BKN000 (in remarks) ... “smoke obscuring five− to seven−eighths of the sky”
(f) When sky conditions include a layer aloft, other than clouds, such as smoke or haze the type of
phenomena, sky cover and height are shown in remarks.
EXAMPLE−
BKN020 (in body) ........ “ceiling two thousand broken”
RMK FU BKN020 ........ “broken layer of smoke aloft, based at two thousand”
(g) Variable ceiling. When a ceiling is below three thousand and is variable, the remark “CIG” will be
shown followed with the lowest and highest ceiling heights separated by a “V .”
EXAMPLE−
CIG 005V010 ............ “ceiling variable between five hundred and one thousand”
(h) Second site sensor. When an automated station uses meteorological discontinuity sensors, remarks
will be shown to identify site specific sky conditions which differ and are lower than conditions reported in the
body.
EXAMPLE−
CIG 020 RY11 ........... “ceiling two thousand at runway one one”
(i) Variable cloud layer. When a layer is varying in sky cover, remarks will show the variability range.
If there is more than one cloud layer, the variable layer will be identified by including the layer height.
EXAMPLE−
SCT V BKN ............. “scattered layer variable to broken”
BKN025 V OVC ......... “broken layer at two thousand five hundred variable to overcast”
(j) Significant clouds. When significant clouds are observed, they are shown in remarks, along with the
specified information as shown below:
(1) Cumulonimbus (CB), or Cumulonimbus Mammatus (CBMAM), distance (if known), direction
from the station, and direction of movement, if known. If the clouds are beyond 10 miles from the airport, DSNT
will indicate distance.
EXAMPLE−
CB W MOV E ....... “cumulonimbus west moving east”
CBMAM DSNT S .... “cumulonimbus mammatus distant south”
(2) Towering Cumulus (TCU), location, (if known), or direction from the station.
EXAMPLE−
TCU OHD ......... “towering cumulus overhead”
TCU W ............ “towering cumulus west”
Meteorology 7−1−71
AIM 2/20/25
(3) Altocumulus Castellanus (ACC), Stratocumulus Standing Lenticular (SCSL), Altocumulus
Standing Lenticular (ACSL), Cirrocumulus Standing Lenticular (CCSL) or rotor clouds, describing the clouds
(if needed) and the direction from the station.
EXAMPLE−
ACC W ............. “altocumulus castellanus west”
ACSL SW−S ......... “standing lenticular altocumulus southwest through south”
APRNT ROTOR CLD S “apparent rotor cloud south”
CCSL OVR MT E ..... “standing lenticular cirrocumulus over the mountains east”
10. Temperature/Dew Point. Temperature and dew point are reported in two, two-digit groups in degrees
Celsius, separated by a solidus (“/”). Temperatures below zero are prefixed with an “M.” If the temperature is
available but the dew point is missing, the temperature is shown followed by a solidus. If the temperature is
missing, the group is omitted from the report.
EXAMPLE−
15/08 .............. “temperature one five, dew point 8”
00/M02 ............ “temperature zero, dew point minus 2”
M05/ ............... “temperature minus five, dew point missing”
11. Altimeter. Altimeter settings are reported in a four-digit format in inches of mercury prefixed with an
“A” to denote the units of pressure.
EXAMPLE−
A2995 − “Altimeter two niner niner five”
12. Remarks. Remarks will be included in all observations, when appropriate. The contraction “RMK”
denotes the start of the remarks section of a METAR report.
Except for precipitation, phenomena located within 5 statute miles of the point of observation will be reported
as at the station. Phenomena between 5 and 10 statute miles will be reported in the vicinity, “VC.” Precipitation
not occurring at the point of observation but within 10 statute miles is also reported as in the vicinity, “VC.”
Phenomena beyond 10 statute miles will be shown as distant, “DSNT.” Distances are in statute miles except for
automated lightning remarks which are in nautical miles. Movement of clouds or weather will be indicated by
the direction toward which the phenomena is moving.
(a) There are two categories of remarks:
(1) Automated, manual, and plain language.
(2) Additive and automated maintenance data.
(b) Automated, Manual, and Plain Language. This group of remarks may be generated from either
manual or automated weather reporting stations and generally elaborate on parameters reported in the body of
the report. (Plain language remarks are only provided by manual stations).
(1) V olcanic eruptions.
(2) Tornado, Funnel Cloud, Waterspout.
(3) Station Type (AO1 or AO2).
(4) PK WND.
(5) WSHFT (FROPA).
(6) TWR VIS or SFC VIS.
(7) VRB VIS.
(8) Sector VIS.
(9) VIS @ 2nd Site.
(10) Lightning. When lightning is observed at a manual location, the frequency and location is
reported.
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When cloud−to−ground lightning is detected by an automated lightning detection system, such as ALDARS:
[a] Within 5 nautical miles (NM) of the Airport Reference Point (ARP), it will be reported as “TS”
in the body of the report with no remark;
[b] Between 5 and 10 NM of the ARP, it will be reported as “VCTS” in the body of the report with
no remark;
[c] Beyond 10 but less than 30 NM of the ARP, it will be reported in remarks as “DSNT” followed
by the direction from the ARP.
EXAMPLE−
LTG DSNT W or LTG DSNT ALQDS
(11) Beginning/Ending of Precipitation/TSTMS.
(12) TSTM Location MVMT.
(13) Hailstone Size (GR).
(14) Virga.
(15) VRB CIG (height).
(16) Obscuration.
(17) VRB Sky Condition.
(18) Significant Cloud Types.
(19) Ceiling Height 2nd Location.
(20) PRESFR PRESRR.
(21) Sea−Level Pressure.
(22) ACFT Mishap (not transmitted).
(23) NOSPECI.
(24) SNINCR.
(25) Other SIG Info.
(c) Additive and Automated Maintenance Data.
(1) Hourly Precipitation.
(2) 3− and 6−Hour Precipitation Amount.
(3) 24−Hour Precipitation.
(4) Snow Depth on Ground.
(5) Water Equivalent of Snow.
(6) Cloud Type.
(7) Duration of Sunshine.
(8) Hourly Temperature/Dew Point (Tenths).
(9) 6−Hour Maximum Temperature.
(10) 6−Hour Minimum Temperature.
(11) 24−Hour Maximum/Minimum Temperature.
(12) Pressure Tendency.
Meteorology 7−1−73
AIM 2/20/25
(13) Sensor Status.
PWINO
FZRANO
TSNO
RVRNO
PNO
VISNO
Examples of METAR reports and explanation:
METAR KBNA 281250Z 33018KT 290V360 1/2SM R31/2700FT SN BLSN FG VV008 00/M03 A2991 RMK
RAE42SNB42
METAR ...... aviation routine weather report
KBNA ........ Nashville, TN
281250Z ...... date 28th, time 1250 UTC
(no modifier) .. This is a manually generated report, due to the absence of “AUTO” and “AO1 or AO2”
in remarks
33018KT ...... wind three three zero at one eight
290V360 ...... wind variable between two nine zero and three six zero
1/2SM ........ visibility one half
R31/2700FT ... Runway three one RVR two thousand seven hundred
SN ........... moderate snow
BLSN FG ..... visibility obscured by blowing snow and fog
VV008 ........ indefinite ceiling eight hundred
00/M03 ....... temperature zero, dew point minus three
A2991 ........ altimeter two niner niner one
RMK ......... remarks
RAE42 ....... rain ended at four two
SNB42 ........ snow began at four two
METAR KSFO 041453Z AUTO VRB02KT 3SM BR CLR 15/12 A3012 RMK AO2
METAR ...... aviation routine weather report
KSFO ........ San Francisco, CA
041453Z ...... date 4th, time 1453 UTC
AUTO ....... fully automated; no human intervention
VRB02KT .... wind variable at two
3SM ......... visibility three
BR .......... visibility obscured by mist
CLR ......... no clouds below one two thousand
15/12 ......... temperature one five, dew point one two
A3012 ........ altimeter three zero one two
RMK ........ remarks
AO2 ......... this automated station has a weather discriminator (for precipitation)
SPECI KCVG 152224Z 28024G36KT 3/4SM +TSRA BKN008 OVC020CB 28/23 A3000 RMK TSRAB24 TS
W MOV E
SPECI ....... (nonroutine) aviation special weather report
KCVG ....... Cincinnati, OH
152228Z ...... date 15th, time 2228 UTC
(no modifier) .. This is a manually generated report due to the absence of “AUTO” and “AO1 or AO2”
in remarks
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28024G36KT .. wind two eight zero at two four gusts three six
3/4SM ........ visibility three fourths
+TSRA ....... thunderstorms, heavy rain
BKN008 ceiling eight hundred broken
OVC020CB ... two thousand overcast cumulonimbus clouds
28/23 ......... temperature two eight, dew point two three
A3000 ........ altimeter three zero zero zero
RMK ........ remarks
TSRAB24 ..... thunderstorm and rain began at two four
TS W MOV E thunderstorm west moving east
c. Aerodrome Forecast (TAF). A concise statement of the expected meteorological conditions at an airport
during a specified period. At most locations, TAFs have a 24 hour forecast period. However, TAFs for some
locations have a 30 hour forecast period. These forecast periods may be shorter in the case of an amended TAF.
TAFs use the same codes as METAR weather reports. They are scheduled four times daily for 24−hour periods
beginning at 0000Z, 0600Z, 1200Z, and 1800Z.
Forecast times in the TAF are depicted in two ways. The first is a 6−digit number to indicate a specific point in
time, consisting of a two−digit date, two−digit hour, and two−digit minute (such as issuance time or FM). The
second is a pair of four−digit numbers separated by a “/” to indicate a beginning and end for a period of time.
In this case, each four−digit pair consists of a two−digit date and a two−digit hour.
TAFs are issued in the following format:
TYPE OF REPORT/ICAO STATION IDENTIFIER/DATE AND TIME OF ORIGIN/V ALID PERIOD DATE
AND TIME/FORECAST METEOROLOGICAL CONDITIONS
NOTE−
The “/” above and in the following descriptions are for separation purposes in this publication and do not appear in the
actual TAFs.
TAF KORD 051130Z 0512/0618 14008KT 5SM BR BKN030
TEMPO 0513/0516 1 1/2SM BR
FM051600 16010KT P6SM SKC
FM052300 20013G20KT 4SM SHRA OVC020
PROB40 0600/0606 2SM TSRA OVC008CB
BECMG 0606/0608 21015KT P6SM NSW SCT040
TAF format observed in the above example:
TAF = type of report
KORD = ICAO station identifier
051130Z = date and time of origin (issuance time)
0512/0618 = valid period date and times
14008KT 5SM BR BKN030 = forecast meteorological conditions
Explanation of TAF elements:
1. Type of Report. There are two types of TAF issuances, a routine forecast issuance (TAF) and an
amended forecast (TAF AMD). An amended TAF is issued when the current TAF no longer adequately describes
the on-going weather or the forecaster feels the TAF is not representative of the current or expected weather.
Corrected (COR) or delayed (RTD) TAFs are identified only in the communications header which precedes the
actual forecasts.
2. ICAO Station Identifier. The TAF code uses ICAO 4−letter location identifiers as described in the
METAR section.
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3. Date and Time of Origin. This element is the date and time the forecast is actually prepared. The format
is a two−digit date and four−digit time followed, without a space, by the letter “Z.”
4. Valid Period Date and Time. The UTC valid period of the forecast consists of two four −digit sets,
separated by a “/”. The first four−digit set is a two−digit date followed by the two−digit beginning hour, and the
second four−digit set is a two−digit date followed by the two−digit ending hour. Although most airports have a
24−hour TAF, a select number of airports have a 30−hour TAF. In the case of an amended forecast, or a forecast
which is corrected or delayed, the valid period may be for less than 24 hours. Where an airport or terminal
operates on a part −time basis (less than 24 hours/day), the TAFs issued for those locations will have the
abbreviated statement “AMD NOT SKED” added to the end of the forecasts. The time observations are
scheduled to end and/or resume will be indicated by expanding the AMD NOT SKED statement. Expanded
statements will include:
(a) Observation ending time (AFT DDHHmm; for example, AFT 120200)
(b) Scheduled observations resumption time (TIL DDHHmm; for example, TIL 171200Z) or
(c) Period of observation unavailability (DDHH/DDHH); for example, 2502/2512).
5. Forecast Meteorological Conditions. This is the body of the TAF. The basic format is:
WIND/VISIBILITY/WEATHER/SKY CONDITION/OPTIONAL DATA (WIND SHEAR)
The wind, visibility, and sky condition elements are always included in the initial time group of the forecast.
Weather is included only if significant to aviation. If a significant, lasting change in any of the elements is
expected during the valid period, a new time period with the changes is included. It should be noted that with
the exception of a “FM” group the new time period will include only those elements which are expected to
change, i.e., if a lowering of the visibility is expected but the wind is expected to remain the same, the new time
period reflecting the lower visibility would not include a forecast wind. The forecast wind would remain the same
as in the previous time period. Any temporary conditions expected during a specific time period are included
with that time period. The following describes the elements in the above format.
(a) Wind. This five (or six) digit group includes the expected wind direction (first 3 digits) and speed
(last 2 digits or 3 digits if 100 knots or greater). The contraction “KT” follows to denote the units of wind speed.
Wind gusts are noted by the letter “G” appended to the wind speed followed by the highest expected gust. A
variable wind direction is noted by “VRB” where the three digit direction usually appears. A calm wind (3 knots
or less) is forecast as “00000KT.”
EXAMPLE−
18010KT ..... wind one eight zero at one zero (wind is blowing from 180).
35012G20KT .. wind three five zero at one two gust two zero.
(b) Visibility. The expected prevailing visibility up to and including 6 miles is forecast in statute miles,
including fractions of miles, followed by “SM” to note the units of measure. Expected visibilities greater than
6 miles are forecast as P6SM (plus six statute miles).
EXAMPLE−
1/2SM − visibility one−half
4SM − visibility four
P6SM − visibility more than six
(c) Weather Phenomena. The expected weather phenomena is coded in TAF reports using the same
format, qualifiers, and phenomena contractions as METAR reports (except UP). Obscurations to vision will be
forecast whenever the prevailing visibility is forecast to be 6 statute miles or less. If no significant weather is
expected to occur during a specific time period in the forecast, the weather phenomena group is omitted for that
time period. If, after a time period in which significant weather phenomena has been forecast, a change to a
forecast of no significant weather phenomena occurs, the contraction NSW (No Significant Weather) will appear
as the weather group in the new time period. (NSW is included only in TEMPO groups).
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NOTE−
It is very important that pilots understand that NSW only refers to weather phenomena, i.e., rain, snow, drizzle, etc. Omitted
conditions, such as sky conditions, visibility, winds, etc., are carried over from the previous time group.
(d) Sky Condition. TAF sky condition forecasts use the METAR format described in the METAR
section. Cumulonimbus clouds (CB) are the only cloud type forecast in TAFs. When clear skies are forecast, the
contraction “SKC” will always be used. The contraction “CLR” is never used in the TAF. When the sky is
obscured due to a surface −based phenomenon, vertical visibility (VV) into the obscuration is forecast. The
format for vertical visibility is “VV” followed by a three−digit height in hundreds of feet.
NOTE−
As in METAR, ceiling layers are not designated in the TAF code. For aviation purposes, the ceiling is the lowest broken or
overcast layer or vertical visibility into a complete obscuration.
SKC .............. “sky clear”
SCT005 BKN025CB . “five hundred scattered, ceiling two thousand five hundred broken cumulonimbus
clouds”
VV008 ............ “indefinite ceiling eight hundred”
(e) Optional Data (Wind Shear). Wind shear is the forecast of nonconvective low level winds (up to
2,000 feet). The forecast includes the letters “WS” followed by the height of the wind shear, the wind direction
and wind speed at the indicated height and the ending letters “KT” (knots). Height is given in hundreds of feet
(AGL) up to and including 2,000 feet. Wind shear is encoded with the contraction “WS,” followed by a
three−digit height, slant character “/,” and winds at the height indicated in the same format as surface winds. The
wind shear element is omitted if not expected to occur.
WS010/18040KT − “LOW LEVEL WIND SHEAR AT ONE THOUSAND, WIND ONE EIGHT ZERO AT
FOUR ZERO”
d. Probability Forecast. The probability or chance of thunderstorms or other precipitation events occurring,
along with associated weather conditions (wind, visibility, and sky conditions). The PROB30 group is used when
the occurrence of thunderstorms or precipitation is 30−39% and the PROB40 group is used when the occurrence
of thunderstorms or precipitation is 40−49%. This is followed by two four−digit groups separated by a “/”, giving
the beginning date and hour, and the ending date and hour of the time period during which the thunderstorms
or precipitation are expected.
NOTE−
NWS does not use PROB 40 in the TAF . However U.S. Military generated TAFS may include PROB40. PROB30 will not
be shown during the first nine hours of a NWS forecast.
EXAMPLE−
PROB40 2221/2302 1/2SM +TSRA “chance between 2100Z and 0200Z of visibility one−half statute mile in thunderstorms
and heavy rain.”
PROB30 3010/3014 1SM RASN . “chance between 1000Z and 1400Z of visibility one statute mile in mixed rain and
snow.”
e. Forecast Change Indicators. The following change indicators are used when either a rapid, gradual, or
temporary change is expected in some or all of the forecast meteorological conditions. Each change indicator
marks a time group within the TAF report.
1. From (FM) group. The FM group is used when a rapid change, usually occurring in less than one hour,
in prevailing conditions is expected. Typically, a rapid change of prevailing conditions to more or less a
completely new set of prevailing conditions is associated with a synoptic feature passing through the terminal
area (cold or warm frontal passage). Appended to the “FM” indicator is the six−digit date, hour, and minute the
change is expected to begin and continues until the next change group or until the end of the current forecast.
A “FM” group will mark the beginning of a new line in a TAF report (indented 5 spaces). Each “FM” group
contains all the required elements−wind, visibility, weather, and sky condition. Weather will be omitted in “FM”
groups when it is not significant to aviation. FM groups will not include the contraction NSW.
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EXAMPLE−
FM210100 14010KT P6SM SKC − “after 0100Z on the 21st, wind one four zero at one zero, visibility more than six, sky
clear .”
2. Becoming (BECMG) group. The BECMG group is used when a gradual change in conditions is expected
over a longer time period, usually two hours. The time period when the change is expected is two four −digit
groups separated by a “/”, with the beginning date and hour, and ending date and hour of the change period which
follows the BECMG indicator. The gradual change will occur at an unspecified time within this time period. Only
the changing forecast meteorological conditions are included in BECMG groups. The omitted conditions are
carried over from the previous time group.
NOTE−
The NWS does not use BECMG in the TAF .
EXAMPLE−
OVC012 BECMG 0114/0116 BKN020 − “ceiling one thousand two hundred overcast. Then a gradual change to ceiling two
thousand broken between 1400Z on the 1st and 1600Z on the 1st.”
3. Temporary (TEMPO) group. The TEMPO group is used for any conditions in wind, visibility, weather,
or sky condition which are expected to last for generally less than an hour at a time (occasional), and are expected
to occur during less than half the time period. The TEMPO indicator is followed by two four −digit groups
separated by a “/”. The first four digit group gives the beginning date and hour, and the second four digit group
gives the ending date and hour of the time period during which the temporary conditions are expected. Only the
changing forecast meteorological conditions are included in TEMPO groups. The omitted conditions are carried
over from the previous time group.
EXAMPLE−
1. SCT030 TEMPO 0519/0523 BKN030 − “three thousand scattered with occasional ceilings three thousand broken
between 1900Z on the 5th and 2300Z on the 5th.”
2. 4SM HZ TEMPO 1900/1906 2SM BR HZ − “visibility four in haze with occasional visibility two in mist and haze between
0000Z on the 19th and 0600Z on the 19th.”
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Section 2. Barometric Altimeter Errors and Setting
Procedures
7−2−1. General
a. Aircraft altimeters are subject to the following errors and weather factors:
1. Instrument error.
2. Position error from aircraft static pressure systems.
3. Nonstandard atmospheric pressure.
4. Nonstandard temperatures.
b. The standard altimeter 29.92 inches Mercury (“Hg.) setting at the higher altitudes eliminates station
barometer errors, some altimeter instrument errors, and errors caused by altimeter settings derived from different
geographical sources.
7−2−2. Barometric Pressure Altimeter Errors
a. High Barometric Pressure: Cold, dry air masses may produce barometric pressures in excess of 31.00 “Hg.
Many aircraft altimeters cannot be adjusted above 31.00 “Hg. When an aircraft’s altimeter cannot be set to
pressure settings above 31.00 “Hg, the aircraft’s true altitude will be higher than the indicated altitude on the
barometric altimeter.
b. Low Barometric Pressure: An abnormal low−pressure condition exists when the barometric pressure is less
than 28.00 “Hg. Flight operations are not recommended when an aircraft’s altimeter is unable to be set below
28.00 “Hg. In this situation, the aircraft’s true altitude is lower than the indicated altitude. This situation may be
exacerbated when operating in extremely cold temperatures, which may result in the aircraft’s true altitude being
significantly lower than the indicated altitude.
NOTE−
EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN
IN LOW PRESSURES AND/OR LOW TEMPERATURES.
7−2−3. Altimeter Errors
a. Manufacturing and installation specifications, along with 14 CFR part 43, Appendix E requirement for
periodic tests and inspections, helps reduce mechanical, elastic, temperature, and installation errors. (See
Instrument Flying Handbook.) Scale error may be observed while performing a ground altimeter check using
the following procedure:
1. Set the current reported airfield altimeter setting on the altimeter setting scale.
2. Read the altitude on the altimeter. The altitude should read the known field elevation if you are located
on the same reference level used to establish the altimeter setting.
3. If the difference from the known field elevation and the altitude read from the altimeter is plus or minus
75 feet or greater, the accuracy of the altimeter is questionable and the problem should be referred to an
appropriately rated repair station for evaluation and possible correction.
b. It is important to set the current altimeter settings for the area of operation when flying at an enroute altitude
that does not require a standard altimeter setting of 29.92 “Hg. If the altimeter is not set to the current altimeter
setting when flying from an area of high pressure into an area of low pressure, the aircraft will be closer to the
surface than the altimeter indicates. An inch Hg. error in the altimeter setting equals 1,000 feet of altitude. For
Barometric Altimeter Errors and Setting Procedures 7−2−1
AIM 2/20/25
example, setting 29.90 “Hg instead of 30.90 “Hg. To quote an old saying: “GOING FROM A HIGH TO A LOW,
LOOK OUT BELOW.”
c. The aircraft cruising altitude or flight level is maintained by referencing the barometric altimeter.
Procedures for setting altimeters during high and low barometric pressure events must be set using the following
procedures:
1. Below 18,000 feet mean sea level (MSL).
(a) Barometric pressure is 31.00 “Hg or less.
(1) Set the altimeter to a current reported altimeter setting from a station along the route and within 100
NM of the aircraft, or;
(2) If there is no station within this area, use the current reported altimeter setting of an appropriate
available station, or;
NOTE−
Air traffic controllers will furnish this information at least once when en route or on an instrument flight plan within their
controlled airspace:
(3) If the aircraft is not equipped with a radio, set the altimeter to the elevation of the departure airport
or use an available appropriate altimeter setting prior to departure.
(b) When the barometric pressure exceeds 31.00 “Hg., a NOTAM will be published to define the affected
geographic area. The NOTAM will also institute the following procedures:
(1) All aircraft: All aircraft will set 31.00 “Hg. for en route operations below 18,000 feet MSL.
Maintain this setting until out of the affected area or until reaching the beginning of the final approach segment
on an instrument approach. Set the current altimeter setting (above 31.00 “Hg.) approaching the final segment,
if possible. If no current altimeter setting is available, or if a setting above 31.00 “Hg. cannot be made on the
aircraft’s altimeter, leave 31.00 “Hg. set in the altimeter and continue the approach.
(2) Set 31.00 “Hg. in the altimeter prior to reaching the lowest of any mandatory/crossing altitudes
or 1,500 feet above ground level (AGL) when on a departure or missed approach.
NOTE−
Air traffic control will issue actual altimeter settings and advise pilots to set 31.00 “Hg. in their altimeters for en route
operations below 18,000 feet MSL in affected areas.
(3) No additional restrictions apply for aircraft operating into an airport that are able to set and measure
altimeter settings above 31.00 “Hg.
(4) Flight operations are restricted to VFR weather conditions to and from an airport that is unable to
accurately measure barometric pressures above 31.00 “Hg. These airports will report the barometric pressure as
“missing” or “in excess of 31.00 “Hg.”.
(5) VFR aircraft. VFR operating aircraft have no additional restrictions. Pilots must use caution when
flight planning and operating in these conditions.
(6) IFR aircraft: IFR aircraft unable to set an altimeter setting above 31.00 “Hg. should apply the
following:
[a] The suitability of departure alternate airports, destination airports, and destination alternate
airports will be determined by increasing the published ceiling and visibility requirements when unable to set
the aircraft altimeter above 31.00 “Hg. Any reported or forecast altimeter setting over 31.00 “Hg. will be rounded
up to the next tenth to calculate the required increases. The ceiling will be increased by 100 feet and the visibility
by 1/4 statute mile for each 1/10 “Hg. over 31.00 “Hg. Use these adjusted values in accordance with operating
regulations and operations specifications.
EXAMPLE−
Destination airport altimeter is 31.21 “Hg. The planned approach is an instrument landing system (ILS) with a decision altitude (DA)
7−2−2 Barometric Altimeter Errors and Setting Procedures
AIM2/20/258/7/25 AIM
200 feet and visibility 1/2 mile (200−1/2). Subtract 31.00 “Hg. from 31.21 “Hg. to get .21 “Hg. .21 “Hg rounds up to .30 “Hg. Calculate
the increased requirement: 100 feet per 1/10 equates to a 300 feet increase for .30 “Hg. 1/4 statute mile per 1/10 equates to a 3/4 statute
mile increase for .30 “Hg. The destination weather requirement is determined by adding the 300−3/4 increase to 200−1/2. The destination
weather requirement is now 500−1 ¼..
[b] 31.00 “Hg. will remain set during the complete instrument approach. The aircraft has arrived at
the DA or minimum descent altitude (MDA) when the published DA or MDA is displayed on the barometric
altimeter.
NOTE−
The aircraft will be approximately 300 feet higher than the indicated barometric altitude using this method.
[c] These restrictions do not apply to authorized Category II/III ILS operations and certificate
holders using approved atmospheric pressure at aerodrome elevation (QFE) altimetry systems.
(7) Air Traffic Organization (ATO) Service Center Directors, in their area of jurisdiction, may
authorize temporary waivers to high barometric pressure flying NOTAM requirements to permit emergency
supply, transport, or medical services, per 14 CFR § 91.144 (b). ATO Service Center contact information can be
found at: https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/mission_support/sc.
2. At or above 18,000 feet MSL. All operators will set 29.92 “Hg. (standard setting) in the barometric
altimeter. The lowest usable flight level is determined by the atmospheric pressure in the area of operation as
shown in TBL 7−2−1. Air Traffic Control (ATC) will assign this flight level.
TBL 7−2−1
Lowest Usable Flight Level
Altimeter Setting Lowest Usable Flight
(Current Reported) Level
29.92 or higher 180
29.91 to 28.92 190
28.91 to 27.92 200
3. When the minimum altitude per 14 CFR section 91.159 and 14 CFR section 91.177 is above 18,000 feet
MSL, the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number
of feet specified in TBL 7−2−2. ATC will accomplish this calculation.
TBL 7−2−2
Lowest Flight Level Correction Factor
Altimeter Factor Correction Factor
29.92 or higher None
29.91 to 29.42 500 feet
29.41 to 28.92 1000 feet
28.91 to 28.42 1500 feet
28.41 to 27.92 2000 feet
27.91 to 27.42 2500 feet
EXAMPLE−
The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.43 “Hg,
the lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude
(TBL 7−2−1) plus 500 feet).
Barometric Altimeter Errors and Setting Procedures 7−2−3
2/20/25 AIM
Section 3. Cold Temperature Barometric Altimeter
Errors, Setting Procedures and Cold Temperature
Airports (CTA)
7−3−1. Effect of Cold Temperature on Barometric Altimeters
a. Temperature has an effect on the accuracy of barometric altimeters, indicated altitude, and true altitude. The
standard temperature at sea level is 15 degrees Celsius (59 degrees Fahrenheit). The temperature gradient from
sea level is minus 2 degrees Celsius (3.6 degrees Fahrenheit) per 1,000 feet. For example, at 5000 feet above sea
level, the ambient temperature on a standard day would be 5 degrees Celsius. When the ambient (at altitude)
temperature is colder than standard, the aircraft’s true altitude is lower than the indicated barometric altitude.
When the ambient temperature is warmer than the standard day, the aircraft’s true altitude is higher than the
indicated barometric altitude.
b. TBL 7−3−1 indicates how much error may exist when operating in non−standard cold temperatures. To use
the table, find the reported temperature in the left column, and read across the top row to locate the height above
the airport (subtract the airport elevation from the flight altitude). Find the intersection of the temperature row
and height above airport column. This number represents how far the aircraft may be below the indicated altitude
due to possible cold temperature induced error.
TBL 7−3−1
ICAO Cold Temperature Error Table
7−3−2. Pre− Flight Planning for Cold Temperature Altimeter Errors
Flight planning into a CTA may be accomplished prior to flight. Use the predicted coldest temperature for plus
or minus 1 hour of the estimated time of arrival and compare against the CTA published temperature. If the
predicted temperature is at or below CTA temperature, calculate an altitude correction using TBL 7−3−1. This
correction may be used at the CTA if the actual arrival temperature is the same as the temperature used to calculate
the altitude correction during preflight planning.
7−3−3. Effects of Cold Temperature on Baro −Vertical Navigation (VNAV) Vertical Guidance
Non−standard temperatures can result in a change to effective vertical paths and actual descent rates when using
aircraft baro−VNA V equipment for vertical guidance on final approach segments. A lower than standard
temperature will result in a shallower descent angle and reduced descent rate. Conversely, a higher than standard
temperature will result in a steeper angle and incr eased descent rate. Pilots should consider potential
Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature 7−3−1
Airports (CTA)
AIM 2/20/25
consequences of these effects on approach minima, power settings, sight picture, visual cues, etc., especially for
high−altitude or terrain−challenged locations and during low−visibility conditions.
REFERENCE−
AIM, Para 5−4−5, Instrument Approach Procedure (IAP) Charts.
a. Uncompensated Baro −VNA V note on 14 CFR part 97 IAPs. The area navigation (RNA V) global
positioning system (GPS) and RNA V required navigation performance (RNP) notes, “For uncompensated
Baro−VNA V systems, lateral navigation (LNAV)/VNA V NA below –XX°C (−XX°F) or above XX°C (XXX°F)”
and “For uncompensated Baro −VNA V systems, procedure NA below –XX °C (−XX°F) or above XX °C
(XXX°F)” apply to baro−VNA V equipped aircraft. These temperatures and how they are used are independent
of the temperature and procedures applied for a Cold Temperature Airport.
1. The uncompensated baro−VNA V chart note and temperature range on an RNA V (GPS) approach is
applicable to the LNA V/VNA V line of minima. Baro −VNA V equipped aircraft without a temperature
compensating system may not use the RNA V (GPS) approach LNA V/VNA V line of minima when the actual
temperature is above or below the charted temperature range.
2. The uncompensated baro−VNA V chart note and temperature range on an RNA V (RNP) approach applies
to the entire procedure. For aircraft without a baro−VNA V and temperature compensating system, the RNA V
(RNP) approach is not authorized when the actual temperature is above or below the charted uncompensated
baro−VNA V temperature range.
b. Baro−VNA V temperature range versus CTA temperature: The baro−VNA V and CTA temperatures are
independent and do not follow the same correction or reporting procedures. However, there are times when both
procedures, each according to its associated temperature, should be accomplished on the approach.
c. Operating and ATC reporting procedures.
1. Do not use the CTA operating or reporting procedure found in this section, 7−3−4a thru 7−3−5e when
complying with the baro−VNA V temperature note on an RNA V (GPS) approach. Correction is not required nor
expected to be applied to procedure altitudes or VNA V paths outside of the final approach segment.
2. Operators must advise ATC when making temperature corrections on RNP authorization required (AR)
approaches while adhering to baro−VNA V temperature note.
3. Reporting altitude corrections is required when complying with CTAs in conjunction with the
baro−VNA V temperature note. The CTA altitude corrections will be reported in this situation. No altitude
correction reporting is required in the final segment.
NOTE−
When executing an approach with vertical guidance at a CTA (i.e., ILS, localizer performance with vertical guidance (LPV),
LNAV/VNAV), pilots are reminded to intersect the glideslope/glidepath at the corrected intermediate altitude (if applicable)
and follow the published glideslope/glidepath to the corrected minima. The ILS glideslope and WAAS generated glidepath
are unaffected by cold temperatures and provide vertical guidance to the corrected DA. Begin descent on the ILS glideslope
or WAAS generated glidepath when directed by aircraft instrumentation. Temperature affects the precise final approach fix
(PFAF) true altitude where a baro−VNAV generated glidepath begins. The PF AF altitude must be corrected when below the
CTA temperature restriction for the intermediate segment or outside of the baro−VNAV temperature restriction when using
the LNAV/VNAV line of minima to the corrected DA.
7−3−4. Cold Temperature Airports (CTA)
a. General: The FAA has determined that operating in cold temperatures has placed some 14 CFR part 97
instrument approach procedures in the United States National Airspace System at risk for loss of required
obstacle clearance (ROC). An airport that is determined to be at risk will have an ICON and temperature
published on the instrument approach procedure (IAP) in the terminal procedures publication (TPP).
b. CTA identification in TPP: A CTA is identified by a “snowflake” icon (
) and temperature limit, in
Celsius, on U.S. Government approach charts.
7−3−2 Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature
Airports (CTA)
2/20/25 AIM
c. A current list of CTAs is located at: https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/
dtpp/search/. Airports are listed by ICAO code, Airport Name, Temperature in Celsius, and affected segment(s).
d. Airport Criteria. The CTA risk analysis is performed on airports that have at least one runway of 2500 ft.
Pilots operating into an airport with a runway length less than 2500 ft may make a cold temperature altitude
correction in cold temperature conditions, if desired. Comply with operating and reporting procedures for CTAs.
e. ATC Reporting Requirements. Pilots must advise ATC with the corrected altitude when applying an altitude
correction on any approach segment with the exception of the final segment.
f. Methods to apply correction: The FAA recommends operators/pilots use either the All Segments Method
or the Individual Segments Method when making corrections at CTAs.
7−3−5. Cold Temperature Airport Procedures
a. PILOTS MUST NOT MAKE AN ALTIMETER CHANGE to accomplish an altitude correction. Pilots
must ensure that the altimeter is set to the current altimeter setting provided by ATC in accordance with 14 CFR
§91.121.
b. Actions on when and where to make corrections: Pilots will make an altitude correction to the published,
“at”, “at or above”, and “at or below” altitudes on all designated segment(s) to all runways for all published
instrument approach procedures when the reported airport temperature is at or below the published CTA
temperature on the approach plate. A pilot may request an altitude correction (if desired) on any approach at any
United States airport when extreme cold temperature is encountered. Pilots making a correction must comply
with ATC reporting requirements.
c. Correctable altitudes: ATC does not apply a cold temperature correction to their Minimum Vectoring
Altitude (MVA) or Minimum IFR Altitude (MIA) charts. Pilots must request approval from ATC to apply a cold
temperature correction to any ATC assigned altitude. Pilots must not correct altitudes published on Standard
Instrument Departures (SIDs), Obstacle Departure Pr ocedures (ODPs), and Standard Terminal Arrivals
(STARs).
d. Use of corrected MDA/DA: Pilots will use the corrected MDA or DA as the minimum altitude for an
approach. Pilots must meet the requirements in 14 CFR part 91.175 in order to operate below the corrected MDA
or DA. Pilots must see and avoid obstacles when descending below the minimum altitude on the approach.
NOTE−
The corrected DA or MDA does not affect the visibility minima published for the approach. With the application of a cold
temperature correction to the DA or MDA, the airplane should be in a position on the glideslope/glidepath or at the published
missed approach point to identify the runway environment.
e. Acceptable use of the table for manual CTA altitude correction (see TBL 7−3−1): Pilots may calculate a
correction with a visual interpolation of the chart when using reported temperature and height above airport. This
calculated altitude correction may then be rounded to the nearest whole hundred or rounded up. For example,
a correction of 130 ft from the chart may be rounded to 100 ft or 200 ft. A correction of 280 ft will be rounded
up to 300 ft. This rounded correction will be added to the appropriate altitudes for the “Individual” or “All”
segment method. The correction calculated from the table for the MDA or DA may be used as is or rounded up,
but never rounded down. This number will be added to the MDA, DA, and all step-down fix altitudes inside of
the FAF/PFAF.
1. No extrapolation above the 5000 ft column is required. Pilots may use the 5000 ft “height above airport
in feet” column for calculating corrections when the calculated altitude is greater than 5000 ft above reporting
station elevation. Pilots must add the correction(s) from the table to the affected segment altitude(s) and fly at
the new corrected altitude. Do not round down when using the 5000 ft column for calculated height above airport
values greater than 5000 ft. Pilots may extrapolate above the 5000 ft column to apply a correction if desired.
2. These techniques have been adopted to minimize pilot distraction by limiting the number of entries into
the table when making corrections. Although not all altitudes on the approach will be corrected back to standard
Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature 7−3−3
Airports (CTA)
AIM 2/20/25
day values, a safe distance above the terrain/obstacle will be maintained on the corrected approach segment(s).
Pilots may calculate a correction for each fix based on the fix altitude if desired.
NOTE−
Pilots may use Real Time Mesoscale Analysis (RTMA): Alternate Report of Surface Temperature, for computing altitude
corrections, when airport temperatures are not available via normal reporting.
f. How to apply Cold Temperature Altitude Corrections on an Approach.
1. All Segments Method: Pilots may correct all segment altitudes from the IAF altitude to the MA final
holding altitude. Pilots familiar with the information in this section and the procedures for accomplishing the all
segments method, only need to use the published “snowflake” icon,
/CTA temperature limit on the approach
chart for making corrections. Pilots are not required to reference the CTA list. The altitude correction is calculated
as follows:
(a) Manual correction: Pilots will make a manual correction when the aircraft is not equipped with a
temperature compensating system or when a compensating system is not used to make the correction. Use
TBL 7−3−1, ICAO Cold Temperature Error Table, to calculate the correction needed for the approach
segment(s).
(1) Correct all altitudes from the FAF/PFAF up to and including the IAF altitude: Calculate the
correction by taking the FAF/PFAF altitude and subtracting the airport elevation. Use this number to enter the
height above airport column in TBL 7−3−1 until reaching the reported temperature from the “Reported
Temperature” row. Round this number as applicable and then add to all altitudes from the FAF altitude through
the IAF altitude.
(2) Correct all altitudes in the final segment: Calculate the correction by taking the MDA or DA for
the approach being flown and subtract the airport elevation. Use this number to enter the height above airport
column in TBL 7−3−1 until reaching the reported temperature from the “Reported Temperature” row. Use this
number or round up to next nearest 100 ft. Add this number to MDA or DA, and any step−down fix altitudes in
the final segment.
(3) Correct final holding altitude in the MA Segment: Calculate the correction by taking the MA
holding altitude and subtract the airport elevation. Use this number to enter the height above airport column in
TBL 7−3−1 until reaching the reported temperature from the “Reported Temperature” row. Round this number
as applicable and then add to the final MA altitude only.
(b) Aircraft with temperature compensating systems: If flying an aircraft equipped with a system capable
of temperature compensation, follow the instructions for applying temperature compensation provided in the
airplane flight manual (AFM), AFM supplement, or system operating manual. Ensure that temperature
compensation system is on and active prior to the IAF and remains active throughout the entire approach and
missed approach.
(1) Pilots that have a system that is able to calculate a temperature-corrected DA or MDA may use the
system for this purpose.
(2) Pilots that have a system unable to calculate a temperature corrected DA or MDA will manually
calculate an altitude correction for the MDA or DA.
NOTE−
Some systems apply temperature compensation only to those altitudes associated with an instrument approach procedure
loaded into the active flight plan, while other systems apply temperature compensation to all procedure altitudes or user
entered altitudes in the active flight plan, including altitudes associated with a Standard Terminal Arrival (STAR). For those
systems that apply temperature compensation to all altitude s in the active flight plan, delay activating temperature
compensation until the aircraft has passed the last altitude constraint associated with the active STAR.
2. Individual Segment(s) Method: Pilots are allowed to correct only the marked segment(s) indicated in the
CTA list (https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dtpp/search/). Pilots using the
Individual Segment(s) Method will reference the CTA list to determine which segment(s) need a correction. (See
FIG 7−3−1.)
7−3−4 Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature
Airports (CTA)
2/20/25 AIM
FIG 7−3−1
Example Cold Temperature Restricted Airport List − Required Segments
Identifier Airport name Temperature Initial Intermediate Final Missed
Montana
KBTM Bert Mooney −25C X X X
KBZN Bozeman Yellowstone Intl −31C X
KEKS Ennis Big Sky −25C X
KGPI Glacier Park Intl −15C X
KHLN Helena Rgnl −17C X X X
(a) Manual Correction: Pilots will make a manual correction when the aircraft is not equipped with a
temperature compensating system or when a compensating system is not used to make the correction. Use
TBL 7−3−1, ICAO Cold Temperature Error Table, to calculate the correction needed for the approach
segment(s).
(1) Initial Segment: All altitudes from the intermediate fix (IF) altitude up to and including the IAF
altitude. The correction may be accomplished by using the IF altitude or by using the All Segments Method (a)
Manual correction (1). To correct the initial segment by using the IF altitude, subtract the airport elevation from
the IF altitude. Use this number to enter the height above airport column in TBL 7−3−1 until reaching the reported
temperature from the “Reported Temperature” row. Round this number as applicable and then add to the IF, IAF,
and any step-down fix altitudes.
(2) Intermediate Segment: All altitudes from the FAF/PFAF up to but not including the IF altitude.
Calculate the correction by taking FAF/PFAF altitude and subtracting the airport elevation. Use this number to
enter the height above airport column in TBL 7−3−1 until reaching the reported temperature from the “Reported
Temperature” row. Round this number as applicable and then add to FAF altitude and all step-down fix altitudes
within the intermediate segment (inside of the waypoint labeled “IF”).
(3) Final segment: Calculate the correction by taking the MDA or DA for the approach flown and
subtract the airport elevation. Use this number to enter the height above airport column in TBL 7 −3−1 until
reaching the reported temperature from the “Reported Temperature” row. Use this number or round up to next
nearest 100 ft. Add this number to MDA or DA and any applicable step-down fix altitudes in the final segment.
(4) Missed Approach Segment: Calculate the correction by taking the final MA holding altitude and
subtract the airport elevation. Use this number to enter the height above airport column in TBL 7 −3−1 until
reaching the reported temperature from the “Reported Temperature” row. Round this number as applicable and
then add to the final MA altitude only.
(b) Aircraft with temperature compensating system: If flying an aircraft equipped with a system capable
of temperature compensation, follow the instructions for applying temperature compensation provided in the
AFM, AFM supplement, or system operating manual. Ensure the temperature compensation system is on and
active prior to the segment(s) being corrected. Manually calculate an altimetry correction for the MDA or DA.
Determine an altimetry correction from the ICAO table based on the reported airport temperature and the height
difference between the MDA or DA, as applicable, and the airport elevation, or use the compensating system
to calculate a temperature corrected altitude for the published MDA or DA if able.
g. Communication: Pilots must request approval from ATC whenever applying a cold temperature altitude
correction. Pilots do not need to inform ATC of the final approach segment correction (i.e., new MDA or DA).
This request should be made on initial radio contact with the ATC facility issuing the approach clearance. ATC
requires this information in order to ensure appropriate vertical separation between known traffic. Pilots should
query ATC when vectored altitudes to a segment are lower than the requested corrected altitude. Pilots are
encouraged to self−announce corrected altitude when flying into a non−towered airfield.
1. The following are examples of appropriate pilot −to−ATC communication when applying
cold−temperature altitude corrections.
Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature 7−3−5
Airports (CTA)
AIM 2/20/25
(a) On initial check−in with ATC providing approach clearance: Missoula, MT (example below).
Vectors to final approach course: Outside of IAFs: “Request 9700 ft for cold temperature
operations.”
Vectors to final approach course: Inside of ODIRE: “Request 7300 ft for cold temperature
operations.”
Missed Approach segment: “Require final holding altitude, 12500 ft on missed approach for cold
temperature operations.”
(b) Pilots cleared by ATC for an instrument approach procedure; “Cleared the RNAV (GPS) Y RWY 12
approach (from any IAF)”. Missoula, MT (example below).
IAF: “Request 9700 ft for cold temperature operations at LANNY, CHARL, or ODIRE.”
7−3−6. Examples for Calculating Altitude Corrections on CTAs
All 14 CFR part 97 IAPs must be corrected at an airport. The following example provides the steps for correcting
the different segments of an approach and will be applied to all 14 CFR part 97 IAPs:
a. Missoula Intl (KMSO). Reported Temperature −12°C: RNA V (GPS) Y RWY 12.
1. All Segments Method: All segments corrected from IAF through MA holding altitude.
(a) Manual Calculation:
(1) Cold Temperature Restricted Airport Temperature Limit: −12°C.
(2) Altitude at the Final Approach Fix (FAF) (SUPPY) = 6200 ft.
(3) Airport elevation = 3206 ft.
(4) Difference: 6200 ft – 3206 ft = 2994 ft.
(5) Use TBL 7−3−1, ICAO Cold Temperature Error Table, a height above airport of 2994 ft and −12°C.
Visual interpolation is approximately 300 ft. Actual interpolation is 300 ft.
(6) Add 300 ft to the FAF and all procedure altitudes outside of the FAF up to and including IAF
altitude(s):
[a] LANNY (IAF), CHARL (IAF), and ODIRE (IAF Holding−in−Lieu): 9400 + 300 = 9700 ft.
[b] CALIP (stepdown fix): 7000 + 300 = 7300 ft.
[c] SUPPY (FAF): 6200 + 300 = 6500 ft.
(7) Correct altitudes within the final segment altitude based on the minima used. LP MDA = 4520 ft.
(8) Difference: 4520 ft – 3206 ft = 1314 ft.
(9) AIM 7−3−1 Table: 1314 ft at −12°C is approximately 150ft. Use 150 ft or round up to 200 ft.
(10) Add corrections to altitudes up to but not including the FAF:
[a] BEGPE (stepdown fix): 4840 + 150 = 4990 ft.
[b] LNA V MDA: 4520 + 150 = 4670 ft.
(11) Correct JENKI/Missed Approach Holding Altitude: MA altitude is 12000:
[a] JENKI: 12000 − 3206 = 8794 ft.
(12) TBL 7−3−1: 8794 ft at −12°C. Enter table at −12°C and intersect the 5000 ft height above airport
column. The approximate value is 500 ft.
(13) Add correction to holding fix final altitude:
7−3−6 Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature
Airports (CTA)
2/20/25 AIM
[a] JENKI: 12000 + 500 = 12500 ft.
b. Temperature Compensating System: Operators using a temperature compensating RNA V system to make
altitude corrections will be set to the current airport temperature (−12°C) and activated prior to passing the IAF.
A manual calculation of the cold temperature altitude correction is required for the MDA/DA.
1. Individual Segments Method: Missoula requires correction in the intermediate and final segments.
However, in this example, the missed approach is also shown.
(a) Manual Calculation: Use the appropriate steps in the All Segments Method above to apply a
correction to the required segment.
(1) Intermediate. Use steps 7−3−6a1(a)(1) thru (6). Do not correct the IAF or IF when using individual
segments method.
(2) Final. Use steps 7−3−6a1(a)(7) thru (10).
(3) Missed Approach. Use steps 7−3−6a1(a)(11) thru (13).
(b) Temperature Compensating System: Operators using a temperature compensating RNA V system to
make altitude corrections will be set to the current airport temperature (−12°C) and activated at a point needed
to correct the altitude for the segment. A manual calculation of the cold temperature altitude correction is required
for the MDA/DA.
Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature 7−3−7
Airports (CTA)
AIM 2/20/25
FIG 7−3−2
Missoula Intl RNA V (GPS) Y RWY 12
7−3−8 Cold Temperature Barometric Altimeter Errors, Setting Procedures and Cold Temperature
Airports (CTA)
2/20/25 AIM
Section 4. Wake Turbulence
7−4−1. General
a. Every aircraft generates wake turbulence while in flight. Wake turbulence is a function of an aircraft
producing lift, resulting in the formation of two counter−rotating vortices trailing behind the aircraft.
b. Wake turbulence from the generating aircraft can affect encountering aircraft due to the strength, duration,
and direction of the vortices. Wake turbulence can impose rolling moments exceeding the roll−control authority
of encountering aircraft, causing possible injury to occupants and damage to aircraft. Pilots should always be
aware of the possibility of a wake turbulence encounter when flying through the wake of another aircraft, and
adjust the flight path accordingly.
7−4−2. Vortex Generation
a. The creation of a pressure differential over the wing surface generates lift. The lowest pressure occurs over
the upper wing surface and the highest pressure under the wing. This pressure differential triggers the roll up of
the airflow at the rear of the wing resulting in swirling air masses trailing downstream of the wing tips. After the
roll up is completed, the wake consists of two counter−rotating cylindrical vortices. (See FIG 7−4−1.) The wake
vortex is formed with most of the energy concentrated within a few feet of the vortex core.
FIG 7−4−1
Wake Vortex Generation
b. More aircraft are being manufactured or retrofitted with winglets. There are several types of winglets, but
their primary function is to increase fuel efficiency by improving the lift−to−drag ratio. Studies have shown that
winglets have a negligible effect on wake turbulence generation, particularly with the slower speeds involved
during departures and arrivals.
7−4−3. Vortex Strength
a. Weight, speed, wingspan, and shape of the generating aircraft’s wing all govern the strength of the vortex.
The vortex characteristics of any given aircraft can also be changed by extension of flaps or other wing
configuring devices. However, the vortex strength from an aircraft increases proportionately to an increase in
operating weight or a decrease in aircraft speed. Since the turbulence from a “dirty” aircraft configuration hastens
wake decay, the greatest vortex strength occurs when the generating aircraft is HEA VY , CLEAN, and SLOW.
b. Induced Roll
Wake Turbulence 7−4−1
AIM 2/20/25
1. In rare instances, a wake encounter could cause catastrophic inflight structural damage to an aircraft.
However, the usual hazard is associated with induced rolling moments that can exceed the roll−control authority
of the encountering aircraft. During inflight testing, aircraft intentionally flew directly up trailing vortex cores
of larger aircraft. These tests demonstrated that the ability of aircraft to counteract the roll imposed by wake
vortex depends primarily on the wingspan and counter −control responsiveness of the encountering aircraft.
These tests also demonstrated the difficulty of an aircraft to remain within a wake vortex. The natural tendency
is for the circulation to eject aircraft from the vortex.
2. Counter control is usually effective and induced roll minimal in cases where the wingspan and ailerons
of the encountering aircraft extend beyond the rotational flow field of the vortex. It is more difficult for aircraft
with short wingspan (relative to the generating aircraft) to counter the imposed roll induced by vortex flow. Pilots
of short span aircraft, even of the high performance type, must be especially alert to vortex encounters. (See
FIG 7−4−2.)
FIG 7−4−2
Wake Encounter Counter Control
COUNTER
CONTROL
7−4−4. Vortex Behavior
a. Trailing vortices have certain behavioral characteristics which can help a pilot visualize the wake location
and thereby take avoidance precautions.
1. An aircraft generates vortices from the moment it rotates on takeoff to touchdown, since trailing vortices
are a by−product of wing lift. Prior to takeoff or touchdown pilots should note the rotation or touchdown point
of the preceding aircraft. (See FIG 7−4−3.)
2. The vortex circulation is outward, upward and around the wing tips when viewed from either ahead or
behind the aircraft. Tests with larger aircraft have shown that the vortices remain spaced a bit less than a wingspan
apart, drifting with the wind, at altitudes greater than a wingspan from the ground. In view of this, if persistent
vortex turbulence is encountered, a slight change of altitude (upward) and lateral position (upwind) should
provide a flight path clear of the turbulence.
3. Flight tests have shown that the vortices from larger aircraft sink at a rate of several hundred feet per
minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft.
Pilots should fly at or above the preceding aircraft’s flight path, altering course as necessary to avoid the area
directly behind and below the generating aircraft. (See FIG 7−4−4.) Pilots, in all phases of flight, must remain
vigilant of possible wake effects created by other aircraft. Studies have shown that atmospheric turbulence
hastens wake breakup, while other atmospheric conditions can transport wake horizontally and vertically.
4. When the vortices of larger aircraft sink close to the ground (within 100 to 200 feet), they tend to move
laterally over the ground at a speed of 2 or 3 knots. (See .FIG 7−4−5)
Wake Turbulence 7−4−2
2/20/25 AIM
FIG 7−4−3
Wake Ends/Wake Begins
Touchdown Rotation
Wake Ends Wake Begins
FIG 7−4−4
Vortex Flow Field
AVOIDAVOID Nominally 500-1000 Ft.Nominally 500-1000 Ft.
Sink Rate
Several Hundred Ft.,/Min.
Sink Rate
Several Hundred Ft.,/Min.
FIG 7−4−5
Vortex Movement Near Ground − No Wind
No WindNo Wind
3K3K 3K3K
Wake Turbulence 7−4−3
6K
AIM 2/20/25
FIG 7−4−6
Vortex Movement Near Ground − with Cross Winds
3K Wind3K Wind
6K 0 (3K - 3K)0 (3K - 3K)
(3K + 3K)(3K + 3K)
5. Pilots should be alert at all times for possible wake vortex encounters when conducting approach and
landing operations. The pilot is ultimately responsible for maintaining an appropriate interval, and should
consider all available information in positioning the aircraft in the terminal area, to avoid the wake turbulence
created by a preceding aircraft. Test data shows that vortices can rise with the air mass in which they are
embedded. The effects of wind shear can cause vortex flow field “tilting.” In addition, ambient thermal lifting
and orographic effects (rising terrain or tree lines) can cause a vortex flow field to rise and possibly bounce.
b. A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the
downwind vortex. Thus, a light wind with a cross−runway component of 1 to 5 knots could result in the upwind
vortex remaining in the touchdown zone for a period of time and hasten the drift of the downwind vortex toward
another runway. (See FIG 7−4−6.) Similarly, a tailwind condition can move the vortices of the preceding aircraft
forward into the touchdown zone. THE LIGHT QUARTERING TAILWIND REQUIRES MAXIMUM
CAUTION. Pilots should be alert to large aircraft upwind from their approach and takeoff flight paths. (See
FIG 7−4−7.)
Wake Turbulence7−4−4
2/20/25 AIM
FIG 7−4−7
Vortex Movement in Ground Effect − Tailwind
Light Quartering
Tailwind
Light Quartering
Tailwind
x
Tail WindTail Wind
Touchdown PointTouchdown Point
7−4−5. Operations Problem Areas
a. A wake turbulence encounter can range from negligible to catastrophic. The impact of the encounter
depends on the weight, wingspan, size of the generating aircraft, distance from the generating aircraft, and point
of vortex encounter. The probability of induced roll increases when the encountering aircraft’s heading is
generally aligned with the flight path of the generating aircraft.
b. A VOID THE AREA BELOW AND BEHIND THE WAKE GENERATING AIRCRAFT, ESPECIALLY
AT LOW ALTITUDE WHERE EVEN A MOMENTARY WAKE ENCOUNTER COULD BE
CATASTROPHIC.
NOTE−
A common scenario for a wake encounter is in terminal airspace after accepting clearance for a visual approach behind
landing traffic. Pilots must be cognizant of their position relative to the traffic and use all means of vertical guidance to
ensure they do not fly below the flight path of the wake generating aircraft.
c. Pilots should be particularly alert in calm wind conditions and situations where the vortices could:
1. Remain in the touchdown area.
2. Drift from aircraft operating on a nearby runway.
3. Sink into the takeoff or landing path from a crossing runway.
4. Sink into the traffic pattern from other airport operations.
5. Sink into the flight path of VFR aircraft operating on the hemispheric altitude 500 feet below.
d. Pilots should attempt to visualize the vortex trail of aircraft whose projected flight path they may encounter.
When possible, pilots of larger aircraft should adjust their flight paths to minimize vortex exposure to other
aircraft.
Wake Turbulence 7−4−5
AIM 2/20/25
7−4−6. Vortex Avoidance Procedures
a. Under certain conditions, airport traffic controllers apply procedures for separating IFR aircraft. If a pilot
accepts a clearance to visually follow a preceding aircraft, the pilot accepts responsibility for separation and wake
turbulence avoidance. The controllers will also provide to VFR aircraft, with whom they are in communication
and which in the tower’s opinion may be adversely affected by wake turbulence from a larger aircraft, the
position, altitude and direction of flight of la rger aircraft followed by the phrase “CAUTION − WAKE
TURBULENCE.” After issuing the caution for wake turbulence, the airport traffic controllers generally do not
provide additional information to the following aircraft unless the airport traffic controllers know the following
aircraft is overtaking the preceding aircraft. WHETHER OR NOT A WARNING OR INFORMATION HAS
BEEN GIVEN, HOWEVER, THE PILOT IS EXPECTED TO ADJUST AIRCRAFT OPERATIONS AND
FLIGHT P ATH AS NECESSARY TO PRECLUDE SERIOUS WAKE ENCOUNTERS. When any doubt exists
about maintaining safe separation distances between aircraft during approaches, pilots should ask the control
tower for updates on separation distance and aircraft groundspeed.
b. The following vortex avoidance procedures are recommended for the various situations:
1. Landing behind a larger aircraft− same runway. Stay at or above the larger aircraft’s final approach
flight path−note its touchdown point−land beyond it.
2. Landing behind a larger aircraft− when parallel runway is closer than 2,500 feet. Consider possible
drift to your runway. Stay at or above the larger aircraft’s final approach flight path− note its touchdown point.
3. Landing behind a larger aircraft− crossing runway. Cross above the larger aircraft’s flight path.
4. Landing behind a departing larger aircraft− same runway. Note the larger aircraft’s rotation point−
land well prior to rotation point.
5. Landing behind a departing larger aircraft − crossing runway. Note the larger aircraft’s rotation
point− if past the intersection− continue the approach− land prior to the intersection. If larger aircraft rotates prior
to the intersection, avoid flight below the larger aircraft’s flight path. Abandon the approach unless a landing is
ensured well before reaching the intersection.
6. Departing behind a larger aircraft. Note the larger aircraft’s rotation point and rotate prior to the larger
aircraft’s rotation point. Continue climbing above the larger aircraft’s climb path until turning clear of the larger
aircraft’s wake. Avoid subsequent headings which will cross below and behind a larger aircraft. Be alert for any
critical takeoff situation which could lead to a vortex encounter.
7. Intersection takeoffs − same runway. Be alert to adjacent larger ai rcraft operations, particularly
upwind of your runway. If intersection takeoff clearance is received, avoid subsequent heading which will cross
below a larger aircraft’s path.
8. Departing or landing after a larger aircraft executing a low approach, missed approach, or
touch−and−go landing. Because vortices settle and move laterally near the ground, the vortex hazard may exist
along the runway and in your flight path after a larger aircraft has executed a low approach, missed approach,
or a touch−and−go landing, particular in light quartering wind conditions. You should ensure that an interval of
at least 2 minutes has elapsed before your takeoff or landing.
9. En route VFR (thousand−foot altitude plus 500 feet). Avoid flight below and behind a large aircraft’s
path. If a larger aircraft is observed above on the same track (meeting or overtaking) adjust your position laterally,
preferably upwind.
7−4−7. Helicopters
In a slow hover taxi or stationary hover near the surface, helicopter main rotor(s) generate downwash producing
high velocity outwash vortices to a distance approximately three times the diameter of the rotor. When rotor
downwash hits the surface, the resulting outwash vortices have behavioral characteristics similar to wing tip
vortices produced by fixed wing aircraft. However, the vortex circulation is outward, upward, around, and away
Wake Turbulence 7−4−6
2/20/25 AIM
from the main rotor(s) in all directions. Pilots of small aircraft should avoid operating within three rotor diameters
of any helicopter in a slow hover taxi or stationary hover. In forward flight, departing or landing helicopters
produce a pair of strong, high−speed trailing vortices similar to wing tip vortices of larger fixed wing aircraft.
Pilots of small aircraft should use caution when operating behind or crossing behind landing and departing
helicopters.
7−4−8. Pilot Responsibility
a. Research and testing have been conducted, in addition to ongoing wake initiatives, in an attempt to mitigate
the effects of wake turbulence. Pilots must exercise vigilance in situations where they are responsible for
avoiding wake turbulence.
b. Pilots are reminded that in operations conducted behind all aircraft, acceptance of instructions from A TC
in the following situations is an acknowledgment that the pilot will ensure safe takeoff and landing intervals and
accepts the responsibility for providing wake turbulence separation.
1. Traffic information.
2. Instructions to follow an aircraft; and
3. The acceptance of a visual approach clearance.
c. For operations conducted behind super or heavy aircraft, ATC will specify the word “super” or “heavy”
as appropriate, when this information is known. Pilots of super or heavy aircraft should always use the word
“super” or “heavy” in radio communications.
d. Super, heavy, and large jet aircraft operators should use the following procedures during an approach to
landing. These procedures establish a dependable baseline from which pilots of in −trail, lighter aircraft may
reasonably expect to make effective flight path adjustments to avoid serious wake vortex turbulence.
1. Pilots of aircraft that produce strong wake vortices should make every attempt to fly on the established
glidepath, not above it; or, if glidepath guidance is not available, to fly as closely as possible to a “3−1” glidepath,
not above it.
EXAMPLE−
Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at 5 miles, 1,200 feet at 4 miles, and so on to touchdown.
2. Pilots of aircraft that produce strong wake vortices should fly as closely as possible to the approach course
centerline or to the extended centerline of the runway of intended landing as appropriate to conditions.
e. Pilots operating lighter aircraft on visual approaches in −trail to aircraft producing strong wake vortices
should use the following procedures to assist in avoiding wake turbulence. These procedures apply only to those
aircraft that are on visual approaches.
1. Pilots of lighter aircraft should fly on or above the glidepath. Glidepath reference may be furnished by
an ILS, by a visual approach slope system, by other ground−based approach slope guidance systems, or by other
means. In the absence of visible glidepath guidance, pilots may very nearly duplicate a 3−degree glideslope by
adhering to the “3 to 1” glidepath principle.
EXAMPLE−
Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at 5 miles, 1,200 feet at 4 miles, and so on to touchdown.
2. If the pilot of the lighter following aircraft has visual contact with the preceding heavier aircraft and also
with the runway, the pilot may further adjust for possible wake vortex turbulence by the following practices:
(a) Pick a point of landing no less than 1,000 feet from the arrival end of the runway.
(b) Establish a line−of−sight to that landing point that is above and in front of the heavier preceding
aircraft.
(c) When possible, note the point of landing of the heavier preceding aircraft and adjust point of intended
landing as necessary.
Wake Turbulence 7−4−7
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
EXAMPLE−
A puff of smoke may appear at the 1,000−foot markings of the runway, showing that touchdown was that point; therefore,
adjust point of intended landing to the 1,500−foot markings.
(d) Maintain the line−of−sight to the point of intended landing above and ahead of the heavier preceding
aircraft; maintain it to touchdown.
(e) Land beyond the point of landing of the preceding heavier aircraft. Ensure you have adequate runway
remaining, if conducting a touch−and−go landing, or adequate stopping distance available for a full stop landing.
f. During visual approaches pilots may ask ATC for updates on separation and groundspeed with respect to
heavier preceding aircraft, especially when there is any question of safe separation from wake turbulence.
g. Pilots should notify A TC when a wake event is encountered. Be as descriptive as possible (i.e., bank angle,
altitude deviations, intensity and duration of event, etc.) when reporting the event. ATC will record the event
through their reporting system. You are also encouraged to use the Aviation Safety Reporting System (ASRS)
to report wake events.
7−4−9. Air Traffic Wake Turbulence Separations
a. Because of the possible effects of wake turbulence, controllers are required to apply no less than minimum
required separation to all aircraft operating behind a Super or Heavy, and to Small aircraft operating behind a
B757, when aircraft are IFR; VFR and receiving Class B, Class C, or TRSA airspace services; or VFR and being
radar sequenced.
1. Typical separation applied to aircraft operating directly behind a super or heavy at the same altitude or
less than 1,000 feet below, and to small aircraft operating directly behind a B757 at the same altitude or less than
500 feet below:
(a) Heavy behind super − 5 miles.
(b) Large behind super − 7 miles.
(c) Small behind super − 8 miles.
(d) Heavy behind heavy −3 miles.
(e) Small/large behind heavy − 5 miles.
(f) Small behind B757 − 4 miles.
2. Also, separation, measured at the time the preceding aircraft is over the landing threshold, is provided
to small aircraft:
(a) Small landing behind heavy − 6 miles.
(b) Small landing behind large, non−B757 − 4 miles.
REFERENCE−
Pilot/Controller Glossary Term− Aircraft Classes.
b. Additionally, appropriate time or distance intervals are provided to departing aircraft when the departure
will be from the same threshold, a parallel runway separated by less than 2,500 feet with less than 500 feet
threshold stagger, or on a crossing runway and projected flight paths will cross:
1. Three minutes or the appropriate radar separation when takeoff will be behind a super aircraft;
2. Two minutes or the appropriate radar separation when takeoff will be behind a heavy aircraft.
3. Two minutes or the appropriate radar separation when a small aircraft will takeoff behind a B757.
NOTE−
Controllers may not reduce or waive these intervals.
c. A 3−minute interval will be provided when a small aircraft will takeoff:
Wake Turbulence 7−4−8
AIM2/20/251/22/26 AIM
1. From an intersection on the same runway (same or opposite direction) behind a departing large aircraft
(except B757), or
2. In the opposite direction on the same runway behind a large aircraft (except B757) takeoff or low/missed
approach.
NOTE−
This 3−minute interval may be waived upon specific pilot request.
d. A 3−minute interval will be provided when a small aircraft will takeoff:
1. From an intersection on the same runway (same or opposite direction) behind a departing B757, or
2. In the opposite direction on the same runway behind a B757 takeoff or low/missed approach.
NOTE−
This 3−minute interval may not be waived.
e. A 4−minute interval will be provided for all aircraft taking off behind a super aircraft, and a 3 −minute
interval will be provided for all aircraft taking off behind a heavy aircraft when the operations are as described
in subparagraphs c1 and c2 above, and are conducted on either the same runway or parallel runways separated
by less than 2,500 feet. Controllers may not reduce or waive this interval.
f. Pilots may request additional separation (i.e., 2 minutes instead of 4 or 5 miles) for wake turbulence
avoidance. This request should be made as soon as practical on ground control and at least before taxiing onto
the runway.
NOTE−
14 CFR section 91.3(a) states: “The pilot−in−command of an aircraft is directly responsible for and is the final authority
as to the operation of that aircraft.”
g. Controllers may anticipate separation and need not withhold a takeoff clearance for an aircraft departing
behind a large, heavy, or super aircraft if there is reasonable assurance the required separation will exist when
the departing aircraft starts takeoff roll.
Wake Turbulence 7−4−9
2/20/25 AIM
Section 5. Bird Hazards and Flight Over National
Refuges, Parks, and Forests
7−5−1. Migratory Bird Activity
a. Bird strike risk increases because of bird migration during the months of March through April, and August
through November.
b. The altitudes of migrating birds vary with winds aloft, weather fronts, terrain elevations, cloud conditions,
and other environmental variables. While over 90 percent of the reported bird strikes occur at or below 3,000
feet AGL, strikes at higher altitudes are common during migration. Ducks and geese are frequently observed up
to 7,000 feet AGL and pilots are cautioned to minimize en route flying at lower altitudes during migration.
c. Considered the greatest potential hazard to aircraft because of their size, abundance, or habit of flying in
dense flocks are gulls, waterfowl, vultures, hawks, owls, egrets, blackbirds, and starlings. Four major migratory
flyways exist in the U.S. The Atlantic flyway parallels the Atlantic Coast. The Mississippi Flyway stretches from
Canada through the Great Lakes and follows the Mississippi River. The Central Flyway represents a broad area
east of the Rockies, stretching from Canada through Central America. The Pacific Flyway follows the west coast
and overflies major parts of Washington, Oregon, and California. There are also numerous smaller flyways which
cross these major north-south migratory routes.
7−5−2. Reducing Bird Strike Risks
a. The most serious strikes are those involving ingestion into an engine (turboprops and turbine jet engines)
or windshield strikes. These strikes can result in emergency situations requiring prompt action by the pilot.
b. Engine ingestions may result in sudden loss of power or engine failure. Review engine out procedures,
especially when operating from airports with known bird hazards or when operating near high bird
concentrations.
c. Windshield strikes have resulted in pilots experiencing confusion, disorientation, loss of communications,
and aircraft control problems. Pilots are encouraged to review their emergency procedures before flying in these
areas.
d. When encountering birds en route, climb to avoid collision, because birds in flocks generally distribute
themselves downward, with lead birds being at the highest altitude.
e. Avoid overflight of known areas of bird concentration and flying at low altitudes during bird migration.
Charted wildlife refuges and other natural areas contain unusually high local concentration of birds which may
create a hazard to aircraft.
7−5−3. Reporting Bird Strikes
Pilots are urged to report any bird or other wildlife strike using FAA Form 5200−7, Bird/Other Wildlife Strike
Report (Appendix 1). Additional forms are available at any FSS; at any FAA Regional Office or at
https://www.faa.gov/airports/airport_safety/wildlife/. The data derived from these reports are used to develop
standards to cope with this potential hazard to aircraft and for documentation of necessary habitat control on
airports.
7−5−4. Reporting Bird and Other Wildlife Activities
If you observe birds or other animals on or near the runway, request airport management to disperse the wildlife
before taking off. Also contact the nearest FAA ARTCC, FSS, or tower (including non−Federal towers) regarding
large flocks of birds and report the:
Bird Hazards and Flight Over National Refuges, Parks, and Forests 7−5−1
AIM 2/20/25
a. Geographic location.
b. Bird type (geese, ducks, gulls, etc.).
c. Approximate numbers.
d. Altitude.
e. Direction of bird flight path.
7−5−5. Pilot Advisories on Bird and Other Wildlife Hazards
Many airports advise pilots of other wildlife hazards caused by large animals on the runway through the Chart
Supplement and the NOTAM system. Collisions of landing and departing aircraft and animals on the runway are
increasing and are not limited to rural airports. These accidents have also occurred at several major airports. Pilots
should exercise extreme caution when warned of the presence of wildlife on and in the vicinity of airports. If you
observe deer or other large animals in close proximity to movement areas, advise the FSS, tower, or airport
management.
7−5−6. Flights Over Charted U.S. Wildlife Refuges, Parks, and Forest Service Areas
a. The landing of aircraft is prohibited on lands or waters administered by the National Park Service, U.S. Fish
and Wildlife Service, or U.S. Forest Service without authorization from the respective agency. Exceptions
include:
1. When forced to land due to an emergency beyond the control of the operator;
2. At officially designated landing sites; or
3. An approved official business of the Federal Government.
b. Pilots are requested to maintain a minimum altitude of 2,000 feet above the surface of the following:
National Parks, Monuments, Seashores, Lakeshores, Recreation Areas and Scenic Riverways administered by
the National Park Service, National Wildlife Refuges, Big Game Refuges, Game Ranges and Wildlife Ranges
administered by the U.S. Fish and Wildlife Service, and Wilderness and Primitive areas administered by the U.S.
Forest Service.
NOTE−
F AA Advisory Circular AC 91−36, Visual Flight Rules (VFR) Flight Near Noise-Sensitive Areas, defines the surface of a
national park area (including parks, forests, primitive areas, wilderness areas, recreational areas, national seashores,
national monuments, national lakeshores, and national wildlife refuge and range areas) as: the highest terrain within 2,000
feet laterally of the route of flight, or the upper-most rim of a canyon or valley.
c. Federal statutes prohibit certain types of flight activity and/or provide altitude restrictions over designated
U.S. Wildlife Refuges, Parks, and Forest Service Areas. These designated areas, for example: Boundary Waters
Canoe Wilderness Areas, Minnesota; Haleakala National Park, Hawaii; Yosemite National Park, California; and
Grand Canyon National Park, Arizona, are charted on Sectional Charts.
d. Federal regulations also prohibit airdrops by parachute or other means of persons, cargo, or objects from
aircraft on lands administered by the three agencies without authorization from the respective agency. Exceptions
include:
1. Emergencies involving the safety of human life; or
2. Threat of serious property loss.
7−5−2 Bird Hazards and Flight Over National Refuges, Parks, and Forests
AIM2/20/258/7/25 AIM
Section 6. Potential Flight Hazards
7−6−1. Accident Causal Factors
a. The 10 most frequent causal factors for general aviation accidents that involve the pilot-in-command are:
1. Inadequate preflight preparation and/or planning.
2. Failure to obtain and/or maintain flying speed.
3. Failure to maintain direction control.
4. Improper level off.
5. Failure to see and avoid objects or obstructions.
6. Mismanagement of fuel.
7. Improper inflight decisions or planning.
8. Misjudgment of distance and speed.
9. Selection of unsuitable terrain.
10. Improper operation of flight controls.
b. This list remains relatively stable and points out the need for continued refresher training to establish a
higher level of flight proficiency for all pilots. A part of the FAA’s continuing effort to promote increased aviation
safety is the Aviation Safety Program. For information on Aviation Safety Program activities contact your nearest
Flight Standards District Office.
c. Alertness. Be alert at all times, especially when the weather is good. Most pilots pay attention to business
when they are operating in full IFR weather conditions, but strangely, air collisions almost invariably have
occurred under ideal weather conditions. Unlimited visibility appears to encourage a sense of security which is
not at all justified. Considerable information of value may be obtained by listening to advisories being issued
in the terminal area, even though controller workload may prevent a pilot from obtaining individual service.
d. Giving Way. If you think another aircraft is too close to you, give way instead of waiting for the other pilot
to respect the right-of-way to which you may be entitled. It is a lot safer to pursue the right-of-way angle after
you have completed your flight.
7−6−2. Reporting Radio/Radar Altimeter Anomalies
a. Background.
1. The radio altimeter (also known as radar altimeter or RADALT) is a safety−critical aircraft system used
to determine an aircraft’s height above terrain. It is the only sensor onboard the aircraft capable of providing a
direct measurement of the clearance height above the terrain and obstacles. Information from radio altimeters
is essential for flight operations as a main enabler of several safety−critical functions and systems on the aircraft.
The receiver on the radio altimeter is highly accurate because it is extremely sensitive, making it susceptible to
radio frequency interference (RFI). RFI in the C−band portion of the spectrum could impact the functions of the
radio altimeter during any phase of flight—most critically during takeoff, approach, and landing phases. This
could pose a serious risk to flight safety.
2. Installed radio altimeters normally supply critical height data to a wide range of automated safety
systems, navigation systems, and cockpit displays. Harmful RFI affecting the radio altimeter can cause these
safety and navigation systems to operate in unexpected ways and display erroneous information to the pilot. RFI
can interrupt, or significantly degrade, radio altimeter functions—precluding radio altimeter−based terrain alerts
Potential Flight Hazards 7−6−1
AIM 2/20/25
and low−visibility approach and landing operations. Systems of concern include Terrain Awareness Warning
Systems (TAWS), Enhanced Ground Proximity Warning Systems (EGPWS), and Traffic Collision Avoidance
Systems (TCAS), to name a few. Pilots of radio altimeter equipped aircraft should become familiar with the radio
altimeter’s interdependence with the other aircraft systems and expected failure modes and indications that may
be associated with harmful interference.
b. Actions. Recognizing interference/anomalies in the radio altimeter can be difficult, as it may present as
inoperative or erroneous data. Pilots need to monitor their automation, as well as their radio altimeters for
discrepancies, and be prepared to take action. Pilots encountering radio altimeter interference/anomalies should
transition to procedures that do not require the radio altimeter, and inform Air Traffic Control (ATC).
c. Inflight Reporting. Pilots should report any radio altimeter anomaly to ATC as soon as practical.
d. Post Flight Reporting.
1. Pilots are encouraged to submit detailed reports of radio altimeter interference/anomalies post flight as
soon as practical, by internet via the Radio Altimeter Anomaly Reporting Form at
https://www.faa.gov/air_traffic/nas/RADALT_reports/.
2. The post flight pilot reports of radio altimeter anomalies should contain as much of the following
information as applicable:
(a) Date and time the anomaly was observed;
(b) Location of the aircraft at the time the anomaly started and ended (e.g., latitude, longitude or
bearing/distance from a reference point or navigational aid);
(c) Magnetic heading;
(d) Altitude (MSL/AGL);
(e) Aircraft Type (make/model);
(f) Flight Number or Aircraft Registration Number;
(g) Meteorological conditions;
(h) Type of radio altimeter in use (e.g., make/model/software series or version), if known;
(i) Event overview;
(j) Consequences/operational impact (e.g., impacted equipment, actions taken to mitigate the disruption
and/or remedy provided by ATC, required post flight pilot and maintenance actions).
7−6−3. VFR in Congested Areas
A high percentage of near midair collisions occur below 8,000 feet AGL and within 30 miles of an airport. When
operating VFR in these highly congested areas, whether you intend to land at an airport within the area or are
just flying through, it is recommended that extra vigilance be maintained and that you monitor an appropriate
control frequency. Normally the appropriate frequency is an approach control frequency. By such monitoring
action you can “get the picture” of the traffic in your area. When the approach controller has radar, radar traffic
advisories may be given to VFR pilots upon request.
REFERENCE−
AIM, Para 4−1−15, Radar Traffic Information Service.
7−6−4. Obstructions To Flight
a. General. Many structures exist that could significantly affect the safety of your flight when operating below
500 feet above ground level (AGL), and particularly below 200 feet AGL. While 14 CFR part 91.119 allows
flight below 500 feet AGL when over sparsely populated areas or open water, such operations involve increased
7−6−2 Potential Flight Hazards
AIM2/20/258/7/25 AIM1/22/26 AIM
safety risks. At and below 200 feet AGL there are numerous power lines, antenna towers, etc., that are not marked
and lighted and/or charted as obstructions and, therefore, may not be seen in time to avoid a collision. Notices
to Airmen (NOTAMs) are issued on those lighted structures experiencing temporary light outages. However,
some time may pass before the FAA is notified of these outages, and the NOTAM issued, thus pilot vigilance
is imperative. Additionally, new obstructions may not be on current charts because the information was not
received prior to the FAA publishing the chart.
b. Antenna Towers. Extreme caution should be exercised when flying less than 2,000 feet AGL because of
numerous skeletal structures, such as radio and television antenna towers, that exceed 1,000 feet AGL with some
extending higher than 2,000 feet AGL. Most skeletal structures are supported by guy wires which are very
difficult to see in good weather and can be invisible at dusk or during periods of reduced visibility. These wires
can extend about 1,500 feet horizontally from a structure; therefore, all skeletal structures should be avoided
horizontally by at least 2,000 feet.
c. Overhead Wires. Overhead transmission and utility lines often span approaches to runways, natural
flyways such as lakes, rivers, gorges, and canyons, and cross other landmarks pilots frequently follow such as
highways, railroad tracks, etc. As with antenna towers, these power transmission and/or utility lines and the
supporting structures of these lines may not always be readily visible. The wires may be virtually impossible to
see under certain conditions. Spherical markers may be used to identify overhead wires and catenary
transmission lines and may be lighted. In some locations, the supporting structures of overhead transmission lines
are equipped with unique sequence flashing white strobe light systems to indicate that there are wires between
the structures. The flash sequence for the wire support structures will be middle, top, and bottom with all lights
on the same level flashing simultaneously. However, not all power transmission and/or utility lines require notice
to the FAA as they do not exceed 200 feet AGL or meet the obstruction standard of 14 CFR part 77 and, therefore,
are not marked and/or lighted. All pilots are cautioned to remain extremely vigilant for power transmission and/or
utility lines and their supporting structures when following natural flyways or during the approach and landing
phase. This is particularly important for seaplane and/or float equipped aircraft when landing on, or departing
from, unfamiliar lakes or rivers.
d. Wind Turbines. The number, size, and height of individual wind turbines and wind turbine farms have
increased over time. The locations of wind turbine farms have also expanded to areas more commonly flown by
VFR pilots and to all regions of the United States. VFR pilots should be aware that many wind turbines are
exceeding 499 feet AGL in height, which may affect minimum safe VFR altitudes in uncontrolled airspace. In
addition, many wind turbines are encroaching on the 700−foot AGL floor of controlled airspace (Class E). Pilots
are cautioned to maintain appropriate safe distance (laterally, vertically, or both). Wind turbines are typically
charted on Visual Flight Rules (VFR) Sectional Charts and/or Terminal Area Charts. For a description of how
wind turbines and wind turbine farms are charted, refer to the FAA Aeronautical Chart User’s Guide. Wind
turbines are normally painted white or light gray to improve daytime conspicuity. They are typically lit with
medium−intensity, flashing red lights, placed as high as possible on the turbine nacelle (not the blade tips), that
should be synchronized to flash together; however, not all wind turbine units within a farm need to be lighted,
depending on their location and height. Sometimes, only the perimeter of the wind turbine farm and an
arrangement of interior wind turbines are lit. Some wind turbine farms use Aircraft Detection Lighting Systems
(ADLS), which are proximity sensor−based systems designed to detect aircraft as they approach the obstruction.
This system automatically activates the appropriate obstruction lights until they are no longer needed based on
the position of the transiting aircraft. This technology reduces the impact of nighttime lighting on nearby
communities and migratory birds and extends the life expectancy of the obstruction lights. For more information
on how obstructions such as wind turbines are marked and lighted, refer to Advisory Circular 70/7460 −1,
Obstruction Marking and Lighting. Pilots should be aware that wind turbines in motion could result in limitations
of air traffic services in the vicinity of the wind turbine farms.
REFERENCE−
AIM, Para 4-5-1, Radar.
e. Meteorological Towers. Meteorological towers are used by wind energy companies to determine feasible
sites for wind turbines. Some of these towers are less than 200 feet AGL. These structures are portable, erected
Potential Flight Hazards 7−6−3
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in a matter of hours, installed with guyed wires, and constructed from a galvanized material often making them
difficult to see in certain atmospheric conditions. Markings for these towers include alternating bands of aviation
orange and white paint, and high −visibility sleeves installed on the outer guy wires. However, not all
Meteorological towers follow these guidelines, and pilots should be vigilant when flying at low altitude in remote
or rural areas.
f. Other Objects/Structures. There are other objects or structures that could adversely affect your flight such
as temporary construction cranes near an airport, newly constructed buildings, new towers, etc. Many of these
structures do not meet charting requirements or may not yet be charted because of the charting cycle. Some
structures do not require obstruction marking and/or lighting, and some may not be marked and lighted even
though the FAA recommended it. VFR pilots should carefully review NOTAMs for temporary or permanent
obstructions along the planned route of flight during their preflight preparations. Particular emphasis should be
given to obstructions in the vicinity of the approach and departure ends of the runway complex or any other areas
where flight below 500 feet AGL is planned or likely to occur.
7−6−5. Avoid Flight Beneath Unmanned Balloons
a. The majority of unmanned free balloons currently being operated have, extending below them, either a
suspension device to which the payload or instrument package is attached, or a trailing wire antenna, or both.
In many instances these balloon subsystems may be invisible to the pilot until the aircraft is close to the balloon,
thereby creating a potentially dangerous situation. Therefore, good judgment on the part of the pilot dictates that
aircraft should remain well clear of all unmanned free balloons and flight below them should be avoided at all
times.
b. Pilots are urged to report any unmanned free balloons sighted to the nearest FAA ground facility with which
communication is established. Such information will assist FAA ATC facilities to identify and flight follow
unmanned free balloons operating in the airspace.
7−6−6. Unmanned Aircraft Systems
a. Unmanned Aircraft Systems (UAS), formerly referred to as “Unmanned Aerial Vehicles” (UA Vs) or
“drones,” are having an increasing operational presence in the NAS. Once the exclusive domain of the military,
UAS are now being operated by various entities. Although these aircraft are “unmanned,” UAS are flown by a
remotely located pilot and crew. Physical and performance characteristics of unmanned aircraft (UA) vary
greatly and unlike model aircraft that typically operate lower than 400 feet AGL, UA may be found operating
at virtually any altitude and any speed. Sizes of UA can be as small as several pounds to as large as a commercial
transport aircraft. UAS come in various categories including airplane, rotorcraft, powered−lift (tilt−rotor), and
lighter−than−air. Propulsion systems of UAS include a broad range of alternatives from piston powered and
turbojet engines to battery and solar−powered electric motors.
b. To ensure segregation of UAS operations from other aircraft, the military typically conducts UAS
operations within restricted or other special use airspace. However, UAS operations are now being approved in
the NAS outside of special use airspace through the use of FAA−issued Certificates of Waiver or Authorization
(COA) or through the issuance of a special airworthiness certificate. COA and special airworthiness approvals
authorize UAS flight operations to be contained within specific geographic boundaries and altitudes, usually
require coordination with an ATC facility, and typically require the issuance of a NOTAM describing the
operation to be conducted. UAS approvals also require observers to provide “see−and−avoid” capability to the
UAS crew and to provide the necessary compliance with 14 CFR section 91.113. For UAS operations approved
at or above FL180, UAS operate under the same requirements as that of manned aircraft (i.e., flights are operated
under instrument flight rules, are in communication with ATC, and are appropriately equipped).
c. UAS operations may be approved at either controlled or uncontrolled airports and are typically
disseminated by NOTAM. In all cases, approved UAS operations must comply with all applicable regulations
and/or special provisions specified in the COA or in the operating limitations of the special airworthiness
7−6−4 Potential Flight Hazards
2/20/25 AIM
certificate. At uncontrolled airports, UAS operations are advised to operate well clear of all known manned
aircraft operations. Pilots of manned aircraft are advised to follow normal operating procedures and are urged
to monitor the CTAF for any potential UAS activity. At controlled airports, local ATC procedures may be in place
to handle UAS operations and should not require any special procedures from manned aircraft entering or
departing the traffic pattern or operating in the vicinity of the airport.
d. In addition to approved UAS operations described above, a recently approved agreement between the FAA
and the Department of Defense authorizes small UAS operations wholly contained within Class G airspace, and
in no instance, greater than 1200 feet AGL over military owned or leased property. These operations do not
require any special authorization as long as the UA remains within the lateral boundaries of the military
installation as well as other provisions including the issuance of a NOTAM. Unlike special use airspace, these
areas may not be depicted on an aeronautical chart.
e. There are several factors a pilot should consider regarding UAS activity in an effort to reduce potential flight
hazards. Pilots are urged to exercise increased vigilance when operating in the vicinity of restricted or other
special use airspace, military operations areas, and any military installation. Areas with a preponderance of UAS
activity are typically noted on sectional charts advising pilots of this activity. Since the size of a UA can be very
small, they may be difficult to see and track. If a UA is encountered during flight, as with manned aircraft, never
assume that the pilot or crew of the UAS can see you, maintain increased vigilance with the UA and always be
prepared for evasive action if necessary. Always check NOTAMs for potential UAS activity along the intended
route of flight and exercise increased vigilance in areas specified in the NOTAM.
7−6−7. Mountain Flying
a. Your first experience of flying over mountainous terrain (particularly if most of your flight time has been
over the flatlands of the Midwest) could be a never-to-be-forgotten nightmare if proper planning is not done and
if you are not aware of the potential hazards awaiting. Those familiar section lines are not present in the
mountains; those flat, level fields for forced landings are practically nonexistent; abrupt changes in wind
direction and velocity occur; severe updrafts and downdrafts are common, particularly near or above abrupt
changes of terrain such as cliffs or rugged areas; even the clouds look different and can build up with startling
rapidity. Mountain flying need not be hazardous if you follow the recommendations below.
b. File a Flight Plan. Plan your route to avoid topography which would prevent a safe forced landing. The
route should be over populated areas and well known mountain passes. Sufficient altitude should be maintained
to permit gliding to a safe landing in the event of engine failure.
c. Don’t fly a light aircraft when the winds aloft, at your proposed altitude, exceed 35 miles per hour. Expect
the winds to be of much greater velocity over mountain passes than reported a few miles from them. Approach
mountain passes with as much altitude as possible. Downdrafts of from 1,500 to 2,000 feet per minute are not
uncommon on the leeward side.
d. Don’t fly near or above abrupt changes in terrain. Severe turbulence can be expected, especially in high
wind conditions.
e. Understand Mountain Obscuration. The term Mountain Obscuration (MTOS) is used to describe a
visibility condition that is distinguished from IFR because ceilings, by definition, are described as “above ground
level” (AGL). In mountainous terrain clouds can form at altitudes significantly higher than the weather reporting
station and at the same time nearby mountaintops may be obscured by low visibility. In these areas the ground
level can also vary greatly over a small area. Beware if operating VFR−on−top. You could be operating closer
to the terrain than you think because the tops of mountains are hidden in a cloud deck below. MTOS areas are
identified daily on The Aviation Weather Center located at: http://www.aviationweather.gov.
f. Navigating in confined terrain when flying through mountain passes can be challenging. For high−traffic
mountain passes, VFR checkpoints may be provided on VFR navigation charts to increase situational awareness
by indicating key landmarks inside confined terrain. A collocated VFR waypoint and checkpoint may be
provided to assist with identifying natural entry points for commonly flown mountain passes. Pilots should
Potential Flight Hazards 7−6−5
AIM 2/20/25
reference the name of the charted VFR checkpoint, wherever possible, when making position reports on CTAF
frequencies to reduce the risk of midair collisions. Pilots should evaluate the terrain along the route they intend
to fly with respect to their aircraft type and performance capabilities, local weather, and their experience level
to avoid flying into confined areas without adequate room to execute a 180 degree turn, should conditions require.
Always fly with a planned escape route in mind.
REFERENCE−
AIM, Para 1−1−17, Global Positioning System (GPS).
g. VFR flight operations may be conducted at night in mountainous terrain with the application of sound
judgment and common sense. Proper pre-flight planning, giving ample consideration to winds and weather,
knowledge of the terrain and pilot experience in mountain flying are prerequisites for safety of flight. Continuous
visual contact with the surface and obstructions is a major concern and flight operations under an overcast or in
the vicinity of clouds should be approached with extreme caution.
h. When landing at a high altitude field, the same indicated airspeed should be used as at low elevation fields.
Remember: that due to the less dense air at altitude, this same indicated airspeed actually results in higher true
airspeed, a faster landing speed, and more important, a longer landing distance. During gusty wind conditions
which often prevail at high altitude fields, a power approach and power landing is recommended. Additionally,
due to the faster groundspeed, your takeoff distance will increase considerably over that required at low altitudes.
i. Effects of Density Altitude. Performance figures in the aircraft owner’s handbook for length of takeoff
run, horsepower, rate of climb, etc., are generally based on standard atmosphere conditions (59 degrees
Fahrenheit (15 degrees Celsius), pressure 29.92 inches of mercury) at sea level. However, inexperienced pilots,
as well as experienced pilots, may run into trouble when they encounter an altogether different set of conditions.
This is particularly true in hot weather and at higher elevations. Aircraft operations at altitudes above sea level
and at higher than standard temperatures are commonplace in mountainous areas. Such operations quite often
result in a drastic reduction of aircraft performance capabilities because of the changing air density. Density
altitude is a measure of air density. It is not to be confused with pressure altitude, true altitude, or absolute altitude.
It is not to be used as a height reference, but as a determining criteria in the performance capability of an aircraft.
Air density decreases with altitude. As air density decreases, density altitude increases. The further effects of high
temperature and high humidity are cumulative, resulting in an increasing high density altitude condition. High
density altitude reduces all aircraft performance parameters. To the pilot, this means that the normal horsepower
output is reduced, propeller efficiency is reduced, and a higher true airspeed is required to sustain the aircraft
throughout its operating parameters. It means an increase in runway length requirements for takeoff and landings,
and decreased rate of climb. An average small airplane, for example, requiring 1,000 feet for takeoff at sea level
under standard atmospheric conditions will require a takeoff run of approximately 2,000 feet at an operational
altitude of 5,000 feet.
NOTE−
A turbo-charged aircraft engine provides a slight advantage in that it provides sea level horsepower up to a specified altitude
above sea level.
1. Density Altitude Advisories. At airports with elevations of 2,000 feet and higher, control towers and
FSSs will broadcast the advisory “Check Density Altitude” when the temperature reaches a predetermined level.
These advisories will be broadcast on appropriate tower frequencies or, where available, ATIS. FSSs will
broadcast these advisories as a part of Local Airport Advisory.
2. These advisories are provided by air traffic facilities, as a reminder to pilots that high temperatures and
high field elevations will cause significant changes in aircraft characteristics. The pilot retains the responsibility
to compute density altitude, when appropriate, as a part of preflight duties.
NOTE−
All FSSs will compute the current density altitude upon request.
j. Mountain Wave. Many pilots go all their lives without understanding what a mountain wave is. Quite a
few have lost their lives because of this lack of understanding. One need not be a licensed meteorologist to
understand the mountain wave phenomenon.
7−6−6 Potential Flight Hazards
2/20/25 AIM
1. Mountain waves occur when air is being blown over a mountain range or even the ridge of a sharp bluff
area. As the air hits the upwind side of the range, it starts to climb, thus creating what is generally a smooth updraft
which turns into a turbulent downdraft as the air passes the crest of the ridge. From this point, for many miles
downwind, there will be a series of downdrafts and updrafts. Satellite photos of the Rockies have shown
mountain waves extending as far as 700 miles downwind of the range. Along the east coast area, such photos
of the Appalachian chain have picked up the mountain wave phenomenon over a hundred miles eastward. All
it takes to form a mountain wave is wind blowing across the range at 15 knots or better at an intersection angle
of not less than 30 degrees.
2. Pilots from flatland areas should understand a few things about mountain waves in order to stay out of
trouble. When approaching a mountain range from the upwind side (generally the west), there will usually be
a smooth updraft; therefore, it is not quite as dangerous an area as the lee of the range. From the leeward side,
it is always a good idea to add an extra thousand feet or so of altitude because downdrafts can exceed the climb
capability of the aircraft. Never expect an updraft when approaching a mountain chain from the leeward. Always
be prepared to cope with a downdraft and turbulence.
3. When approaching a mountain ridge from the downwind side, it is recommended that the ridge be
approached at approximately a 45 degree angle to the horizontal direction of the ridge. This permits a safer retreat
from the ridge with less stress on the aircraft should severe turbulence and downdraft be experienced. If severe
turbulence is encountered, simultaneously reduce power and adjust pitch until aircraft approaches maneuvering
speed, then adjust power and trim to maintain maneuvering speed and fly away from the turbulent area.
7−6−8. Use of Runway Half−way Signs at Unimproved Airports
When installed, runway half−way signs provide the pilot with a reference point to judge takeoff acceleration trends.
Assuming that the runway length is appropriate for takeoff (considering runway condition and slope, elevation,
aircraft weight, wind, and temperature), typical takeoff acceleration should allow the airplane to reach 70 percent
of lift−off airspeed by the midpoint of the runway. The “rule of thumb” is that should airplane acceleration not allow
the airspeed to reach this value by the midpoint, the takeoff should be aborted, as it may not be possible to liftoff
in the remaining runway.
Several points are important when considering using this “rule of thumb”:
a. Airspeed indicators in small airplanes are not required to be evaluated at speeds below stalling, and may not
be usable at 70 percent of liftoff airspeed.
b. This “rule of thumb” is based on a uniform surface condition. Puddles, soft spots, areas of tall and/or wet
grass, loose gravel, etc., may impede acceleration or even cause deceleration. Even if the airplane achieves 70
percent of liftoff airspeed by the midpoint, the condition of the remainder of the runway may not allow further
acceleration. The entire length of the runway should be inspected prior to takeoff to ensure a usable surface.
c. This “rule of thumb” applies only to runway required for actual liftoff. In the event that obstacles affect the
takeoff climb path, appropriate distance must be available after liftoff to accelerate to best angle of climb speed
and to clear the obstacles. This will, in effect, require the airplane to accelerate to a higher speed by midpoint,
particularly if the obstacles are close to the end of the runway. In addition, this technique does not take into account
the effects of upslope or tailwinds on takeoff performance. These factors will also require greater acceleration than
normal and, under some circumstances, prevent takeoff entirely.
d. Use of this “rule of thumb” does not alleviate the pilot’s responsibility to comply with applicable Federal
Aviation Regulations, the limitations and performance data provided in the FAA approved Airplane Flight Manual
(AFM), or, in the absence of an FAA approved AFM, other data provided by the aircraft manufacturer.
In addition to their use during takeoff, runway half−way signs offer the pilot increased awareness of his or her
position along the runway during landing operations.
NOTE−
No F AA standard exists for the appearance of the runway half−way sign. FIG 7−6−1 shows a graphical depiction of a typical
runway half−way sign.
Potential Flight Hazards 7−6−7
AIM 2/20/25
FIG 7−6−1
Typical Runway Half−way Sign
7−6−9. Seaplane Safety
a. Acquiring a seaplane class rating affords access to many areas not available to landplane pilots. Adding a
seaplane class rating to your pilot certificate can be relatively uncomplicated and inexpensive. However, more
effort is required to become a safe, efficient, competent “bush” pilot. The natural hazards of the backwoods have
given way to modern man-made hazards. Except for the far north, the available bodies of water are no longer
the exclusive domain of the airman. Seaplane pilots must be vigilant for hazards such as electric power lines,
power, sail and rowboats, rafts, mooring lines, water skiers, swimmers, etc.
b. Seaplane pilots must have a thorough understanding of the right-of-way rules as they apply to aircraft
versus other vessels. Seaplane pilots are expected to know and adhere to both the U.S. Coast Guard’s (USCG)
Navigation Rules, International−Inland, and 14 CFR section 91.115, Right−of−Way Rules; Water Operations.
The navigation rules of the road are a set of collision avoidance rules as they apply to aircraft on the water. A
seaplane is considered a vessel when on the water for the purposes of these collision avoidance rules. In general,
a seaplane on the water must keep well clear of all vessels and avoid impeding their navigation. The CFR requires,
in part, that aircraft operating on the water “. . . shall, insofar as possible, keep clear of all vessels and avoid
impeding their navigation, and shall give way to any vessel or other aircraft that is given the right−of−way . . .
.” This means that a seaplane should avoid boats and commercial shipping when on the water. If on a collision
course, the seaplane should slow, stop, or maneuver to the right, away from the bow of the oncoming vessel. Also,
while on the surface with an engine running, an aircraft must give way to all nonpowered vessels. Since a seaplane
in the water may not be as maneuverable as one in the air, the aircraft on the water has right-of-way over one
in the air, and one taking off has right-of-way over one landing. A seaplane is exempt from the USCG safety
equipment requirements, including the requirements for Personal Flotation Devices (PFD). Requiring seaplanes
on the water to comply with USCG equipment requirements in addition to the FAA equipment requirements
would be an unnecessary burden on seaplane owners and operators.
c. Unless they are under Federal jurisdiction, navigable bodies of water are under the jurisdiction of the state,
or in a few cases, privately owned. Unless they are specifically restricted, aircraft have as much right to operate
on these bodies of water as other vessels. To avoid problems, check with Federal or local officials in advance
of operating on unfamiliar waters. In addition to the agencies listed in TBL 7−6−1, the nearest Flight Standards
District Office can usually offer some practical suggestions as well as regulatory information. If you land on a
restricted body of water because of an inflight emergency, or in ignorance of the restrictions you have violated,
report as quickly as practical to the nearest local official having jurisdiction and explain your situation.
d. When operating a seaplane over or into remote areas, appropriate attention should be given to survival gear.
Minimum kits are recommended for summer and winter, and are required by law for flight into sparsely settled
7−6−8 Potential Flight Hazards
AIM2/20/251/22/26 AIM
areas of Canada and Alaska. Alaska State Department of Transportation and Canadian Ministry of Transport
officials can provide specific information on survival gear requirements. The kit should be assembled in one
container and be easily reachable and preferably floatable.
TBL 7−6−1
Jurisdictions Controlling Navigable Bodies of Water
Authority to Consult For Use of a Body of Water
Location Authority Contact
Wilderness Area U.S. Department of Agriculture, Forest Service Local forest ranger
National Forest USDA Forest Service Local forest ranger
National Park U.S. Department of the Interior (USDI), National Park Service Local park ranger
Indian Reservation USDI, Bureau of Indian Affairs Local Bureau office
State Park State government or state forestry or park service Local state aviation office for
further information
Canadian National and
Provincial Parks
Supervised and restricted on an individual basis from province to
province and by different departments of the Canadian
government; consult Canadian Flight Information Manual and/or
Water Aerodrome Supplement
Park Superintendent in an
emergency
e. The FAA recommends that each seaplane owner or operator provide flotation gear for occupants any time
a seaplane operates on or near water. 14 CFR section 91.205(b)(12) requires approved flotation gear for aircraft
operated for hire over water and beyond power-off gliding distance from shore. FAA-approved gear differs from
that required for navigable waterways under USCG rules. FAA-approved life vests are inflatable designs as
compared to the USCG’s noninflatable PFD’s that may consist of solid, bulky material. Such USCG PFDs are
impractical for seaplanes and other aircraft because they may block passage through the relatively narrow exits
available to pilots and passengers. Life vests approved under Technical Standard Order (TSO) TSO −C13E
contain fully inflatable compartments. The wearer inflates the compartments (AFTER exiting the aircraft)
primarily by independent CO2 cartridges, with an oral inflation tube as a backup. The flotation gear also contains
a water-activated, self-illuminating signal light. The fact that pilots and passengers can easily don and wear
inflatable life vests (when not inflated) provides maximum effectiveness and allows for unrestricted movement.
It is imperative that passengers are briefed on the location and proper use of available PFDs prior to leaving the
dock.
f. The FAA recommends that seaplane owners and operators obtain Advisory Circular (AC) 91−69, Seaplane
Safety for 14 CFR part 91 Operations, free from the U.S. Department of Transportation, Subsequent Distribution
Office, SVC−121.23, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785; fax: (301)
386−5394. The USCG Navigation Rules International−Inland (COMDTINSTM 16672.2B) is available for a fee
from the Government Publishing Office by facsimile request to (202) 512 −2250, and can be ordered using
Mastercard or Visa.
7−6−10. Flight Operations in Volcanic Ash
a. Severe volcanic eruptions which send ash and sulphur dioxide (SO2) gas into the upper atmosphere occur
somewhere around the world several times each year. Flying into a volcanic ash cloud can be exceedingly
dangerous. A B747−200 lost all four engines after such an encounter and a B747 −400 had the same nearly
catastrophic experience. Piston−powered aircraft are less likely to lose power but severe damage is almost certain
to ensue after an encounter with a volcanic ash cloud which is only a few hours old.
b. Most important is to avoid any encounter with volcanic ash. The ash plume may not be visible, especially
in instrument conditions or at night; and even if visible, it is difficult to distinguish visually between an ash cloud
and an ordinary weather cloud. V olcanic ash clouds are not displayed on airborne or ATC radar. The pilot must
rely on reports from air traffic controllers and other pilots to determine the location of the ash cloud and use that
information to remain well clear of the area. Additionally, the presence of a sulphur-like odor throughout the
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AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
cabin may indicate the presence of SO2 emitted by volcanic activity, but may or may not indicate the presence
of volcanic ash. Every attempt should be made to remain on the upwind side of the volcano.
c. It is recommended that pilots encountering an ash cloud should immediately reduce thrust to idle (altitude
permitting), and reverse course in order to escape from the cloud. Ash clouds may extend for hundreds of miles
and pilots should not attempt to fly through or climb out of the cloud. In addition, the following procedures are
recommended:
1. Disengage the autothrottle if engaged. This will prevent the autothrottle from increasing engine thrust;
2. Turn on continuous ignition;
3. Turn on all accessory airbleeds including all air conditioning packs, nacelles, and wing anti-ice. This will
provide an additional engine stall margin by reducing engine pressure.
d. The following has been reported by flightcrews who have experienced encounters with volcanic dust
clouds:
1. Smoke or dust appearing in the cockpit.
2. An acrid odor similar to electrical smoke.
3. Multiple engine malfunctions, such as compressor stalls, increasing Exhaust Gas Temperature (EGT),
torching from tailpipe, and flameouts.
4. At night, St. Elmo’s fire or other static discharges accompanied by a bright orange glow in the engine
inlets.
5. A fire warning in the forward cargo area.
e. It may become necessary to shut down and then restart engines to prevent exceeding EGT limits. V olcanic
ash may block the pitot system and result in unreliable airspeed indications.
f. If you see a volcanic eruption and have not been previously notified of it, you may have been the first person
to observe it. In this case, immediately contact ATC and alert them to the existence of the eruption. If possible,
use the V olcanic Activity Reporting form (V AR) depicted in Appendix 2 of this manual. Items 1 through 8 of
the V AR should be transmitted immediately. The information requested in items 9 through 16 should be passed
after landing. If a V AR form is not immediately available, relay enough information to identify the position and
nature of the volcanic activity. Do not become unnecessarily alarmed if there is merely steam or very low-level
eruptions of ash.
g. When landing at airports where volcanic ash has been deposited on the runway, be aware that even a thin
layer of dry ash can be detrimental to braking action. Wet ash on the runway may also reduce effectiveness of
braking. It is recommended that reverse thrust be limited to minimum practical to reduce the possibility of
reduced visibility and engine ingestion of airborne ash.
h. When departing from airports where volcanic ash has been deposited, it is recommended that pilots avoid
operating in visible airborne ash. Allow ash to settle before initiating takeoff roll. It is also recommended that
flap extension be delayed until initiating the before takeoff checklist and that a rolling takeoff be executed to
avoid blowing ash back into the air.
7−6−11. Emergency Airborne Inspection of Other Aircraft
a. Providing airborne assistance to another aircraft may involve flying in very close proximity to that aircraft.
Most pilots receive little, if any, formal training or instruction in this type of flying activity. Close proximity
flying without sufficient time to plan (i.e., in an emergency situation), coupled with the stress involved in a
perceived emergency can be hazardous.
b. The pilot in the best position to assess the situation should take the responsibility of coordinating the
airborne intercept and inspection, and take into account the unique flight characteristics and differences of the
category(s) of aircraft involved.
7−6−10 Potential Flight Hazards
2/20/25 AIM
c. Some of the safety considerations are:
1. Area, direction and speed of the intercept;
2. Aerodynamic effects (i.e., rotorcraft downwash);
3. Minimum safe separation distances;
4. Communications requirements, lost communications procedures, coordination with ATC;
5. Suitability of diverting the distressed aircraft to the nearest safe airport; and
6. Emergency actions to terminate the intercept.
d. Close proximity, inflight inspection of another aircraft is uniquely hazardous. The pilot−in−command of
the aircraft experiencing the problem/emergency must not relinquish control of the situation and/or jeopardize
the safety of their aircraft. The maneuver must be accomplished with minimum risk to both aircraft.
7−6−12. Precipitation Static
a. Precipitation static is caused by aircraft in flight coming in contact with uncharged particles. These particles
can be rain, snow, fog, sleet, hail, volcanic ash, dust; any solid or liquid particles. When the aircraft strikes these
neutral particles the positive element of the particle is reflected away from the aircraft and the negative particle
adheres to the skin of the aircraft. In a very short period of time a substantial negative charge will develop on
the skin of the aircraft. If the aircraft is not equipped with static dischargers, or has an ineffective static discharger
system, when a sufficient negative voltage level is reached, the aircraft may go into “CORONA.” That is, it will
discharge the static electricity from the extremities of the aircraft, such as the wing tips, horizontal stabilizer,
vertical stabilizer, antenna, propeller tips, etc. This discharge of static electricity is what you will hear in your
headphones and is what we call P−static.
b. A review of pilot reports often shows different symptoms with each problem that is encountered. The
following list of problems is a summary of many pilot reports from many different aircraft. Each problem was
caused by P−static:
1. Complete loss of VHF communications.
2. Erroneous magnetic compass readings (30 percent in error).
3. High pitched squeal on audio.
4. Motor boat sound on audio.
5. Loss of all avionics in clouds.
6. VLF navigation system inoperative most of the time.
7. Erratic instrument readouts.
8. Weak transmissions and poor receptivity of radios.
9. “St. Elmo’s Fire” on windshield.
c. Each of these symptoms is caused by one general problem on the airframe. This problem is the inability
of the accumulated charge to flow easily to the wing tips and tail of the airframe, and properly discharge to the
airstream.
d. Static dischargers work on the principal of creating a relatively easy path for discharging negative charges
that develop on the aircraft by using a discharger with fine metal points, carbon coated rods, or carbon wicks
rather than wait until a large charge is developed and discharged off the trailing edges of the aircraft that will
interfere with avionics equipment. This process offers approximately 50 decibels (dB) static noise reduction
which is adequate in most cases to be below the threshold of noise that would cause interference in avionics
equipment.
Potential Flight Hazards 7−6−11
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
e. It is important to remember that precipitation static problems can only be corrected with the proper number
of quality static dischargers, properly installed on a properly bonded aircraft. P−static is indeed a problem in the
all weather operation of the aircraft, but there are effective ways to combat it. All possible methods of reducing
the effects of P− static should be considered so as to provide the best possible performance in the flight
environment.
f. A wide variety of discharger designs is available on the commercial market. The inclusion of well−designed
dischargers may be expected to improve airframe noise in P−static conditions by as much as 50 dB. Essentially,
the discharger provides a path by which accumulated charge may leave the airframe quietly. This is generally
accomplished by providing a group of tiny corona points to permit onset of corona−current flow at a low aircraft
potential. Additionally, aerodynamic design of dischargers to permit corona to occur at the lowest possible
atmospheric pressure also lowers the corona threshold. In addition to permitting a low−potential discharge, the
discharger will minimize the radiation of radio frequency (RF) energy that accompanies the corona discharge,
in order to minimize effects of RF components at communications and navigation frequencies on avionics
performance. These effects are reduced through resistive attachment of the corona point(s) to the airframe,
preserving direct current connection but attenuating the higher−frequency components of the discharge.
g. Each manufacturer of static dischargers offers information concerning appropriate discharger location on
specific airframes. Such locations emphasize the trailing outboard surfaces of wings and horizontal tail surfaces,
plus the tip of the vertical stabilizer, where charge tends to accumulate on the airframe. Sufficient dischargers
must be provided to allow for current−carrying capacity which will maintain airframe potential below the corona
threshold of the trailing edges.
h. In order to achieve full performance of avionic equipment, the static discharge system will require periodic
maintenance. A pilot knowledgeable of P−static causes and effects is an important element in assuring optimum
performance by early recognition of these types of problems.
7−6−13. Light Amplification by Stimulated Emission of Radiation (Laser) Operations and
Reporting Illumination of Aircraft
a. Lasers have many applications. Of concern to users of the National Airspace System are those laser events
that may affect pilots, e.g., outdoor laser light shows or demonstrations for entertainment and advertisements at
special events and theme parks. Generally, the beams from these events appear as bright blue−green in color;
however, they may be red, yellow, or white. However, some laser systems produce light which is invisible to the
human eye.
b. FAA regulations prohibit the disruption of aviation activity by any person on the ground or in the air. The
FAA and the Food and Drug Administration (the Federal agency that has the responsibility to enforce compliance
with Federal requirements for laser systems and laser light show products) are working together to ensure that
operators of these devices do not pose a hazard to aircraft operators.
c. Pilots should be aware that illumination from these laser operations are able to create temporary vision
impairment miles from the actual location. In addition, these operations can produce permanent eye damage.
Pilots should make themselves aware of where these activities are being conducted and avoid these areas if
possible.
d. Recent and increasing incidents of unauthorized illumination of aircraft by lasers, as well as the
proliferation and increasing sophistication of laser devices available to the general public, dictates that the FAA,
in coordination with other government agencies, take action to safeguard flights from these unauthorized
illuminations.
e. Pilots should report laser illumination activity to the controlling Air Traffic Control facilities, Federal
Contract Towers or Flight Service Stations as soon as possible after the event. The following information should
be included:
1. UTC Date and Time of Event.
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AIM2/20/251/22/26 AIM
2. Call Sign or Aircraft Registration Number.
3. Type Aircraft.
4. Nearest Major City.
5. Altitude.
6. Location of Event (Latitude/Longitude and/or Fixed Radial Distance (FRD)).
7. Brief Description of the Event and any other Pertinent Information.
f. Pilots are also encouraged to complete the Laser Beam Exposure Questionnaire located on the FAA Laser
Safety Initiative website at http://www.faa.gov/about/in itiatives/lasers/ and submit electronically per the
directions on the questionnaire, as soon as possible after landing.
g. When a laser event is reported to an air traffic facility, a general caution warning will be broadcasted on
all appropriate frequencies every five minutes for 20 minutes and broadcasted on the ATIS for one hour following
the report.
PHRASEOLOGY−
UNAUTHORIZED LASER ILLUMINATION EVENT, (UTC time), (location), (altitude), (color), (direction).
EXAMPLE−
“Unauthorized laser illumination event, at 0100z, 8 mile final runway 18R at 3,000 feet, green laser from the southwest.”
REFERENCE−
F AA Order JO 7110.65, Para 10−2−14, Unauthorized Laser Illumination of Aircraft.
F AA Order JO 7210.3, Para 2−1−27, Reporting Unauthorized Laser Illumination of Aircraft.
h. When these activities become known to the FAA, Notices to Airmen (NOTAMs) are issued to inform the
aviation community of the events. Pilots should consult NOTAMs or the Chart Supplement for information
regarding these activities.
7−6−14. Flying in Flat Light, Brown Out Conditions, and White Out Conditions
a. Flat Light. Flat light is an optical illusion, also known as “sector or partial white out.” It is not as severe
as “white out” but the condition causes pilots to lose their depth −of−field and contrast in vision. Flat light
conditions are usually accompanied by overcast skies inhibiting any visual clues. Such conditions can occur
anywhere in the world, primarily in snow covered areas but can also occur in dust, sand, mud flats, or on glassy
water. Flat light can completely obscure features of the terrain, creating an inability to distinguish distances and
closure rates. As a result of this reflected light, it can give pilots the illusion that they are ascending or descending
when they may actually be flying level. However, with good judgment and proper training and planning, it is
possible to safely operate an aircraft in flat light conditions.
b. Brown Out. A brownout (or brown−out) is an in−flight visibility restriction due to dust or sand in the air.
In a brownout, the pilot cannot see nearby objects which provide the outside visual references necessary to
control the aircraft near the ground. This can cause spatial disorientation and loss of situational awareness leading
to an accident.
1. The following factors will affect the probability and severity of brownout: rotor disk loading, rotor
configuration, soil composition, wind, approach speed, and approach angle.
2. The brownout phenomenon causes accidents during helicopter landing and take−off operations in dust,
fine dirt, sand, or arid desert terrain. Intense, blinding dust clouds stirred up by the helicopter rotor downwash
during near−ground flight causes significant flight safety risks from aircraft and ground obstacle collisions, and
dynamic rollover due to sloped and uneven terrain.
3. This is a dangerous phenomenon experienced by many helicopters when making landing approaches in
dusty environments, whereby sand or dust particles become swept up in the rotor outwash and obscure the pilot’s
vision of the terrain. This is particularly dangerous because the pilot needs those visual cues from their
surroundings in order to make a safe landing.
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4. Blowing sand and dust can cause an illusion of a tilted horizon. A pilot not using the flight instruments
for reference may instinctively try to level the aircraft with respect to the false horizon, resulting in an accident.
Helicopter rotor wash also causes sand to blow around outside the cockpit windows, possibly leading the pilot
to experience an illusion where the helicopter appears to be turning when it is actually in a level hover. This can
also cause the pilot to make incorrect control inputs which can quickly lead to disaster when hovering near the
ground. In night landings, aircraft lighting can enhance the visual illusions by illuminating the brownout cloud.
c. White Out. As defined in meteorological terms, white out occurs when a person becomes engulfed in a
uniformly white glow. The glow is a result of being surrounded by blowing snow, dust, sand, mud or water. There
are no shadows, no horizon or clouds and all depth−of−field and orientation are lost. A white out situation is
severe in that there are no visual references. Flying is not recommended in any white out situation. Flat light
conditions can lead to a white out environment quite rapidly, and both atmospheric conditions are insidious; they
sneak up on you as your visual references slowly begin to disappear. White out has been the cause of several
aviation accidents.
d. Self Induced White Out. This effect typically occurs when a helicopter takes off or lands on a
snow−covered area. The rotor downwash picks up particles and re−circulates them through the rotor downwash.
The effect can vary in intensity depending upon the amount of light on the surface. This can happen on the
sunniest, brightest day with good contrast everywhere. However, when it happens, there can be a complete loss
of visual clues. If the pilot has not prepared for this immediate loss of visibility, the results can be disastrous.
Good planning does not prevent one from encountering flat light or white out conditions.
e. Never take off in a white out situation.
1. Realize that in flat light conditions it may be possible to depart but not to return to that site. During takeoff,
make sure you have a reference point. Do not lose sight of it until you have a departure reference point in view.
Be prepared to return to the takeoff reference if the departure reference does not come into view.
2. Flat light is common to snow skiers. One way to compensate for the lack of visual contrast and
depth−of−field loss is by wearing amber tinted lenses (also known as blue blockers). Special note of caution:
Eyewear is not ideal for every pilot. Take into consideration personal factors—age, light sensitivity, and ambient
lighting conditions.
3. So what should a pilot do when all visual references are lost?
(a) Trust the cockpit instruments.
(b) Execute a 180 degree turnaround and start looking for outside references.
(c) Above all − fly the aircraft.
f. Landing in Low Light Conditions. When landing in a low light condition − use extreme caution. Look
for intermediate reference points, in addition to checkpoints along each leg of the route for course confirmation
and timing. The lower the ambient light becomes, the more reference points a pilot should use.
g. Airport Landings.
1. Look for features around the airport or approach path that can be used in determining depth perception.
Buildings, towers, vehicles or other aircraft serve well for this measurement. Use something that will provide
you with a sense of height above the ground, in addition to orienting you to the runway.
2. Be cautious of snowdrifts and snow banks − anything that can distinguish the edge of the runway. Look
for subtle changes in snow texture or shading to identify ridges or changes in snow depth.
h. Off−Airport Landings.
1. In the event of an off−airport landing, pilots have used a number of different visual cues to gain reference.
Use whatever you must to create the contrast you need. Natural references seem to work best (trees, rocks, snow
ribs, etc.)
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(a) Over flight.
(b) Use of markers.
(c) Weighted flags.
(d) Smoke bombs.
(e) Any colored rags.
(f) Dye markers.
(g) Kool−aid.
(h) Trees or tree branches.
2. It is difficult to determine the depth of snow in areas that are level. Dropping items from the aircraft to
use as reference points should be used as a visual aid only and not as a primary landing reference. Unless your
marker is biodegradable, be sure to retrieve it after landing. Never put yourself in a position where no visual
references exist.
3. Abort landing if blowing snow obscures your reference. Make your decisions early. Don’t assume you
can pick up a lost reference point when you get closer.
4. Exercise extreme caution when flying from sunlight into shade. Physical awareness may tell you that you
are flying straight but you may actually be in a spiral dive with centrifugal force pressing against you. Having
no visual references enhances this illusion. Just because you have a good visual reference does not mean that it’s
safe to continue. There may be snow−covered terrain not visible in the direction that you are traveling. Getting
caught in a no visual reference situation can be fatal.
i. Flying Around a Lake.
1. When flying along lakeshores, use them as a reference point. Even if you can see the other side, realize
that your depth perception may be poor. It is easy to fly into the surface. If you must cross the lake, check the
altimeter frequently and maintain a safe altitude while you still have a good reference. Don’t descend below that
altitude.
2. The same rules apply to seemingly flat areas of snow. If you don’t have good references, avoid going
there.
j. Other Traffic. Be on the look out for other traffic in the area. Other aircraft may be using your same
reference point. Chances are greater of colliding with someone traveling in the same direction as you, than
someone flying in the opposite direction.
k. Ceilings. Low ceilings have caught many pilots off guard. Clouds do not always form parallel to the
surface, or at the same altitude. Pilots may try to compensate for this by flying with a slight bank and thus creating
a descending turn.
l. Glaciers. Be conscious of your altitude when flying over glaciers. The glaciers may be rising faster than
you are climbing.
7−6−15. Operations in Ground Icing Conditions
a. The presence of aircraft airframe icing during takeoff, typically caused by improper or no deicing of the
aircraft being accomplished prior to flight has contributed to many recent accidents in turbine aircraft. The
General Aviation Joint Steering Committee (GAJSC) is the primary vehicle for government− industry
cooperation, communication, and coordination on General Aviation (GA) accident mitigation. The Turbine
Aircraft Operations Subgroup (TAOS) works to mitigate accidents in turbine accident aviation. While there is
sufficient information and guidance currently available regarding the effects of icing on aircraft and methods for
deicing, the TAOS has developed a list of recommended actions to further assist pilots and operators in this area.
Potential Flight Hazards 7−6−15
AIM 2/20/25
While the efforts of the TAOS specifically focus on turbine aircraft, it is recognized that their recommendations
are applicable to and can be adapted for the pilot of a small, piston powered aircraft too.
b. The following recommendations are offered:
1. Ensure that your aircraft’s lift−generating surfaces are COMPLETELY free of contamination before
flight through a tactile (hands on) check of the critical surfaces when feasible. Even when otherwise permitted,
operators should avoid smooth or polished frost on lift−generating surfaces as an acceptable preflight condition.
2. Review and refresh your cold weather standard operating procedures.
3. Review and be familiar with the Airplane Flight Manual (AFM) limitations and procedures necessary
to deal with icing conditions prior to flight, as well as in flight.
4. Protect your aircraft while on the ground, if possible, from sleet and freezing rain by taking advantage
of aircraft hangars.
5. Take full advantage of the opportunities available at airports for deicing. Do not refuse deicing services
simply because of cost.
6. Always consider canceling or delaying a flight if weather conditions do not support a safe operation.
c. If you haven’t already developed a set of Standard Operating Procedures for cold weather operations, they
should include:
1. Procedures based on information that is applicable to the aircraft operated, such as AFM limitations and
procedures;
2. Concise and easy to understand guidance that outlines best operational practices;
3. A systematic procedure for recognizing, evaluating and addressing the associated icing risk, and offer
clear guidance to mitigate this risk;
4. An aid (such as a checklist or reference cards) that is readily available during normal day−to−day aircraft
operations.
d. There are several sources for guidance relating to airframe icing, including:
1. http://aircrafticing.grc.nasa.gov/index.html
2. Advisory Circular (AC) 91−74, Pilot Guide, Flight in Icing Conditions.
3. AC 135−17, Pilot Guide Small Aircraft Ground Deicing.
4. AC 135−9, FAR Part 135 Icing Limitations.
5. AC 120−60, Ground Deicing and Anti−icing Program.
6. AC 135−16, Ground Deicing and Anti −icing Training and Checking. The FAA Approved Deicing
Program Updates is published annually as a Flight Standards Information Bulletin for Air Transportation and
contains detailed information on deicing and anti−icing procedures and holdover times. It may be accessed at
the following website by selecting the current year’s information bulletins:
https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/deicing/.
7−6−16. Avoid Flight in the Vicinity of Exhaust Plumes (Smoke Stacks and Cooling Towers)
a. Flight Hazards Exist Around Exhaust Plumes. Exhaust plumes are defined as visible or invisible
emissions from power plants, industrial production facilities, or other industrial systems that release large
amounts of vertically directed unstable gases (effluent). High temperature exhaust plumes can cause significant
air disturbances such as turbulence and vertical shear. Other identified potential hazards include, but are not
necessarily limited to: reduced visibility, oxygen depletion, engine particulate contamination, exposure to
gaseous oxides, and/or icing. Results of encountering a plume may include airframe damage, aircraft upset,
7−6−16 Potential Flight Hazards
2/20/25 AIM
and/or engine damage/failure. These hazards are most critical during low altitude flight in calm and cold air,
especially in and around approach and departure corridors or airport traffic areas.
Whether plumes are visible or invisible, the total extent of their turbulent affect is difficult to predict. Some
studies do predict that the significant turbulent effects of an exhaust plume can extend to heights of over 1,000
feet above the height of the top of the stack or cooling tower. Any effects will be more pronounced in calm stable
air where the plume is very hot and the surrounding area is still and cold. Fortunately, studies also predict that
any amount of crosswind will help to dissipate the effects. However, the size of the tower or stack is not a good
indicator of the predicted effect the plume may produce. The major effects are related to the heat or size of the
plume effluent, the ambient air temperature, and the wind speed affecting the plume. Smaller aircraft can expect
to feel an effect at a higher altitude than heavier aircraft.
b. When able, a pilot should steer clear of exhaust plumes by flying on the upwind side of smokestacks
or cooling towers. When a plume is visible via smoke or a condensation cloud, remain clear and realize a plume
may have both visible and invisible characteristics. Exhaust stacks without visible plumes may still be in full
operation, and airspace in the vicinity should be treated with caution. As with mountain wave turbulence or clear
air turbulence, an invisible plume may be encountered unexpectedly. Cooling towers, power plant stacks, exhaust
fans, and other similar structures are depicted in FIG 7−6−2.
Pilots are encouraged to exercise caution when flying in the vicinity of exhaust plumes. Pilots are also
encouraged to reference the Chart Supplement where amplifying notes may caution pilots and identify the
location of structure(s) emitting exhaust plumes.
The best available information on this phenomenon must come from pilots via the PIREP reporting procedures.
All pilots encountering hazardous plume conditions are urgently requested to report time, location, and intensity
(light, moderate, severe, or extreme) of the element to the FAA facility with which they are maintaining radio
contact. If time and conditions permit, elements should be reported according to the standards for other PIREPs
and position reports (AIM paragraph 7−1−21, PIREPS Relating to Turbulence).
FIG 7−6−2
Plumes
7−6−17. Space Launch and Reentry Area
Locations where commercial space launch and/or reentry operations occur. Hazardous operations occur in space
launch and reentry areas, and for pilot awareness, a rocket−shaped symbol is used to depict them on sectional
aeronautical charts. These locations may have vertical launches from launch pads, horizontal launches from
runways, and/or reentering vehicles coming back to land. Because of the wide range of hazards associated with
space launch and reentry areas, pilots are expected to check NOTAMs for the specific area prior to flight to
determine the location and lateral boundaries of the associated hazard area, and the active time. NOTAMs may
include terms such as “rocket launch activity,” “space launch,” or “space reentry,” depending upon the type of
Potential Flight Hazards 7−6−17
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operation. Space launch and reentry areas are not established for amateur rocket operations conducted per 14
CFR part 101.
FIG 7−6−3
Space Launch and Reentry Area Depicted on a Sectional Chart
7−6−18. Automatic Landing Operations
Prior to conducting automatic landing operations, pilots are expected to determine that the flight control and
instrument approach guidance systems being used permit safe, automatically flown landings to be conducted at
that runway. The analysis should include, but not be limited to, ILS classification code where applicable, suitable
threshold crossing height, runway slope, and pre−threshold terrain. The FAA only evaluates runways and other
ground infrastructure for suitability to support automatic landing operations for those facilities associated with
published CA T II, SA CAT II, and CAT III instrument approach procedures. When conducting automatic landing
operations, pilots must ensure that the runway, associated procedure, navigation source, and other infrastructure
have no outstanding NOTAMs or chart notes that would preclude automatic landing operations (e.g., “Localizer
unusable inside the threshold,” or “Glide slope unusable below xxx feet”). Pilots should advise ATC of their
intent to conduct an automatic landing, remain alert to any unsuitable system performance, and be prepared to
disengage the automatic landing system when necessary. During automatic landing operations using an ILS
facility, pilots should understand and observe the provisions of AIM, subparagraph 1–1–9k, ILS Course and
Glideslope Distortion.
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Section 7. Safety, Accident, and Hazard Reports
7−7−1. Aviation Safety Reporting Program
a. The FAA has established a voluntary Aviation Safety Reporting Program designed to stimulate the free and
unrestricted flow of information concerning deficiencies and discrepancies in the aviation system. This is a
positive program intended to ensure the safest possible system by identifying and correcting unsafe conditions
before they lead to accidents. The primary objective of the program is to obtain information to evaluate and
enhance the safety and efficiency of the present system.
b. This cooperative safety reporting program invites pilots, controllers, flight attendants, maintenance
personnel and other users of the airspace system, or any other person, to file written reports of actual or potential
discrepancies and deficiencies involving the safety of aviation operations. The operations covered by the
program include departure, en route, approach, and landing operations and procedures, air traffic control
procedures and equipment, crew and air traffic control communications, aircraft cabin operations, aircraft
movement on the airport, near midair collisions, aircraft maintenance and record keeping and airport conditions
or services.
c. The report should give the date, time, location, persons and aircraft involved (if applicable), nature of the
event, and all pertinent details.
d. To ensure receipt of this information, the program provides for the waiver of certain disciplinary actions
against persons, including pilots and air traffic controllers, who file timely written reports concerning potentially
unsafe incidents. To be considered timely, reports must be delivered or postmarked within 10 days of the incident
unless that period is extended for good cause. Reports should be submitted on NASA ARC Forms 277, which
are available free of charge, postage prepaid, at FAA Flight Standards District Offices and Flight Service
Stations, and from NASA, ASRS, PO Box 189, Moffet Field, CA 94035.
e. The FAA utilizes the National Aeronautics and Space Administration (NASA) to act as an independent
third party to receive and analyze reports submitted under the program. This program is described in AC 00−46,
Aviation Safety Reporting Program.
7−7−2. Aircraft Accident and Incident Reporting
a. Occurrences Requiring Notification. The operator of an aircraft must immediately, and by the most
expeditious means available, notify the nearest National Transportation Safety Board (NTSB) Field Office
when:
1. An aircraft accident or any of the following listed incidents occur:
(a) Flight control system malfunction or failure.
(b) Inability of any required flight crew member to perform their normal flight duties as a result of injury
or illness.
(c) Failure of structural components of a turbine engine excluding compressor and turbine blades and
vanes.
(d) Inflight fire.
(e) Aircraft collide in flight.
(f) Damage to property, other than the aircraft, es timated to exceed $25,000 for repair (including
materials and labor) or fair market value in the event of total loss, whichever is less.
(g) For large multi-engine aircraft (more than 12,500 pounds maximum certificated takeoff weight):
(1) Inflight failure of electrical systems which requires the sustained use of an emergency bus powered
by a back-up source such as a battery, auxiliary power unit, or air-driven generator to retain flight control or
essential instruments;
Safety, Accident, and Hazard Reports 7−7−1
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(2) Inflight failure of hydraulic systems that results in sustained reliance on the sole remaining
hydraulic or mechanical system for movement of flight control surfaces;
(3) Sustained loss of the power or thrust produced by two or more engines; and
(4) An evacuation of aircraft in which an emergency egress system is utilized.
2. An aircraft is overdue and is believed to have been involved in an accident.
b. Manner of Notification.
1. The most expeditious method of notification to the NTSB by the operator will be determined by the
circumstances existing at that time. The NTSB has advised that any of the following would be considered
examples of the type of notification that would be acceptable:
(a) Direct telephone notification.
(b) Telegraphic notification.
(c) Notification to the FAA who would in turn notify the NTSB by direct communication; i.e., dispatch
or telephone.
c. Items to be Included in Notification. The notification required above must contain the following
information, if available:
1. Type, nationality, and registration marks of the aircraft.
2. Name of owner and operator of the aircraft.
3. Name of the pilot-in-command.
4. Date and time of the accident, or incident.
5. Last point of departure, and point of intended landing of the aircraft.
6. Position of the aircraft with reference to some easily defined geographical point.
7. Number of persons aboard, number killed, and number seriously injured.
8. Nature of the accident, or incident, the weather, and the extent of damage to the aircraft so far as is known;
and
9. A description of any explosives, radioactive materials, or other dangerous articles carried.
d. Follow−up Reports.
1. The operator must file a report on NTSB Form 6120.1 or 6120.2, available from NTSB Field Offices or
from the NTSB, Washington, DC, 20594:
(a) Within 10 days after an accident;
(b) When, after 7 days, an overdue aircraft is still missing;
(c) A report on an incident for which notification is required as described in subparagraph a(1) must be
filed only as requested by an authorized representative of the NTSB.
2. Each crewmember, if physically able at the time the report is submitted, must attach a statement setting
forth the facts, conditions, and circumstances relating to the accident or incident as they appeared. If the
crewmember is incapacitated, a statement must be submitted as soon as physically possible.
e. Where to File the Reports.
1. The operator of an aircraft must file with the NTSB Field Office nearest the accident or incident any
report required by this section.
2. The NTSB Field Offices are listed under U.S. Government in the telephone directories in the following
cities: Anchorage, AK; Atlanta, GA; Chicago, IL; Denver, CO; Fort Worth, TX; Los Angeles, CA; Miami, FL;
Parsippany, NJ; Seattle, WA.
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AIM2/20/251/22/26 AIM
7−7−3. Near Midair Collision Reporting
a. Purpose and Data Uses. The primary purpose of the Near Midair Collision (NMAC) Reporting Program
is to provide information for use in enhancing the safety and efficiency of the National Airspace System. Data
obtained from NMAC reports are used by the FAA to improve the quality of FAA services to users and to develop
programs, policies, and procedures aimed at the reduction of NMAC occurrences. All NMAC reports are
thoroughly investigated by Flight Standards Facilities in coordination with Air Traffic Facilities. Data from these
investigations are transmitted to FAA Headquarters in Washington, DC, where they are compiled and analyzed,
and where safety programs and recommendations are developed.
b. Definition. A near midair collision is defined as an incident associated with the operation of an aircraft
in which a possibility of collision occurs as a result of proximity of less than 500 feet to another aircraft, or a report
is received from a pilot or a flight crew member stating that a collision hazard existed between two or more
aircraft.
c. Reporting Responsibility. It is the responsibility of the pilot and/or flight crew to determine whether a
near midair collision did actually occur and, if so, to initiate a NMAC report. Be specific, as ATC will not interpret
a casual remark to mean that a NMAC is being reported. The pilot should state “I wish to report a near midair
collision.”
d. Where to File Reports. Pilots and/or flight crew members involved in NMAC occurrences are urged to
report each incident immediately:
1. By radio or telephone to the nearest FAA ATC facility or FSS.
2. In writing, in lieu of the above, to the nearest Flight Standards District Office (FSDO).
e. Items to be Reported.
1. Date and time (UTC) of incident.
2. Location of incident and altitude.
3. Identification and type of reporting aircraft, aircrew destination, name and home base of pilot.
4. Identification and type of other aircraft, aircrew destination, name and home base of pilot.
5. Type of flight plans; station altimeter setting used.
6. Detailed weather conditions at altitude or flight level.
7. Approximate courses of both aircraft: indicate if one or both aircraft were climbing or descending.
8. Reported separation in distance at first sighting, proximity at closest point horizontally and vertically,
and length of time in sight prior to evasive action.
9. Degree of evasive action taken, if any (from both aircraft, if possible).
10. Injuries, if any.
f. Investigation. The FSDO in whose area the incident occurred is responsible for the investigation and
reporting of NMACs.
g. Existing radar, communication, and weather data will be examined in the conduct of the investigation.
When possible, all cockpit crew members will be interviewed regarding factors involving the NMAC incident.
Air traffic controllers will be interviewed in cases where one or more of the involved aircraft was provided ATC
service. Both flight and ATC procedures will be evaluated. When the investigation reveals a violation of an FAA
regulation, enforcement action will be pursued.
7−7−4. Unidentified Anomalous Phenomena (UAP) Reports
a. Persons wanting to report UAP/unexplained phenomena activity should visit the All−Domain Anomaly
Resolution Office (AARO) website at https://www.aaro.mil/.
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b. If concern is expressed that life or property might be endangered by unidentified anomalous phenomena
(UAP) activity, report the activity to the local law enforcement department.
7−7−5. Safety Alerts For Operators (SAFO) and Information For Operators (InFO)
a. SAFOs contain important safety information that is often time-critical. A SAFO may contain information
and/or recommended (non-regulatory) action to be taken by the respective operators or parties identified in the
SAFO. The audience for SAFOs varies with each subject and may include: Air carrier certificate holders, air
operator certificate holders, general aviation operators, directors of safety, directors of operations, directors of
maintenance, fractional ownership program managers, training center managers, accountable managers at repair
stations, and other parties as applicable.
b. InFOs are similar to SAFOs, but contain valuable information for operators that should help them meet
administrative requirements or certain regulatory requirements with relatively low urgency or impact in safety.
c. The SAFO and InFO system provides a means to rapidly distribute this information to operators and can
be found at the following website:
http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo and
http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info or search keyword FAA
SAFO or FAA INFO. Free electronic subscription is available on the “ALL SAFOs” or “ALL InFOs” page of
the website.
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Chapter 8. Medical Facts for Pilots
Section 1. Fitness for Flight
8−1−1. Fitness For Flight
a. Medical Certification.
1. All pilots except those flying gliders and free air balloons must possess valid medical certificates in order
to exercise the privileges of their airman certificates. The periodic medical examinations required for medical
certification are conducted by designated Aviation Medical Examiners, who are physicians with a special interest
in aviation safety and training in aviation medicine.
2. The standards for medical certification are contained in 14 CFR part 67. Pilots who have a history of
certain medical conditions described in these standards are mandatorily disqualified from flying. These medical
conditions include a personality disorder manifested by overt acts, a psychosis, alcoholism, drug dependence,
epilepsy, an unexplained disturbance of consciousness, myocardial infarction, angina pectoris and diabetes
requiring medication for its control. Other medical conditions may be temporarily disqualifying, such as acute
infections, anemia, and peptic ulcer. Pilots who do not meet medical standards may still be qualified under special
issuance provisions or the exemption process. This may require that either additional medical information be
provided or practical flight tests be conducted.
3. Student pilots should visit an Aviation Medical Examiner as soon as possible in their flight training in
order to avoid unnecessary training expenses should they not meet the medical standards. For the same reason,
the student pilot who plans to enter commercial aviation should apply for the highest class of medical certificate
that might be necessary in the pilot’s career.
CAUTION−
The CFRs prohibit a pilot who possesses a current medical certificate from performing crewmember duties while the pilot
has a known medical condition or increase of a known medical condition that would make the pilot unable to meet the
standards for the medical certificate.
b. Illness.
1. Even a minor illness suffered in day-to-day living can seriously degrade performance of many piloting
tasks vital to safe flight. Illness can produce fever and distracting symptoms that can impair judgment, memory,
alertness, and the ability to make calculations. Although symptoms from an illness may be under adequate
control with a medication, the medication itself may decrease pilot performance.
2. The safest rule is not to fly while suffering from any illness. If this rule is considered too stringent for
a particular illness, the pilot should contact an Aviation Medical Examiner for advice.
c. Medication.
1. Pilot performance can be seriously degraded by both prescribed and over-the-counter medications, as
well as by the medical conditions for which they are taken. Many medications, such as tranquilizers, sedatives,
strong pain relievers, and cough-suppressant preparations, have primary effects that may impair judgment,
memory, alertness, coordination, vision, and the ability to make calculations. Others, such as antihistamines,
blood pressure drugs, muscle relaxants, and agents to control diarrhea and motion sickness, have side effects that
may impair the same critical functions. Any medication that depresses the nervous system, such as a sedative,
tranquilizer or antihistamine, can make a pilot much more susceptible to hypoxia.
2. The CFRs prohibit pilots from performing crewmember duties while using any medication that affects
the faculties in any way contrary to safety. The safest rule is not to fly as a crewmember while taking any
medication, unless approved to do so by the FAA.
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d. Alcohol.
1. Extensive research has provided a number of facts about the hazards of alcohol consumption and flying.
As little as one ounce of liquor, one bottle of beer or four ounces of wine can impair flying skills, with the alcohol
consumed in these drinks being detectable in the breath and blood for at least 3 hours. Even after the body
completely destroys a moderate amount of alcohol, a pilot can still be severely impaired for many hours by
hangover. There is simply no way of increasing the destruction of alcohol or alleviating a hangover. Alcohol also
renders a pilot much more susceptible to disorientation and hypoxia.
2. A consistently high alcohol related fatal aircraft accident rate serves to emphasize that alcohol and flying
are a potentially lethal combination. The CFRs prohibit pilots from performing crewmember duties within 8
hours after drinking any alcoholic beverage or while under the influence of alcohol. However, due to the slow
destruction of alcohol, a pilot may still be under influence 8 hours after drinking a moderate amount of alcohol.
Therefore, an excellent rule is to allow at least 12 to 24 hours between “bottle and throttle,” depending on the
amount of alcoholic beverage consumed.
e. Fatigue.
1. Fatigue continues to be one of the most treacherous hazards to flight safety, as it may not be apparent to
a pilot until serious errors are made. Fatigue is best described as either acute (short-term) or chronic (long-term).
2. A normal occurrence of everyday living, acute fatigue is the tiredness felt after long periods of physical
and mental strain, including strenuous muscular effort, immobility, heavy mental workload, strong emotional
pressure, monotony, and lack of sleep. Consequently, coordination and alertness, so vital to safe pilot
performance, can be reduced. Acute fatigue is prevented by adequate rest and sleep, as well as by regular exercise
and proper nutrition.
3. Chronic fatigue occurs when there is not enough time for full recovery between episodes of acute fatigue.
Performance continues to fall off, and judgment becomes impaired so that unwarranted risks may be taken.
Recovery from chronic fatigue requires a prolonged period of rest.
4. OBSTRUCTIVE SLEEP APNEA (OSA). OSA is now recognized as an important preventable factor
identified in transportation accidents. OSA interrupt s the normal restorative sleep necessary for normal
functioning and is associated with chronic illnesses such as hypertension, heart attack, stroke, obesity, and
diabetes. Symptoms include snoring, excessive daytime sleepiness, intermittent prolonged breathing pauses
while sleeping, memory impairment and lack of concentration. There are many available treatments which can
reverse the day time symptoms and reduce the chance of an accident. OSA can be easily treated. Most treatments
are acceptable for medical certification upon demonstrating effective treatment. If you have any symptoms
described above, or neck size over 17 inches in men or 16 inches in women, or a body mass index greater than
30 you should be evaluated for sleep apnea by a sleep medicine specialist.
(https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calcula tor/bmi_calculator.html )
With treatment you can avoid or delay the onset of these chronic illnesses and prolong a quality life.
f. Stress.
1. Stress from the pressures of everyday living can impair pilot performance, often in very subtle ways.
Difficulties, particularly at work, can occupy thought processes enough to markedly decrease alertness.
Distraction can so interfere with judgment that unwarranted risks are taken, such as flying into deteriorating
weather conditions to keep on schedule. Stress and fatigue (see above) can be an extremely hazardous
combination.
2. Most pilots do not leave stress “on the ground.” Therefore, when more than usual difficulties are being
experienced, a pilot should consider delaying flight until these difficulties are satisfactorily resolved.
g. Emotion.
Certain emotionally upsetting events, including a serious argument, death of a family member, separation or
divorce, loss of job, and financial catastrophe, can render a pilot unable to fly an aircraft safely. The emotions
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of anger, depression, and anxiety from such events not only decrease alertness but also may lead to taking risks
that border on self-destruction. Any pilot who experiences an emotionally upsetting event should not fly until
satisfactorily recovered from it.
h. Personal Checklist. Aircraft accident statistics show that pilots should be conducting preflight checklists
on themselves as well as their aircraft for pilot impairment contributes to many more accidents than failures of
aircraft systems. A personal checklist, which includes all of the categories of pilot impairment as discussed in
this section, that can be easily committed to memory is being distributed by the FAA in the form of a wallet-sized
card.
i. PERSONAL CHECKLIST. I’m physically and mentally safe to fly; not being impaired by:
Illness
Medication
Stress
Alcohol
Fatigue
Emotion
8−1−2. Effects of Altitude
a. Hypoxia.
1. Hypoxia is a state of oxygen deficiency in the body sufficient to impair functions of the brain and other
organs. Hypoxia from exposure to altitude is due only to the reduced barometric pressures encountered at
altitude, for the concentration of oxygen in the atmosphere remains about 21 percent from the ground out to
space.
2. Although a deterioration in night vision occurs at a cabin pressure altitude as low as 5,000 feet, other
significant effects of altitude hypoxia usually do not occur in the normal healthy pilot below 12,000 feet. From
12,000 to 15,000 feet of altitude, judgment, memory, alertness, coordination and ability to make calculations are
impaired, and headache, drowsiness, dizziness and either a sense of well-being (euphoria) or belligerence occur.
The effects appear following increasingly shorter periods of exposure to increasing altitude. In fact, pilot
performance can seriously deteriorate within 15 minutes at 15,000 feet.
3. At cabin pressure altitudes above 15,000 feet, the periphery of the visual field grays out to a point where
only central vision remains (tunnel vision). A blue coloration (cyanosis) of the fingernails and lips develops. The
ability to take corrective and protective action is lost in 20 to 30 minutes at 18,000 feet and 5 to 12 minutes at
20,000 feet, followed soon thereafter by unconsciousness.
4. The altitude at which significant effects of hypoxia occur can be lowered by a number of factors. Carbon
monoxide inhaled in smoking or from exhaust fumes, lowered hemoglobin (anemia), and certain medications
can reduce the oxygen-carrying capacity of the blood to the degree that the amount of oxygen provided to body
tissues will already be equivalent to the oxygen provided to the tissues when exposed to a cabin pressure altitude
of several thousand feet. Small amounts of alcohol and low doses of certain drugs, such as antihistamines,
tranquilizers, sedatives and analgesics can, through their depressant action, render the brain much more
susceptible to hypoxia. Extreme heat and cold, fever, and anxiety increase the body’s demand for oxygen, and
hence its susceptibility to hypoxia.
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5. The effects of hypoxia are usually quite difficult to recognize, especially when they occur gradually.
Since symptoms of hypoxia do not vary in an individual, the ability to recognize hypoxia can be greatly improved
by experiencing and witnessing the effects of hypoxia during an altitude chamber “flight.” The FAA provides
this opportunity through aviation physiology training, which is conducted at the FAA Civil Aeromedical Institute
and at many military facilities across the U.S. To attend the Physiological Training Program at the Civil
Aeromedical Institute, Mike Monroney Aeronautical Center, Oklahoma City, OK, contact by telephone (405)
954−6212, or by writing Aerospace Medical Education Division, AAM −400, CAMI, Mike Monroney
Aeronautical Center, P.O. Box 25082, Oklahoma City, OK 73125.
NOTE−
To attend the physiological training program at one of the military installations having the training capability, an
application form and a fee must be submitted. Full particulars about location, fees, scheduling procedures, course content,
individual requirements, etc., are contained in the Physiological Training Application, Form Number AC 3150−7, which
is obtained by contacting the accident prevention specialist or the office forms manager in the nearest F AA office.
6. Hypoxia is prevented by heeding factors that reduce tolerance to altitude, by enriching the inspired air
with oxygen from an appropriate oxygen system, and by maintaining a comfortable, safe cabin pressure altitude.
For optimum protection, pilots are encouraged to use supplemental oxygen above 10,000 feet during the day,
and above 5,000 feet at night. The CFRs require that at the minimum, flight crew be provided with and use
supplemental oxygen after 30 minutes of exposure to cabin pressure altitudes between 12,500 and 14,000 feet
and immediately on exposure to cabin pressure altitudes above 14,000 feet. Every occupant of the aircraft must
be provided with supplemental oxygen at cabin pressure altitudes above 15,000 feet.
b. Ear Block.
1. As the aircraft cabin pressure decreases during ascent, the expanding air in the middle ear pushes the
eustachian tube open, and by escaping down it to the nasal passages, equalizes in pressure with the cabin pressure.
But during descent, the pilot must periodically open the eustachian tube to equalize pressure. This can be
accomplished by swallowing, yawning, tensing muscles in the throat, or if these do not work, by a combination
of closing the mouth, pinching the nose closed, and attempting to blow through the nostrils (Valsalva maneuver).
2. Either an upper respiratory infection, such as a cold or sore throat, or a nasal allergic condition can
produce enough congestion around the eustachian tube to make equalization difficult. Consequently, the
difference in pressure between the middle ear and aircraft cabin can build up to a level that will hold the
eustachian tube closed, making equalization difficult if not impossible. The problem is commonly referred to
as an “ear block.”
3. An ear block produces severe ear pain and loss of hearing that can last from several hours to several days.
Rupture of the ear drum can occur in flight or after landing. Fluid can accumulate in the middle ear and become
infected.
4. An ear block is prevented by not flying with an upper respiratory infection or nasal allergic condition.
Adequate protection is usually not provided by decongestant sprays or drops to reduce congestion around the
eustachian tubes. Oral decongestants have side effects that can significantly impair pilot performance.
5. If an ear block does not clear shortly after landing, a physician should be consulted.
c. Sinus Block.
1. During ascent and descent, air pressure in the sinuses equalizes with the aircraft cabin pressure through
small openings that connect the sinuses to the nasal passages. Either an upper respiratory infection, such as a cold
or sinusitis, or a nasal allergic condition can produce enough congestion around an opening to slow equalization,
and as the difference in pressure between the sinus and cabin mounts, eventually plug the opening. This “sinus
block” occurs most frequently during descent.
2. A sinus block can occur in the frontal sinuses, located above each eyebrow, or in the maxillary sinuses,
located in each upper cheek. It will usually produce excruciating pain over the sinus area. A maxillary sinus block
can also make the upper teeth ache. Bloody mucus may discharge from the nasal passages.
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3. A sinus block is prevented by not flying with an upper respiratory infection or nasal allergic condition.
Adequate protection is usually not provided by decongestant sprays or drops to reduce congestion around the
sinus openings. Oral decongestants have side effects that can impair pilot performance.
4. If a sinus block does not clear shortly after landing, a physician should be consulted.
d. Decompression Sickness After Scuba Diving.
1. A pilot or passenger who intends to fly after scuba diving should allow the body sufficient time to rid
itself of excess nitrogen absorbed during diving. If not, altitude decompression sickness due to evolved nitrogen
gas can occur during exposure to reduced barometric pressure (i.e., low cabin pressure) associated with increased
altitude and may lead to a serious inflight emergency.
2. The recommended wait time before going to flight altitudes up to 8,000 feet is at least 12 hours after
diving that did not require a controlled ascent (i.e., non−decompression stop diving), and at least 24 hours after
diving that required a controlled ascent (i.e., decompression stop diving). The recommended wait time before
going to flight altitudes above 8,000 feet is at least 24 hours after any SCUBA dive. These recommended altitudes
are actual flight altitudes above mean sea level (AMSL) and not pressurized cabin altitudes. This takes into
consideration the risk of aircraft decompression during flight.
8−1−3. Hyperventilation in Flight
a. Hyperventilation, or an abnormal increase in the volume of air breathed in and out of the lungs, can occur
subconsciously when a stressful situation is encountered in flight. As hyperventilation “blows off” excessive
carbon dioxide from the body, a pilot can experience symptoms of lightheadedness, suffocation, drowsiness,
tingling in the extremities, and coolness and react to them with even greater hyperventilation. Incapacitation can
eventually result from incoordination, disorientation, and painful muscle spasms. Finally, unconsciousness can
occur.
b. The symptoms of hyperventilation subside within a few minutes after the rate and depth of breathing are
consciously brought back under control. The buildup of carbon dioxide in the body can be hastened by controlled
breathing in and out of a paper bag held over the nose and mouth.
c. Early symptoms of hyperventilation and hypoxia are similar. Moreover, hyperventilation and hypoxia can
occur at the same time. Therefore, if a pilot is using an oxygen system when symptoms are experienced, the
oxygen regulator should immediately be set to deliver 100 percent oxygen, and then the system checked to assure
that it has been functioning effectively before giving attention to rate and depth of breathing.
8−1−4. Carbon Monoxide Poisoning in Flight
a. Carbon monoxide is a colorless, odorless, and tasteless gas contained in exhaust fumes. When breathed
even in minute quantities over a period of time, it can significantly reduce the ability of the blood to carry oxygen.
Consequently, effects of hypoxia occur.
b. Most heaters in light aircraft work by air flowing over the manifold. Use of these heaters while exhaust
fumes are escaping through manifold cracks and seals is responsible every year for several nonfatal and fatal
aircraft accidents from carbon monoxide poisoning.
c. A pilot who detects the odor of exhaust or experiences symptoms of headache, drowsiness, or dizziness
while using the heater should suspect carbon monoxide poisoning, and immediately shut off the heater and open
air vents. If symptoms are severe or continue after landing, medical treatment should be sought.
8−1−5. Illusions in Flight
a. Introduction. Many different illusions can be experienced in flight. Some can lead to spatial
disorientation. Others can lead to landing errors. Illusions rank among the most common factors cited as
contributing to fatal aircraft accidents.
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b. Illusions Leading to Spatial Disorientation.
1. Various complex motions and forces and certain visual scenes encountered in flight can create illusions
of motion and position. Spatial disorientation from these illusions can be prevented only by visual reference to
reliable, fixed points on the ground or to flight instruments.
2. The leans. An abrupt correction of a banked attitude, which has been entered too slowly to stimulate
the motion sensing system in the inner ear, can create the illusion of banking in the opposite direction. The
disoriented pilot will roll the aircraft back into its original dangerous attitude, or if level flight is maintained, will
feel compelled to lean in the perceived vertical plane until this illusion subsides.
(a) Coriolis illusion. An abrupt head movement in a prolonged constant-rate turn that has ceased
stimulating the motion sensing system can create the illusion of rotation or movement in an entirely different axis.
The disoriented pilot will maneuver the aircraft into a dangerous attitude in an attempt to stop rotation. This most
overwhelming of all illusions in flight may be prevented by not making sudden, extreme head movements,
particularly while making prolonged constant-rate turns under IFR conditions.
(b) Graveyard spin. A proper recovery from a spin that has ceased stimulating the motion sensing
system can create the illusion of spinning in the opposite direction. The disoriented pilot will return the aircraft
to its original spin.
(c) Graveyard spiral. An observed loss of altitude during a coordinated constant-rate turn that has
ceased stimulating the motion sensing system can create the illusion of being in a descent with the wings level.
The disoriented pilot will pull back on the controls, tightening the spiral and increasing the loss of altitude.
(d) Somatogravic illusion. A rapid acceleration during takeoff can create the illusion of being in a nose
up attitude. The disoriented pilot will push the aircraft into a nose low, or dive attitude. A rapid deceleration by
a quick reduction of the throttles can have the opposite effect, with the disoriented pilot pulling the aircraft into
a nose up, or stall attitude.
(e) Inversion illusion. An abrupt change from climb to straight and level flight can create the illusion
of tumbling backwards. The disoriented pilot will push the aircraft abruptly into a nose low attitude, possibly
intensifying this illusion.
(f) Elevator illusion. An abrupt upward vertical acceleration, usually by an updraft, can create the
illusion of being in a climb. The disoriented pilot will push the aircraft into a nose low attitude. An abrupt
downward vertical acceleration, usually by a downdraft, has the opposite effect, with the disoriented pilot pulling
the aircraft into a nose up attitude.
(g) False horizon. Sloping cloud formations, an obscured horizon, a dark scene spread with ground
lights and stars, and certain geometric patterns of ground light can create illusions of not being aligned correctly
with the actual horizon. The disoriented pilot will place the aircraft in a dangerous attitude.
(h) Autokinesis. In the dark, a static light will appear to move about when stared at for many seconds.
The disoriented pilot will lose control of the aircraft in attempting to align it with the light.
3. Illusions Leading to Landing Errors.
(a) Various surface features and atmospheric conditions encountered in landing can create illusions of
incorrect height above and distance from the runway threshold. Landing errors from these illusions can be
prevented by anticipating them during approaches, aerial visual inspection of unfamiliar airports before landing,
using electronic glide slope or V ASI systems when available, and maintaining optimum proficiency in landing
procedures.
(b) Runway width illusion. A narrower-than-usual runway can create the illusion that the aircraft is at
a higher altitude than it actually is. The pilot who does not recognize this illusion will fly a lower approach, with
the risk of striking objects along the approach path or landing short. A wider-than-usual runway can have the
opposite effect, with the risk of leveling out high and landing hard or overshooting the runway.
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(c) Runway and terrain slopes illusion. An upsloping runway, upsloping terrain, or both, can create
the illusion that the aircraft is at a higher altitude than it actually is. The pilot who does not recognize this illusion
will fly a lower approach. A downsloping runway, downsloping approach terrain, or both, can have the opposite
effect.
(d) Featureless terrain illusion. An absence of ground features, as when landing over water, darkened
areas, and terrain made featureless by snow, can create the illusion that the aircraft is at a higher altitude than it
actually is. The pilot who does not recognize this illusion will fly a lower approach.
(e) Atmospheric illusions. Rain on the windscreen can create the illusion of greater height, and
atmospheric haze the illusion of being at a greater distance from the runway. The pilot who does not recognize
these illusions will fly a lower approach. Penetration of fog can create the illusion of pitching up. The pilot who
does not recognize this illusion will steepen the approach, often quite abruptly.
(f) Ground lighting illusions. Lights along a straight path, such as a road, and even lights on moving
trains can be mistaken for runway and approach lights. Bright runway and approach lighting systems, especially
where few lights illuminate the surrounding terrain, may create the illusion of less distance to the runway. The
pilot who does not recognize this illusion will fly a higher approach. Conversely, the pilot overflying terrain
which has few lights to provide height cues may make a lower than normal approach.
8−1−6. Vision in Flight
a. Introduction. Of the body senses, vision is the most important for safe flight. Major factors that determine
how effectively vision can be used are the level of illumination and the technique of scanning the sky for other
aircraft.
b. Vision Under Dim and Bright Illumination.
1. Under conditions of dim illumination, small print and colors on aeronautical charts and aircraft
instruments become unreadable unless adequate cockpit lighting is available. Moreover, another aircraft must
be much closer to be seen unless its navigation lights are on.
2. In darkness, vision becomes more sensitive to light, a process called dark adaptation. Although exposure
to total darkness for at least 30 minutes is required for complete dark adaptation, a pilot can achieve a moderate
degree of dark adaptation within 20 minutes under dim red cockpit lighting. Since red light severely distorts
colors, especially on aeronautical charts, and can cause serious difficulty in focusing the eyes on objects inside
the aircraft, its use is advisable only where optimum outside night vision capability is necessary. Even so, white
cockpit lighting must be available when needed for map and instrument reading, especially under IFR conditions.
Dark adaptation is impaired by exposure to cabin pressure altitudes above 5,000 feet, carbon monoxide inhaled
in smoking and from exhaust fumes, deficiency of Vitamin A in the diet, and by prolonged exposure to bright
sunlight. Since any degree of dark adaptation is lost within a few seconds of viewing a bright light, a pilot should
close one eye when using a light to preserve some degree of night vision.
3. Excessive illumination, especially from light reflected off the canopy, surfaces inside the aircraft, clouds,
water, snow, and desert terrain, can produce glare, with uncomfortable squinting, watering of the eyes, and even
temporary blindness. Sunglasses for protection from glare should absorb at least 85 percent of visible light (15
percent transmittance) and all colors equally (neutral transmittance), with negligible image distortion from
refractive and prismatic errors.
c. Scanning for Other Aircraft.
1. Scanning the sky for other aircraft is a key factor in collision avoidance. It should be used continuously
by the pilot and copilot (or right seat passenger) to cover all areas of the sky visible from the cockpit. Although
pilots must meet specific visual acuity requirements, the ability to read an eye chart does not ensure that one will
be able to efficiently spot other aircraft. Pilots must develop an effective scanning technique which maximizes
one’s visual capabilities. The probability of spotting a potential collision threat obviously increases with the time
spent looking outside the cockpit. Thus, one must use timesharing techniques to efficiently scan the surrounding
airspace while monitoring instruments as well.
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2. While the eyes can observe an approximate 200 degree arc of the horizon at one glance, only a very small
center area called the fovea, in the rear of the eye, has the ability to send clear, sharply focused messages to the
brain. All other visual information that is not processed directly through the fovea will be of less detail. An
aircraft at a distance of 7 miles which appears in sharp focus within the foveal center of vision would have to
be as close as 7/10 of a mile in order to be recognized if it were outside of foveal vision. Because the eyes can
focus only on this narrow viewing area, effective scanning is accomplished with a series of short, regularly
spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should
not exceed 10 degrees, and each area should be observed for at least 1 second to enable detection. Although
horizontal back-and-forth eye movements seem preferred by most pilots, each pilot should develop a scanning
pattern that is most comfortable and then adhere to it to assure optimum scanning.
3. Studies show that the time a pilot spends on visual tasks inside the cabin should represent no more that
1/4 to 1/3 of the scan time outside, or no more than 4 to 5 seconds on the instrument panel for every 16 seconds
outside. Since the brain is already trained to process sight information that is presented from left to right, one
may find it easier to start scanning over the left shoulder and proceed across the windshield to the right.
4. Pilots should realize that their eyes may require several seconds to refocus when switching views
between items in the cockpit and distant objects. The eyes will also tire more quickly when forced to adjust to
distances immediately after close-up focus, as required for scanning the instrument panel. Eye fatigue can be
reduced by looking from the instrument panel to the left wing past the wing tip to the center of the first scan
quadrant when beginning the exterior scan. After having scanned from left to right, allow the eyes to return to
the cabin along the right wing from its tip inward. Once back inside, one should automatically commence the
panel scan.
5. Effective scanning also helps avoid “empty-field myopia.” This condition usually occurs when flying
above the clouds or in a haze layer that provides nothing specific to focus on outside the aircraft. This causes the
eyes to relax and seek a comfortable focal distance which may range from 10 to 30 feet. For the pilot, this means
looking without seeing, which is dangerous.
8−1−7. Aerobatic Flight
a. Pilots planning to engage in aerobatics should be aware of the physiological stresses associated with
accelerative forces during aerobatic maneuvers. Many prospective aerobatic trainees enthusiastically enter
aerobatic instruction but find their first experiences with G forces to be unanticipated and very uncomfortable.
To minimize or avoid potential adverse effects, the aerobatic instructor and trainee must have a basic
understanding of the physiology of G force adaptation.
b. Forces experienced with a rapid push-over maneuver result in the blood and body organs being displaced
toward the head. Depending on forces involved and individual tolerance, a pilot may experience discomfort,
headache, “red-out,” and even unconsciousness.
c. Forces experienced with a rapid pull-up maneuver result in the blood and body organ displacement toward
the lower part of the body away from the head. Since the brain requires continuous blood circulation for an
adequate oxygen supply, there is a physiologic limit to the time the pilot can tolerate higher forces before losing
consciousness. As the blood circulation to the brain decreases as a result of forces involved, a pilot will
experience “narrowing” of visual fields, “gray-out,” “black-out,” and unconsciousness. Even a brief loss of
consciousness in a maneuver can lead to improper control movement causing structural failure of the aircraft or
collision with another object or terrain.
d. In steep turns, the centrifugal forces tend to push the pilot into the seat, thereby resulting in blood and body
organ displacement toward the lower part of the body as in the case of rapid pull-up maneuvers and with the same
physiologic effects and symptoms.
e. Physiologically, humans progressively adapt to imposed strains and stress, and with practice, any maneuver
will have decreasing effect. Tolerance to G forces is dependent on human physiology and the individual pilot.
These factors include the skeletal anatomy, the cardiovascular architecture, the nervous system, the quality of
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the blood, the general physical state, and experience and recency of exposure. The pilot should consult an
Aviation Medical Examiner prior to aerobatic training and be aware that poor physical condition can reduce
tolerance to accelerative forces.
f. The above information provides pilots with a brief summary of the physiologic effects of G forces. It does
not address methods of “counteracting” these effects. There are numerous references on the subject of G forces
during aerobatics available to pilots. Among these are “G Effects on the Pilot During Aerobatics,”
FAA−AM−72−28, and “G Incapacitation in Aerobatic Pilots: A Flight Hazard” FAA −AM−82−13. These are
available from the National Technical Information Service, Springfield, Virginia 22161.
REFERENCE−
F AA AC 91−61, A Hazard in Aerobatics: Effects of G−forces on Pilots.
8−1−8. Judgment Aspects of Collision Avoidance
a. Introduction. The most important aspects of vision and the techniques to scan for other aircraft are
described in paragraph 8−1−6, Vision in Flight. Pilots should also be familiar with the following information to
reduce the possibility of mid-air collisions.
b. Determining Relative Altitude. Use the horizon as a reference point. If the other aircraft is above the
horizon, it is probably on a higher flight path. If the aircraft appears to be below the horizon, it is probably flying
at a lower altitude.
c. Taking Appropriate Action. Pilots should be familiar with rules on right-of-way, so if an aircraft is on
an obvious collision course, one can take immediate evasive action, preferably in compliance with applicable
Federal Aviation Regulations.
d. Consider Multiple Threats. The decision to climb, descend, or turn is a matter of personal judgment, but
one should anticipate that the other pilot may also be making a quick maneuver. Watch the other aircraft during
the maneuver and begin your scanning again immediately since there may be other aircraft in the area.
e. Collision Course Targets. Any aircraft that appears to have no relative motion and stays in one scan
quadrant is likely to be on a collision course. Also, if a target shows no lateral or vertical motion, but increases
in size, take evasive action.
f. Recognize High Hazard Areas.
1. Airways, especially near VORs, and Class B, Class C, Class D, and Class E surface areas are places
where aircraft tend to cluster.
2. Remember, most collisions occur during days when the weather is good. Being in a “radar environment”
still requires vigilance to avoid collisions.
g. Cockpit Management. Studying maps, checklists, and manuals before flight, with other proper preflight
planning; e.g., noting necessary radio frequencies and organizing cockpit materials, can reduce the amount of
time required to look at these items during flight, permitting more scan time.
h. Windshield Conditions. Dirty or bug-smeared windshields can greatly reduce the ability of pilots to see
other aircraft. Keep a clean windshield.
i. Visibility Conditions. Smoke, haze, dust, rain, and flying towards the sun can also greatly reduce the
ability to detect targets.
j. Visual Obstructions in the Cockpit.
1. Pilots need to move their heads to see around blind spots caused by fixed aircraft structures, such as door
posts, wings, etc. It will be necessary at times to maneuver the aircraft; e.g., lift a wing, to facilitate seeing.
2. Pilots must ensure curtains and other cockpit objects; e.g., maps on glare shield, are removed and stowed
during flight.
Fitness for Flight 8−1−9
AIM 2/20/25
k. Lights On.
1. Day or night, use of exterior lights can greatly increase the conspicuity of any aircraft.
2. Keep interior lights low at night.
l. ATC Support. ATC facilities often provide radar traffic advisories on a workload-permitting basis. Flight
through Class C and Class D airspace requires communication with ATC. Use this support whenever possible
or when required.
8−1−10 Fitness for Flight
AIM2/20/258/7/25 AIM
Chapter 9. Aeronautical Charts and
Related Publications
Section 1. Types of Charts Available
9−1−1. General
Civil aeronautical charts for the U.S. and its te rritories, and possessions are produced by Aeronautical
Information Services (AIS), https://www.faa.gov/air_traffic/flight_info/aeronav/safety_alerts/ which is part of
FAA’s Air Traffic Organization, Mission Support Services.
9−1−2. Obtaining Aeronautical Charts
Public sales of charts and publications are available through a network of FAA approved print providers. A listing
of products, dates of latest editions and agents is available on the AIS website at:
https://www.faa.gov/air_traffic/flight_info/aeronav/safety_alerts/.
9−1−3. Safety Alerts, Charting Notices, and Data Product Notices
a. Safety Alerts (SAs) are published to notify users of an error that was reported or discovered in one of our
digital products. The specific product and effective date(s) are provided.
b. Charting Notices (CNs) are published to notify users of a planned chart/publication enhancement and the
effective date on which the enhancement will be implemented.
c. Data Product Notices (DPNs) are published to notify users of a system outage. DPNs may also be used to
notify users of a developmental upgrade to one of our digital products and the effective date on which the upgrade
will be implemented.
d. A listing of these notices is found on the AIS website at:
https://www.faa.gov/air_traffic/flight_info/aeronav/safety_alerts/.
9−1−4. Selected Charts and Products Available
VFR Navigation Charts
IFR Navigation Charts
Planning Charts
Supplementary Charts and Publications
Digital Products
9−1−5. General Description of Each Chart Series
a. VFR Navigation Charts.
1. Sectional Aeronautical Charts. Sectional Charts are designed for visual navigation of slow to medium
speed aircraft. The topographic information consists of contour lines, shaded relief, drainage patterns, and an
extensive selection of visual checkpoints and landmarks used for flight under VFR. Cultural features include
cities and towns, roads, railroads, and other distinct landmarks. The aeronautical information includes visual and
radio aids to navigation, airports, controlled airspace, special−use airspace, obstructions, and related data. Scale
1 inch = 6.86 nm/1:500,000. 60 x 20 inches folded to 5 x 10 inches. Revised every 56 days. (See FIG 9−1−1 and
FIG 9−1−2.)
2. VFR Terminal Area Charts (TAC). TACs depict the airspace designated as Class B airspace. While
similar to sectional charts, TACs have more detail because the scale is larger. The TAC should be used by pilots
Types of Charts Available 9−1−1
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/253/15/077110.65R CHG 2AIM 8/7/25
intending to operate to or from airfields within or near Class B or Class C airspace. Areas with TAC coverage
are indicated by a • on the Sectional Chart indexes. VFR Transition Routes may be depicted and/or described
on this chart. Scale 1 inch = 3.43 nm/1:250,000. Revised every 56 days. (See FIG 9−1−1 and FIG 9−1−2.)
3. U.S. Gulf Coast VFR Aeronautical Chart. The Gulf Coast Chart is designed primarily for helicopter
operation in the Gulf of America area. Information depicted includes offshore mineral leasing areas and blocks,
oil drilling platforms, and high density helicopter activity areas. Scale 1 inch = 13.7 nm/1:1,000,000. 55 x 27
inches folded to 5 x 10 inches. Revised every 56 days.
4. Grand Canyon VFR Aeronautical Chart. Covers the Grand Canyon National Park area and is
designed to promote aviation safety, flight free zones, and facilitate VFR navigation in this popular area. The
chart contains aeronautical information for general aviation VFR pilots on one side and commercial VFR air tour
operators on the other side. Revised every 56 days.
FIG 9−1−1
Sectional and VFR Terminal Area Charts for the Conterminous U.S., Hawaii, Puerto Rico, and Virgin Islands
9−1−2 Types of Charts Available
AIM2/20/258/7/25 AIM
FIG 9−1−2
Sectional and VFR Terminal Area Charts for Alaska
5. Caribbean VFR Aeronautical Charts. Caribbean 1 and 2 (CAC−1 and CAC−2) are designed for visual
navigation to assist familiarization of foreign aeronautical and topographic information. The aeronautical
information includes visual and radio aids to navigation, airports, controlled airspace, special −use airspace,
obstructions, and related data. The topographic information consists of contour lines, shaded relief, drainage
patterns, and a selection of landmarks used for flight under VFR. Cultural features include cities and towns,
roads, railroads, and other distinct landmarks. Scale 1 inch = 13.7 nm/1:1,000,000. CAC−1 consists of two sides
measuring 30” x 60” each. CAC−2 consists of two sides measuring 20” x 60” each. Revised every 56 days. (See
FIG 9−1−3.)
Types of Charts Available 9−1−3
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
FIG 9−1−3
Caribbean VFR Aeronautical Charts
6. Helicopter Route Charts. A three−color chart series which shows current aeronautical information
useful to helicopter pilots navigating in areas with high concentrations of helicopter activity. Information
depicted includes helicopter routes, four classes of heliports with associated frequency and lighting capabilities,
NA V AIDs, and obstructions. In addition, pictorial symbols, roads, and easily identified geographical features are
portrayed. Scale 1 inch = 1.71 nm/1:125,000. 34 x 30 inches folded to 5 x 10 inches. Revised every 56 days. (See
FIG 9−1−4.)
9−1−4 Types of Charts Available
AIM2/20/258/7/25 AIM
FIG 9−1−4
Helicopter Route Charts
b. IFR Navigation Charts.
1. IFR En Route Low Altitude Charts (Conterminous U.S. and Alaska). En route low altitude charts
provide aeronautical information for navigation under IFR conditions below 18,000 feet MSL. This four−color
chart series includes airways; limits of controlled airspace; VHF NA V AIDs with frequency, identification,
channel, geographic coordinates; airports with terminal air/ground communications; minimum en route and
obstruction clearance altitudes; airway distances; reporting points; special use airspace; and military training
routes. Scales vary from 1 inch = 5nm to 1 inch = 20 nm. 50 x 20 inches folded to 5 x 10 inches. Charts revised
every 56 days. Area charts show congested terminal areas at a large scale. They are included with subscriptions
to any conterminous U.S. Set Low (Full set, East or West sets). (See FIG 9−1−5 and FIG 9−1−6.)
Types of Charts Available 9−1−5
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
FIG 9−1−5
En Route Low Altitude Instrument Charts for the Conterminous U.S. (Includes Area Charts)
FIG 9−1−6
Alaska En Route Low Altitude Chart
2. IFR En Route High Altitude Charts (Conterminous U.S. and Alaska). En route high altitude charts
are designed for navigation at or above 18,000 feet MSL. This four −color chart series includes the jet route
structure; VHF NA V AIDs with frequency, identification, channel, geographic coordinates; selected airports;
reporting points. Scales vary from 1 inch = 45 nm to 1 inch = 18 nm. 55 x 20 inches folded to 5 x 10 inches.
Revised every 56 days. (See FIG 9−1−7 and FIG 9−1−8.)
9−1−6 Types of Charts Available
AIM2/20/258/7/25 AIM
FIG 9−1−7
En Route High Altitude Charts for the Conterminous U.S.
FIG 9−1−8
Alaskan En Route High Altitude Chart
3. U.S. Terminal Procedures Publication (TPP). TPPs are published in 24 loose−leaf or perfect bound
volumes covering the conterminous U.S., Puerto Rico and the Virgin Islands. A Change Notice is published at
the midpoint between revisions in bound volume format and is available on the internet for free download at the
AIS website. (See FIG 9−1−15.) The TPPs include:
(a) Instrument Approach Procedure (IAP) Charts. IAP charts portray the aeronautical data that is
required to execute instrument approaches to airports. Each chart depicts the IAP, all related navigation data,
communications information, and an airport sketch. Each procedure is designated for use with a specific
electronic navigational aid, such as ILS, VOR, NDB, RNA V , etc.
Types of Charts Available 9−1−7
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
(b) Instrument Departure Procedure (DP) Charts. DP charts are designed to expedite clearance
delivery and to facilitate transition between takeoff and en route operations. They furnish pilots’ departure
routing clearance information in graphic and textual form.
(c) Standard Terminal Arrival (STAR) Charts. STAR charts are designed to expedite ATC arrival
procedures and to facilitate transition between en route and instrument approach operations. They depict
preplanned IFR ATC arrival procedures in graphic and textual form. Each STAR procedure is presented as a
separate chart and may serve either a single airport or more than one airport in a given geographic area.
(d) Airport Diagrams. Full page airport diagrams are designed to assist in the movement of ground
traffic at locations with complex runway/taxiway configurations and provide information for updating geodetic
position navigational systems aboard aircraft. Airport diagrams are available for free download at the AIS
website.
4. Alaska Terminal Procedures Publication. This publication contains all terminal flight procedures for
civil and military aviation in Alaska. Included are IAP charts, DP charts, STAR charts, airport diagrams, radar
minimums, and supplementary support data such as IFR alternate minimums, take −off minimums, rate of
descent tables, rate of climb tables and inoperative components tables. Volume is 5−3/8 x 8−1/4 inch top bound.
Publication revised every 56 days with provisions for a Terminal Change Notice, as required.
c. Planning Charts.
1. U.S. IFR/VFR Low Altitude Planning Chart. This chart is designed for prefight and en route flight
planning for IFR/VFR flights. Depiction includes low altitude airways and mileage, NA VAIDs, airports, special
use airspace, cities, times zones, major drainage, a directory of airports with their airspace classification, and a
mileage table showing great circle distances between major airports. Scale 1 inch = 47nm/1:3,400,000. Chart
revised annually, and is available either folded or unfolded for wall mounting. (See FIG 9−1−10.)
2. Gulf of America and Caribbean Planning Chart. This is a VFR planning chart on the reverse side
of the Puerto Rico − Virgin Islands VFR Terminal Area Chart. Information shown includes mileage between
airports of entry, a selection of special use airspace and a directory of airports with their available services. Scale
1 inch = 85nm/1:6,192,178. 60 x 20 inches folded to 5 x 10 inches. Revised every 56 days. (See FIG 9−1−10.)
3. Alaska VFR Wall Planning Chart. This chart is designed for VFR preflight planning and chart
selection. It includes aeronautical and topographic information of the state of Alaska. The aeronautical
information includes public and military airports; radio aids to navigation; and Class B, Class C, TRSA and
special−use airspace. The topographic information includes city tint, populated places, principal roads, and
shaded relief. Scale 1 inch = 27.4 nm/1:2,000,000. The one sided chart is 58.5 x 40.75 inches and is designed
for wall mounting. Revised annually. (See FIG 9−1−9.)
FIG 9−1−9
Alaska VFR Wall Planning Chart
9−1−8 Types of Charts Available
AIM2/20/251/22/26 AIM
FIG 9−1−10
Planning Charts
4. U.S. VFR Wall Planning Chart. This chart is designed for VFR preflight planning and chart selection.
It includes aeronautical and topographic information of the conterminous U.S. The aeronautical information
includes airports, radio aids to navigation, Class B airspace and special use airspace. The topographic
information includes city tint, populated places, principal roads, drainage patterns, and shaded relief. Scale 1 inch
= 43 nm/ 1:3,100,000. The one−sided chart is 59 x 36 inches and ships unfolded for wall mounting. Revised
annually. (See FIG 9−1−11.)
FIG 9−1−11
U.S. VFR Wall Planning Chart
5. VFR Flyway Planning Charts. This chart is printed on the reverse side of selected TAC charts. The
coverage is the same as the associated TAC. Flyway planning charts depict flight paths and altitudes
recommended for use to bypass high traffic areas. Gr ound references are provided as a guide for visual
orientation. Flyway planning charts are designed for use in conjunction with TACs and sectional charts and are
not to be used for navigation. VFR Transition Routes may be depicted and/or described on this chart. Chart scale
1 inch = 3.43 nm/1:250,000.
Types of Charts Available 9−1−9
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
d. Supplementary Charts and Publications.
1. Chart Supplement refers to a series of civil/military flight information publications issued by FAA every
56 days consisting of the Chart Supplement U.S., Chart Supplement Alaska, and Chart Supplement Pacific.
2. Chart Supplement U.S. This is a civil/military flight information publication. This 7 −volume book
series is designed for use with appropriate IFR or VFR charts and contains data including, but not limited to,
airports, NA V AIDs, communications data, weather data sources, special notices, non−regulatory operational
procedures, and airport diagrams. Coverage includes the conterminous U.S., Puerto Rico, and the Virgin Islands.
The Chart Supplement U.S. shows data that cannot be readily depicted in graphic form; for example, airport
hours of operations, types of fuel available, run widths, and lighting codes. (See FIG 9−1−12.)
FIG 9−1−12
Chart Supplement U.S. Geographic Areas
3. Chart Supplement Alaska. This is a civil/military flight information publication. This single−volume
book is designed for use with appropriate IFR or VFR charts. The Chart Supplement Alaska contains data
including, but not limited to, airports, NA V AIDs, communications data, weather data sources, special notices,
non−regulatory operational procedures, and airport di agrams. The publication also includes uniquely
geographical operational requirements as area notices and emergency procedures.
4. Chart Supplement Pacific. This is a civil/military flight information publication. This single volume
book is designed for use with appropriate IFR or VFR charts. The Chart Supplement Pacific contains data
including, but not limited to, airports, NA V AIDs, communications data, weather data sources, special notices,
non−regulatory operational procedures, and airport di agrams. The publication also includes airspace,
navigational facilities, non− regulatory Pacific area procedures, Instrument Approach Procedures (IAP),
Departure Procedures (DP), Standard Terminal Arrival (STAR) charts, radar minimums, supporting data for the
Hawaiian and Pacific Islands, and uniquely geographica l operational requirements as area notices and
emergency procedures.
5. North Atlantic Route Chart. Designed for FAA controllers to monitor transatlantic flights, this
5−color chart shows oceanic control areas, coastal navigation aids, oceanic reporting points, and NA V AID
geographic coordinates. Full Size Chart: Scale 1 inch = 113.1 nm/1:8,250,000. Chart is shipped flat only. Half
Size Chart: Scale 1 inch = 150.8 nm/1:11,000,000. Chart is 29−3/4 x 20−1/2 inches, shipped folded to 5 x 10
inches only. Chart revised every 56 days. (See FIG 9−1−13.)
9−1−10 Types of Charts Available
AIM2/20/258/7/25 AIM
FIG 9−1−13
North Atlantic Route Charts
6. North Pacific Route Charts. These charts are designed for FAA controllers to monitor transoceanic
flights. They show established intercontinental air routes, including reporting points with geographic positions.
Composite Chart: Scale 1 inch = 164 nm/1:12,000,000. 48 x 41−1/2 inches. Area Charts: Scale 1 inch = 95.9
nm/1:7,000,000. 52 x 40− 1/2 inches. All charts shipped unfolded. Charts revised every 56 days. (See
FIG 9−1−14.)
FIG 9−1−14
North Pacific Oceanic Route Charts
7. Airport Obstruction Charts (OC). The OC is a 1:12,000 scale graphic depicting 14 CFR part 77,
Objects Affecting Navigable Airspace, surfaces, a representation of objects that penetrate these surfaces, aircraft
movement and apron areas, navigational aids, prominent airport buildings, and a selection of roads and other
planimetric detail in the airport vicinity. Also included are tabulations of runway and other operational data.
Types of Charts Available 9−1−11
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
8. FAA Aeronautical Chart User’s Guide. A booklet designed to be used as a teaching aid and reference
document. It describes the substantial amount of information provided on FAA’s aeronautical charts and
publications. It includes explanations and illustrations of chart terms and symbols organized by chart type. The
users guide is available for free download at the AIS website.
e. Digital Products.
1. The Digital Aeronautical Information CD (DAICD). The DAICD is a combination of the NA V AID
Digital Data File, the Digital Chart Supplement, and the Digital Obstacle File on one Compact Disk. These three
digital products are no longer sold separately. The files are updated every 56 days and are available by
subscription only.
(a) The NA V AID Digital Data File. This file contains a current listing of NA VAIDs that are compatible
with the National Airspace System. This file contains all NA V AIDs including ILS and its components, in the
U.S., Puerto Rico, and the Virgin Islands plus bordering facilities in Canada, Mexico, and the Atlantic and Pacific
areas.
(b) The Digital Obstacle File. This file describes all known obstacles of interest to aviation users in the
U.S., with limited coverage of the Pacific, Caribbean, Canada, and Mexico. The obstacles are assigned unique
numerical identifiers, accuracy codes, and listed in order of ascending latitude within each state or area.
2. The Coded Instrument Flight Procedures (CIFP) (ARINC 424 [Ver 13 & 15]). The CIFP is a basic
digital dataset, modeled to an international standard, which can be used as a basis to support GPS navigation.
Initial data elements included are: Airport and Helicopter Records, VHF and NDB Navigation aids, en route
waypoints and airways. Additional data elements will be added in subsequent releases to include: departure
procedures, standard terminal arrivals, and GPS/RNA V instrument approach procedures. The database is
updated every 28 days. The data is available by subscription only and is distributed on CD −ROM or by ftp
download.
3. digital−Visual Charts (d−VC). These digital VFR charts are geo−referenced images of FAA Sectional
Aeronautical, TAC, and Helicopter Route charts. Additional digital data may easily be overlaid on the raster
image using commonly available Geographic Information System software. Data such as weather, temporary
flight restrictions, obstacles, or other geospatial data can be combined with d−VC data to support a variety of
needs. The file resolution is 300 dots per inch and the data is 8−bit color. The data is provided as a GeoTIFF and
distributed on DVD−R media and on the AIS website. The root mean square error of the transformation will not
exceed two pixels. Digital−VCs are updated every 56 days and are available by subscription only.
9−1−12 Types of Charts Available
8/7/25 AIM
FIG 9−1−15
U.S. Terminal Publication Volumes
AIM2/20/25
Types of Charts Available 9−1−13
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
9−1−6. Where and How to Get Charts of Foreign Areas
a. National Geospatial− Intelligence Agency (NGA) Products. For the latest information regarding
publication availability visit the NGA website: https://www.nga.mil/ProductsServices/Pages/default.aspx.
1. Flight Information Publication (FLIP) Planning Documents.
General Planning (GP) Area Planning Area Planning Special Use Airspace Planning Charts
2. FLIP En Route Charts and Chart Supplements.
Pacific, Australasia, and Antarctica U.S. − IFR and VFR Supplements Flight Information Handbook Caribbean
and South America − Low Altitude Caribbean and South America − High Altitude Europe, North Africa, and
Middle East − Low Altitude Europe, North Africa, and Middle East High Altitude Africa Eastern Europe and
Asia Area Arrival Charts
3. FLIP Instrument Approach Procedures (IAPs).
Africa
Canada and North Atlantic
Caribbean and South America
Eastern Europe and Asia
Europe, North Africa, and Middle East
Pacific, Australasia, and Antarctica
VFR Arrival/Departure Routes − Europe and Korea
U.S.
4. Miscellaneous DoD Charts and Products.
Aeronautical Chart Updating Manual (CHUM)
DoD Weather Plotting Charts (WPC)
Tactical Pilotage Charts (TPC)
Operational Navigation Charts (ONC)
Global Navigation and Planning Charts (GNC)
Jet Navigation Charts (JNC) and Universal Jet Navigation Charts (JNU)
Jet Navigation Charts (JNCA)
Aerospace Planning Charts (ASC)
Oceanic Planning Charts (OPC)
Joint Operations Graphics − Air (JOG−A)
Standard Index Charts (SIC)
Universal Plotting Sheet (VP−OS)
Sight Reduction Tables for Air Navigation (PUB249)
Plotting Sheets (VP−30)
Dial−Up Electronic CHUM
b. Canadian Charts. Information on available Canadian charts and publications may be obtained by
contacting the:
NA V CANADA
Aeronautical Publications
Sales and Distribution Unit
P.O. Box 9840, Station T
Ottawa, Ontario K1G 6S8 Canada
Telephone: 613−744−6393 or 1−866−731−7827
Fax: 613−744−7120 or 1−866−740−9992
9−1−14 Types of Charts Available
AIM2/20/258/7/25 AIM
c. Mexican Charts. Information on available Mexican charts and publications may be obtained by
contacting:
Dirección de Navigacion Aereo
Blvd. Puerto Aereo 485
Zona Federal Del Aeropuerto Int’l
15620 Mexico D.F.
Mexico
d. International Civil Aviation Organization (ICAO). A list of free ICAO publications and catalogs is
available at the following website: https://www.icao.int/publications/Pages/default.aspx.
Types of Charts Available 9−1−15
2/20/25 AIM
Chapter 10. Helicopter Operations
Section 1. Helicopter IFR Operations
10−1−1. Helicopter Flight Control Systems
a. The certification requirements for helicopters to operate under Instrument Flight Rules (IFR) are contained
in 14 CFR part 27, Airworthiness Standards: Normal Category Rotorcraft, and 14 CFR part 29, Airworthiness
Standards: Transport Category Rotorcraft. To meet these requirements, helicopter manufacturers usually utilize
a set of stabilization and/or Automatic Flight Control Systems (AFCSs).
b. Typically, these systems fall into the following categories:
1. Aerodynamic surfaces, which impart some stability or control capability not found in the basic VFR
configuration.
2. Trim systems, which provide a cyclic centering effect. These systems typically involve a magnetic
brake/spring device, and may also be controlled by a four−way switch on the cyclic. This is a system that supports
“hands on” flying of the helicopter by the pilot.
3. Stability Augmentation Systems (SASs), which provide short−term rate damping control inputs to
increase helicopter stability. Like trim systems, SAS supports “hands on” flying.
4. Attitude Retention Systems (ATTs), which return the helicopter to a selected attitude after a disturbance.
Changes in desired attitude can be accomplished usually through a four−way “beep” switch, or by actuating a
“force trim” switch on the cyclic, setting the attitude manually, and releasing. Attitude retention may be a SAS
function, or may be the basic “hands off” autopilot function.
5. Autopilot Systems (APs), which provide for “hands off” flight along specified lateral and vertical paths,
including heading, altitude, vertical speed, navigation tracking, and approach. These systems typically have a
control panel for mode selection, and system for indication of mode status. Autopilots may or may not be installed
with an associated Flight Director System (FD). Autopilots typically control the helicopter about the roll and
pitch axes (cyclic control) but may also include yaw axis (pedal control) and collective control servos.
6. FDs, which provide visual guidance to the pilot to fly specific selected lateral and vertical modes of
operation. The visual guidance is typically provided as either a “dual cue” (commonly known as a
“cross−pointer”) or “single cue” (commonly known as a “vee−bar”) presentation superimposed over the attitude
indicator. Some FDs also include a collective cue. The pilot manipulates the helicopter’s controls to satisfy these
commands, yielding the desired flight path, or may couple the flight director to the autopilot to perform automatic
flight along the desired flight path. Typically, flight director mode control and indication is shared with the
autopilot.
c. In order to be certificated for IFR operation, a specific helicopter may require the use of one or more of these
systems, in any combination.
d. In many cases, helicopters are certificated for IFR operations with either one or two pilots. Certain
equipment is required to be installed and functional for two pilot operations, and typically, additional equipment
is required for single pilot operation. These requirements are usually described in the limitations section of the
Rotorcraft Flight Manual (RFM).
e. In addition, the RFM also typically defines systems and functions that are required to be in operation or
engaged for IFR flight in either the single or two pilot configuration. Often, particularly in two pilot operation,
this level of augmentation is less than the full capability of the installed systems. Likewise, single pilot operation
may require a higher level of augmentation.
f. The RFM also identifies other specific limitations associated with IFR flight. Typically, these limitations
include, but are not limited to:
Helicopter IFR Operations 10−1−1
AIM 2/20/25
1. Minimum equipment required for IFR flight (in some cases, for both single pilot and two pilot
operations).
2. Vmini (minimum speed − IFR).
NOTE−
The manufacturer may also recommend a minimum IFR airspeed during instrument approach.
3. Vnei (never exceed speed − IFR).
4. Maximum approach angle.
5. Weight and center of gravity limits.
6. Aircraft configuration limitations (such as aircraft door positions and external loads).
7. Aircraft system limitations (generators, inverters, etc.).
8. System testing requirements (many avionics and AFCS/AP/FD systems incorporate a self−test feature).
9. Pilot action requirements (such as the pilot must have his/her hands and feet on the controls during certain
operations, such as during instrument approach below certain altitudes).
g. It is very important that pilots be familiar with the IFR requirements for their particular helicopter. Within
the same make, model and series of helicopter, variations in the installed avionics may change the required
equipment or the level of augmentation for a particular operation.
h. During flight operations, pilots must be aware of the mode of operation of the augmentation systems, and
the control logic and functions employed. For example, during an ILS approach using a particular system in the
three−cue mode (lateral, vertical and collective cues), the flight director collective cue responds to glideslope
deviation, while the horizontal bar of the “cross−pointer” responds to airspeed deviations. The same system,
while flying an ILS in the two−cue mode, provides for the horizontal bar to respond to glideslope deviations.
This concern is particularly significant when operating using two pilots. Pilots should have an established set
of procedures and responsibilities for the control of flight director/autopilot modes for the various phases of
flight. Not only does a full understanding of the system modes provide for a higher degree of accuracy in control
of the helicopter, it is the basis for crew identification of a faulty system.
i. Relief from the prohibition to takeoff with any inoperative instruments or equipment may be provided
through a Minimum Equipment List (see 14 CFR section 91.213 and 14 CFR section 135.179, Inoperative
Instruments and Equipment). In many cases, a helicopter configured for single pilot IFR may depart IFR with
certain equipment inoperative, provided a crew of two pilots is used. Pilots are cautioned to ensure the
pilot−in−command and second−in−command meet the requirements of 14 CFR section 61.58, Pilot−in−Com-
mand Proficiency Check: Operation of Aircraft Requiring More Than One Pilot Flight Crewmember, and 14
CFR section 61.55, Second −in−Command Qualifications, or 14 CFR part 135, Operating Requirements:
Commuter and On− Demand Operations, Subpart E, Flight Crewmember Requirements, and Subpart G,
Crewmember Testing Requirements, as appropriate.
j. Experience has shown that modern AFCS/AP/FD equipment installed in IFR helicopters can, in some cases,
be very complex. This complexity requires the pilot(s) to obtain and maintain a high level of knowledge of system
operation, limitations, failure indications and reversionary modes. In some cases, this may only be reliably
accomplished through formal training.
10−1−2. Helicopter Instrument Approaches
a. Instrument flight procedures (IFPs) permit helicopter operations to heliports and runways during periods
of low ceilings and reduced visibility (e.g. approach/SID/STAR/en route). IFPs can be designed for both public
and private heliports using FAA instrument criteria. The FAA does recognize there are non −FAA service
providers with proprietary special criteria. Special IFPs are reviewed and approved by Flight Technologies and
Procedures Division and may have specified aircraft performance or equipment requirements, special crew
10−1−2 Helicopter IFR Operations
2/20/25 AIM
training, airport facility equipment, waivers from published standards, proprietary criteria and restricted access.
Special IFPs are not published in the Federal Register or printed in government Flight Information Publications.
b. Helicopters are capable of flying any published IFPs, for which they are properly equipped, subject to the
following limitations and conditions:
1. Helicopters flying conventional (i.e. non−Copter) IAPs may reduce the visibility minima to not less than
one−half the published Category A landing visibility minima, or 1/4 statute mile visibility/1200 RVR, whichever
is greater, unless the procedure is annotated with “Visibility Reduction by Helicopters NA.” This annotation
means that there are penetrations of the final approach obstacle identification surface (OIS) and that the 14 CFR
section 97.3 visibility reduction rule does not apply and you must take precaution to avoid any obstacles in the
visual segment. No reduction in MDA/DA is permitted at any time. The helicopter may initiate the final approach
segment at speeds up to the upper limit of the highest approach category authorized by the procedure, but must
be slowed to no more than 90 KIAS at the missed approach point (MAP) in order to apply the visibility reduction.
Pilots are cautioned that such a decelerating approach may make early identification of wind shear on the
approach path difficult or impossible. If required, use the Inoperative Components and Visual Aids Table
provided inside the front cover of the U.S. Terminal Procedures Publication to derive the Category A minima
before applying the 14 CFR section 97.3 rule.
2. Helicopters flying Copter IAPs should use the published minima, with no reductions allowed. Unless
otherwise specified on the instrument procedure chart, 90 KIAS is the maximum speed on the approach.
3. Pilots flying Area Navigation (RNA V) Copter IAPs should also limit their speed to 90 KIAS unless
otherwise specified on the instrument procedure chart. The final and missed approach segment speeds must be
limited to no more than 70 KIAS unless otherwise charted. Military RNA V Copter IAPs are limited to no more
than 90 KIAS throughout the procedure. Use the published minima; no reductions allowed.
NOTE−
Obstruction clearance surfaces are based on the aircraft speed identified on the approach chart and have been designed
on RNAV approaches for 70 knots unless otherwise indicated. If the helicopter is flown at higher speeds, it may fly outside
of protected airspace. Some helicopters have a VMINI greater than 70 knots; therefore, they cannot meet the 70 knot limitation
to conduct these RNAV approaches. Some helicopter autopilots, when used in the “go−around” mode, are programmed with
a VYI greater than 70 knots. Therefore, those helicopters when using the autopilot “go−around” mode, cannot meet the 70
knot limitation for the RNAV approach. It may be possible to use the autopilot for the missed approach in other than the
“go−around” mode and meet the 70 knot limitation. When operating at speeds other than VYI or VY, performance data may
not be available in the RFM to predict compliance with climb gradient requirements. Pilots may use observed performance
in similar weight/altitude/temperature/speed conditions to evaluate the suitability of performance. Pilots are cautioned to
monitor climb performance to ensure compliance with procedure requirements.
NOTE−
VMINI − Instrument flight minimum speed, utilized in complying with minimum limit speed requirements for instrument flight
VYI − Instrument climb speed, utilized instead of VY for compliance with the climb requirements for instrument flight
VY − Speed for best rate of climb
4. TBL 10−1−1 summarizes these requirements.
5. Even with weather conditions reported at or above minimums, under some combinations of reduced
cockpit cutoff angle, approach/runway lighting, and high MDA/DH (coupled with a low visibility minima), the
pilot may not be able to identify the required visual reference(s), or those references may only be visible in a very
small portion of the available field of view. Even if identified by the pilot, the visual references may not support
normal maneuvering and normal rates of descent to landing. The effect of such a combination may be exacerbated
by other conditions such as rain on the windshield, or incomplete windshield defogging coverage.
6. Pilots should always be prepared to execute a missed approach even though weather conditions may be
reported at or above minimums.
NOTE−
See paragraph 5−4−21, Missed Approach, for additional information on missed approach procedures.
Helicopter IFR Operations 10−1−3
AIM 2/20/25
TBL 10−1−1
Helicopter Use of Standard Instrument Approach Procedures
Procedure Helicopter Visibility
Minima
Helicopter MDA/DA Maximum Speed Limitations
Conventional
(non−Copter)
The greater of: one half
the Category A visibility
minima, 1/4 statute mile
visibility, or 1200 RVR
As published for
Category A
The helicopter may initiate the final
approach segment at speeds up to
the upper limit of the highest
approach category authorized by the
procedure, but must be slowed to no
more than 90 KIAS at the MAP in
order to apply the visibility
reduction.
Copter Procedure As published As published 90 KIAS maximum when on a
published route/track.
RNA V (GPS) Copter
Procedure
As published As published The maximum speed for a Copter
approach will be 90 KIAS or as
published on the chart. Note: Higher
approach angles may require a
lower approach speed and aircraft
VMINI. Military procedures are
limited to 90 KIAS for all segments.
NOTE−
Several factors affect the ability of the pilot to acquire and maintain the visual references specified in 14 CFR
section 91.175(c), even in cases where the flight visibility may be at the minimum derived from the criteria in TBL 10−1−1.
These factors include, but are not limited to:
1. Cockpit cutoff angle (the angle at which the cockpit or other airframe structure limits downward visibility below the
horizon).
2. Combinations of high MDA/DH and low visibility minimum, such as approaches with reduced helicopter visibility
minima (per 14 CFR section 97.3).
3. Type, configuration, and intensity of approach and runway/heliport lighting systems.
4. Type of obscuring phenomenon and/or windshield contamination.
10−1−3. Helicopter Approach Procedures to VFR Heliports
a. The FAA may develop helicopter instrument approaches for heliports that do not meet the design standards
for an IFR heliport. The majority of IFR approaches to VFR heliports are developed in support of Helicopter Air
Ambulance (HAA) operators. These approaches may require use of conventional NA VAIDS or a RNA V system
(e.g., GPS). They may be developed either as a special approach (pilot training is required for special procedures
due to their unique characteristics) or a public approach (no special training required). These instrument
procedures may be designed to guide the helicopter to a specific landing area (Proceed Visually) or to a
point−in−space with a “Proceed VFR” segment.
1. An approach to a specific landing area. This type of approach is aligned to a missed approach point
from which a landing can be accomplished with a maximum course change of 30 degrees. The visual segment
from the MAP to the landing area is evaluated for obstacle hazards. These procedures are annotated: “PROCEED
VISUALLY FROM (named MAP) OR CONDUCT THE SPECIFIED MISSED APPROACH.”
(a) “Proceed Visually” requires the pilot to acquire and maintain visual contact with the landing area at
or prior to the MAP, or execute a missed approach. The visibility minimum is based on the distance from the MAP
to the landing area, among other factors.
(b) The pilot is required to have the published minimum visibility throughout the visual segment flying
the path described on the approach chart.
(c) Similar to an approach to a runway, the pilot is responsible for obstacle or terrain avoidance from the
MAP to the landing area.
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AIM2/20/258/7/25 AIM7/9/26 AIM
(d) Upon reaching the published MAP, or as soon as practicable thereafter, the pilot should advise ATC
whether proceeding visually and canceling IFR or complying with the missed approach instructions. See
paragraph 5−1−15, Canceling IFR Flight Plan.
(e) Where any necessary visual reference requirements are specified by the FAA, at least one of the
following visual references for the intended heliport is visible and identifiable before the pilot may proceed
visually:
(1) FATO or FATO lights.
(2) TLOF or TLOF lights.
(3) Heliport Instrument Lighting System (HILS).
(4) Heliport Approach Lighting System (HALS).
(5) Visual Glideslope Indicator (VGSI).
(6) Windsock or windsock light.
(7) Heliport beacon.
(8) Other facilities or systems approved by the Flight Technologies and Procedures Division
(AFS−400).
2. Approach to a Point−in−Space (PinS). At locations where the MAP is located more than 2 SM from
the landing area, or the path from the MAP to the landing area is populated with obstructions which require
avoidance actions or requires turn greater than 30 degrees, a PinS Proceed VFR procedure may be developed.
These approaches are annotated “PROCEED VFR FROM (named MAP) OR CONDUCT THE SPECIFIED
MISSED APPROACH.”
(a) These procedures require the pilot, at or prior to the MAP, to determine if the published minimum
visibility, or the weather minimums required by the operating rule (e.g., part 91, part 135, etc.), or operations
specifications (whichever is higher) is available to safely transition from IFR to VFR flight. If not, the pilot must
execute a missed approach. For part 135 operations, pilots may not begin the instrument approach unless the
latest weather report indicates that the weather conditions are at or above the authorized IFR minimums or the
VFR weather minimums (as required by the class of airspace, operating rule and/or Operations Specifications)
whichever is higher.
(b) Visual contact with the landing site is not required; however, the pilot must have the appropriate VFR
weather minimums throughout the visual segment. The visibility is limited to no lower than that published in
the procedure, until canceling IFR.
(c) IFR obstruction clearance areas are not applied to the VFR segment between the MAP and the landing
site. Pilots are responsible for obstacle or terrain avoidance from the MAP to the landing area.
(d) Upon reaching the MAP defined on the approach procedure, or as soon as practicable thereafter, the
pilot should advise ATC whether proceeding VFR and canceling IFR, or complying with the missed approach
instructions. See paragraph 5−1−15, Canceling IFR Flight Plan.
(e) If the visual segment penetrates Class B, C, or D airspace, pilots are responsible for obtaining a Special
VFR clearance, when required.
10−1−4. The Gulf of America Grid System
a. The Gulf of America Grid System navigational route structure is completely independent of
ground−based navigation aids (NA V AID) and was designed to facilitate helicopter IFR operations to offshore
destinations. The Grid System is defined by over 300 offshore waypoints located 20 minutes apart (latitude and
longitude). Flight plan routes are routinely defined by just 4 segments: departure point (lat/long), first en route
grid waypoint, last en route grid waypoint prior to approach procedure, and destination point (lat/long). There
Helicopter IFR Operations 10−1−5
AIM 2/20/253/15/077110.65R CHG 2AIM 7/9/26
are over 4,000 possible offshore landing sites (oil rigs, wind turbines, etc.). Upon reaching the waypoint prior
to the destination, the pilot may execute an Offshore Standard Approach Procedure (OSAP), a Helicopter En
Route Descent Areas (HEDA) approach, or an Airborne Radar Approach (ARA). For more information on these
helicopter instrument procedures, refer to FAA AC 90−80, Approval of Offshore Standard Approach Procedures,
Airborne Radar Approaches, and Helicopter En Route Descent Areas, on the FAA Advisory Circulars website
at https://www.faa.gov/regulations_policies/advisory_circulars/. The return flight plan is just the reverse with
the requested stand−alone GPS approach contained in the remarks section.
b. The large number (over 300) of waypoints in the grid system makes it difficult to assign phonetically
pronounceable names to the waypoints that would be meaningful to pilots and controllers. A unique naming
system was adopted that enables pilots and controllers to derive the fix position from the name. The five−letter
names are derived as follows:
1. The waypoints are divided into sets of 3 columns each. A three −letter identifier, identifying a
geographical area or a NA V AID to the north, represents each set.
2. Each column in a set is named after its position, i.e., left (L), center (C), and right (R).
3. The rows of the grid are named alphabetically from north to south, starting with A for the northern most
row.
EXAMPLE−
LCHRC would be pronounced “Lake Charles Romeo Charlie.” The waypoint is in the right −hand column of the Lake
Charles VOR set, in row C (third south from the northern most row).
c. An infrastructure of ADS−B ground stations, weather stations (AWOS), and VHF remote communication
outlets (RCO) exists throughout a large area of the Gulf of America. This infrastructure allows the FAA’s Houston
ARTCC to provide “domestic−like” air traffic control service in the offshore area beyond 12 NM from the
coastline to hundreds of miles offshore to aircraft equipped with ADS−B. Properly equipped aircraft can now
be authorized to receive more direct routing, domestic en route separation minima, and real time flight following.
Operators who do not have authorization to receive ATC separation services using ADS−B, will continue to use
the low altitude grid system and receive procedural separation from Houston ARTCC. Non−ADS−B equipped
aircraft also benefit from improved VHF communication and expanded weather information coverage.
d. Three requirements must be met for operators to file IFR flight plans utilizing the grid:
1. The helicopter must be equipped for IFR operations and equipped with IFR approved GPS navigational
units.
2. The operator must obtain prior written approval from the appropriate Flight Standards District Office
through a Letter of Authorization or Operations Specification, as appropriate.
3. The operator must be a signatory to the Houston ARTCC Letter of Agreement.
e. Operators utilizing ADS− B−based ATC separation services must meet the following additional
requirements:
1. The Operator’s installed ADS−B must comply with the requirements of 14 CFR sections 91.225 and
91.227.
2. Flight crews must comply with the procedures prescribed in the Houston ARTCC Letter of Agreement
dated March 1, 2018, or later.
NOTE−
The unique ADS−B architecture in the Gulf of America depends upon reception of an aircraft’ s Mode C in addition to the
other message elements described in 14 CFR 91.227. Flight crews must be made aware that loss of Mode C also means that
ATC will not receive the aircraft’ s ADS−B signal.
f. FAA/AIS publishes the grid system waypoints on the IFR Gulf of America Vertical Flight Reference Chart.
A commercial equivalent is also available. The chart is updated annually and is available from an FAA print
provider or for free download on the AIS website under “Supplemental Charts/Pubs”:
https://www.faa.gov/air_traffic/flight_info/aeronav/productcatalog/.
10−1−6 Helicopter IFR Operations
AIM2/20/258/7/25 AIM7/9/26 AIM
10−1−5. Departure Procedures
a. When departing from a location on a point−in−space (PinS) SID with a visual segment indicated and the
departure instruction describes the visual segment the aircraft must cross the initial departure fix (IDF) outbound
at−or−above the altitude depicted on the chart. The helicopter will initially establish a hover at or above the
heliport crossing height (HCH) specified on the chart. The HCH specifies a minimum hover height to begin the
climb to assist in avoiding obstacles. The helicopter will leave the departure location on the published outbound
heading/course specified, climbing at least 400 ft/per NM (or as depicted on the chart), remaining clear of
clouds, crossing at or above the IDF altitude specified, prior to proceeding outbound on the
procedure. For example the chart may include these instructions: “Hover at 15 ft AGL, then climb on track 005,
remaining clear of clouds, to cross PAWLY at or above 700.”
b. When flying a PinS SID procedure containing a segment with instructions to “proceed VFR,” the pilot
must keep the aircraft clear of the clouds and cross the IDF outbound at or above the altitude depicted. Departure
procedures that support multiple departure locations will have a Proceed VFR segment leading to the
IDF. The chart will provide a bearing and distance to the IDF from the heliport. That bearing and distance are
for pilot orientation purposes only and are not a required procedure track. The helicopter will leave the departure
location via pilot navigation in order to align with the departure route and comply with the altitude specified at
the IDF. For example, the chart may include these instructions: “VFR Climb to WEBBB, Cross WEBBB at or
above 800.”
c. Once the aircraft reaches the IDF, the aircraft should proceed out the described route as specified on the
chart, crossing each consecutive fix at or above the indicated altitude(s) until reaching the end of the departure or
as directed by ATC.
Helicopter IFR Operations 10−1−7
AIM 2/20/25
FIG 10−1−1
Departure Charts
10−1−8 Helicopter IFR Operations
AIM2/20/258/7/25 AIM
Section 2. Special Operations
10−2−1. Offshore Helicopter Operations
a. Introduction
The offshore environment offers unique applications and challenges for helicopter pilots. The mission demands,
the nature of oil and gas exploration and production facilities, and the flight environment (weather, terrain,
obstacles, traffic), demand special practices, techniques and procedures not found in other flight operations.
Several industry organizations have risen to the task of reducing risks in offshore operations, including the
Helicopter Safety Advisory Conference (HSAC) (http://www.hsac.org), and the Offshore Committee of the
Helicopter Association International (HAI) (https://rotor.org/). The following recommended practices for
offshore helicopter operations are based on guidance developed by HSAC for use in the Gulf of America, and
provided here with their permission. While not regulatory, these recommended practices provide aviation and
oil and gas industry operators with useful information in developing procedures to avoid certain hazards of
offshore helicopter operations.
NOTE−
Like all aviation practices, these recommended practices are under constant review. In addition to normal procedures for
comments, suggested changes, or corrections to the AIM (contained in the Preface), any questions or feedback concerning
these recommended procedures may also be directed to the HSAC through the feedback feature of the HSAC website
(http://www.hsac.org).
b. Passenger Management on and about Heliport Facilities
1. Background. Several incidents involving offshore helicopter passengers have highlighted the potential
for incidents and accidents on and about the heliport area. The following practices will minimize risks to
passengers and others involved in heliport operations.
2. Recommended Practices
(a) Heliport facilities should have a designated and posted passenger waiting area which is clear of the
heliport, heliport access points, and stairways.
(b) Arriving passengers and cargo should be unloaded and cleared from the heliport and access route
prior to loading departing passengers and cargo.
(c) Where a flight crew consists of more than one pilot, one crewmember should supervise the
unloading/loading process from outside the aircraft.
(d) Where practical, a designated facility employee should assist with loading/unloading, etc.
c. Crane−Helicopter Operational Procedures
1. Background. Historical experience has shown that catastrophic consequences can occur when industry
safe practices for crane/helicopter operations are not observed. The following recommended practices are
designed to minimize risks during crane and helicopter operations.
2. Recommended Practices
(a) Personnel awareness
(1) Crane operators and pilots should develop a mutual understanding and respect of the others’
operational limitations and cooperate in the spirit of safety;
(2) Pilots need to be aware that crane operators sometimes cannot release the load to cradle the crane
boom, such as when attached to wire line lubricators or supporting diving bells; and
(3) Crane operators need to be aware that helicopters require warm up before takeoff, a two−minute
cool down before shutdown, and cannot circle for extended lengths of time because of fuel consumption.
Special Operations 10−2−1
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
(b) It is recommended that when helicopters are approaching, maneuvering, taking off, or running on
the heliport, cranes be shutdown and the operator leave the cab. Cranes not in use must have their booms cradled,
if feasible. If in use, the crane’s boom(s) are to be pointed away from the heliport and the crane shutdown for
helicopter operations.
(c) Pilots will not approach, land on, takeoff, or have rotor blades turning on heliports of structures not
complying with the above practice.
(d) It is recommended that cranes on offshore platforms, rigs, vessels, or any other facility, which could
interfere with helicopter operations (including approach/departure paths):
(1) Be equipped with a red rotating beacon or red high intensity strobe light connected to the system
powering the crane, indicating the crane is under power;
(2) Be designed to allow the operator a maximum view of the helideck area and should be equipped
with wide−angle mirrors to eliminate blind spots; and
(3) Have their boom tips, headache balls, and hooks painted with high visibility international orange.
d. Helicopter/Tanker Operations
1. Background. The interface of helicopters and tankers during shipboard helicopter operations is
complex and may be hazardous unless appropriate procedures are coordinated among all parties. The following
recommended practices are designed to minimize risks during helicopter/tanker operations:
2. Recommended Practices
(a) Management, flight operations personnel, and pilots should be familiar with and apply the operating
safety standards set forth in “Guide to Helicopter/Ship Operations”, International Chamber of Shipping, Third
Edition, 5−89 (as amended), establishing operational guide lines/standards and safe practices sufficient to
safeguard helicopter/tanker operations.
(b) Appropriate plans, approvals, and communications must be accomplished prior to reaching the
vessel, allowing tanker crews sufficient time to perform required safety preparations and position crew members
to receive or dispatch a helicopter safely.
(c) Appropriate approvals and direct communications with the bridge of the tanker must be maintained
throughout all helicopter/tanker operations.
(d) Helicopter/tanker operations, including landings/departures, must not be conducted until the
helicopter pilot−in−command has received and acknowledged permission from the bridge of the tanker.
(e) Helicopter/tanker operations must not be conducted during product/cargo transfer.
(f) Generally, permission will not be granted to land on tankers during mooring operations or while
maneuvering alongside another tanker.
e. Helideck/Heliport Operational Hazard Warning(s) Procedures
1. Background
(a) A number of operational hazards can develop on or near offshore helidecks or onshore heliports that
can be minimized through procedures for proper notification or visual warning to pilots. Examples of hazards
include but are not limited to:
(1) Perforating operations: subparagraph f.
(2) H2S gas presence: subparagraph g.
(3) Gas venting: subparagraph h; or,
(4) Closed helidecks or heliports: subparagraph i (unspecified cause).
(b) These and other operational hazards are currently minimized through timely dissemination of a
written Notice to Airmen (NOTAM) for pilots by helicopter companies and operators. A NOTAM provides a
10−2−2 Special Operations
AIM2/20/258/7/25 AIM
written description of the hazard, time and duration of occurrence, and other pertinent information. ANY
POTENTIAL HAZARD should be communicated to helicopter operators or company aviation departments as
early as possible to allow the NOTAM to be activated.
(c) To supplement the existing NOTA M procedure and furt her assist in reduc ing these hazards, a
standardized visual signal(s) on the helideck/heliport will provide a positive indication to an approaching
helicopter of the status of the landing area. Recommended Practice(s) have been developed to reinforce the
NOTAM procedures and standardize visual signals.
f. Drilling Rig Perforating Operations: Helideck/Heliport Operational Hazard
Warning(s)/Procedure(s)
1. Background. A critical step in the oil well completion process is perforation, which involves the use
of explosive charges in the drill pipe to open the pipe to oil or gas deposits. Explosive charges used in conjunction
with perforation operations offshore can potentially be prematurely detonated by radio transmissions, including
those from helicopters. The following practices are recommended.
2. Recommended Practices
(a) Personnel Conducting Perforating Operations. Whenever perforating operations are scheduled
and operators are con cerned that radio transmissions from helicopters in the vicinity may jeopardize the
operation, personnel conducting perforating operations should take the following precautionary measures:
(1) Notify company aviation departments, helicopter operators or bases, and nearby manned platforms
of the pending perforation operation so the Notice to Airmen (NOTAM) system can be activated for the
perforation operation and the temporary helideck closure.
(2) Close the deck and make the radio warning clearly visible to passing pilots, install a temporary
marking (described in subparagraph 10−2−1i1(b)) with the words “NO RADIO” stenciled in red on the legs of
the diagonals. The letters should be 24 inches high and 12 inches wide.
(See FIG 10−2−1.)
(3) The marker should be installed during the time that charges may be affected by radio transmissions.
(b) Pilots
(1) When operating within 1,000 feet of a known perforation operation or observing the white X with
red “NO RADIO” warning indicating perforation operations are underway, pilots will avoid radio transmissions
from or near the helideck (within 1,000 feet) and will not land on the deck if the X is present. In addition to
communications radios, radio transmissions are also emitted by aircraft radar, transponders, ADS−B equipment,
radar altimeters, and DME equipment, and ELTs.
(2) Whenever possible, make radio calls to the platform being approached or to the Flight Following
Communications Center at least one mile out on approach. Ensure all communications are complete outside the
1,000 foot hazard distance. If no response is received, or if the platform is not radio equipped, further radio
transmissions should not be made until visual contact with the deck indicates it is open for operation (no white
“X”).
g. Hydrogen Sulfide Gas Helideck/Heliport Operational Hazard Warning(s)/Procedures
1. Background. Hydrogen sulfide (H2S) gas: Hydrogen sulfide gas in higher concentrations (300 −500
ppm) can cause loss of consciousness within a few seconds and presents a hazard to pilots on/near offshore
helidecks. When operating in offshore areas that have been identified to have concentrations of hydrogen sulfide
gas, the following practices are recommended.
2. Recommended Practices
(a) Pilots
(1) Ensure approved protective air packs are available for emergency use by the crew on the helicopter.
Special Operations 10−2−3
AIM 2/20/25
(2) If shutdown on a helideck, request the supervisor in charge provide a briefing on location of
protective equipment and safety procedures.
(3) If while flying near a helideck and the visual red beacon alarm is observed or an unusually strong
odor of “rotten eggs” is detected, immediately don the protective air pack, exit to an area upwind, and notify the
suspected source field of the hazard.
FIG 10−2−1
Closed Helideck Marking − No Radio
(b) Oil Field Supervisors
(1) If presence of hydrogen sulfide is detected, a red rotating beacon or red high intensity strobe light
adjacent to the primary helideck stairwell or wind indicator on the structure should be turned on to provide visual
warning of hazard. If the beacon is to be located near the stairwell, the State of Louisiana “Offshore Heliport
Design Guide” and FAA Advisory Circular (AC) 150/5390−2A, Heliport Design Guide, should be reviewed to
ensure proper clearance on the helideck.
(2) Notify nearby helicopter operators and bases of the hazard and advise when hazard is cleared.
(3) Provide a safety briefing to include location of protective equipment to all arriving personnel.
(4) Wind socks or indicator should be clearly visible to provide upwind indication for the pilot.
h. Gas V enting Helideck/Heliport Operational Hazard Warning(s)/Procedures − Operations Near Gas
Vent Booms
1. Background. Ignited flare booms can release a large volume of natural gas and create a hot fire and
intense heat with little time for the pilot to react. Likewise, unignited gas vents can release reasonably large
volumes of methane gas under certain conditions. Thus, operations conducted very near unignited gas vents
require precautions to prevent inadvertent ingestion of combustible gases by the helicopter engine(s). The
following practices are recommended.
2. Pilots
(a) Gas will drift upwards and downwind of the vent. Plan the approach and takeoff to observe and avoid
the area downwind of the vent, remaining as far away as practicable from the open end of the vent boom.
(b) Do not attempt to start or land on an offshore helideck when the deck is downwind of a gas vent unless
properly trained personnel verify conditions are safe.
3. Oil Field Supervisors
(a) During venting of large amounts of unignited raw gas, a red rotating beacon or red high intensity
strobe light adjacent to the primary helideck stairwell or wind indicator should be turned on to provide visible
warning of hazard. If the beacon is to be located near the stairwell, the State of Louisiana “Offshore Heliport
Design Guide” and FAA AC 150/5390 −2A, Heliport Design Guide, should be reviewed to ensure proper
clearance from the helideck.
(b) Notify nearby helicopter operators and bases of the hazard for planned operations.
10−2−4 Special Operations
2/20/25 AIM
(c) Wind socks or indicator should be clearly visible to provide upward indication for the pilot.
i. Helideck/Heliport Operational Warning(s)/Procedure(s) − Closed Helidecks or Heliports
1. Background. A white “X” marked diagonally from corner to corner across a helideck or heliport
touchdown area is the universally accepted visual indicator that the landing area is closed for safety of other
reasons and that helicopter operations are not permitted. The following practices are recommended.
(a) Permanent Closing. If a helideck or heliport is to be permanently closed, X diagonals of the same
size and location as indicated above should be used, but the markings should be painted on the landing area.
NOTE−
White Decks: If a helideck is painted white, then international orange or yellow markings can be used for the temporary
or permanent diagonals.
(b) Temporary Closing. A temporary marker can be used for hazards of an interim nature. This marker
could be made from vinyl or other durable material in the shape of a diagonal “X.” The marker should be white
with legs at least 20 feet long and 3 feet in width. This marker is designed to be quickly secured and removed
from the deck using grommets and rope ties. The duration, time, location, and nature of these temporary closings
should be provided to and coordinated with company aviation departments, nearby helicopter bases, and
helicopter operators supporting the area. These markers MUST be removed when the hazard no longer exists.
(See FIG 10−2−2.)
FIG 10−2−2
Closed Helideck Marking
j. Offshore (VFR) Operating Altitudes for Helicopters
1. Background. Mid−air collisions constitute a significant percentage of total fatal offshore helicopter
accidents. A method of reducing this risk is the use of coordinated VFR cruising altitudes. To enhance safety
through standardized vertical separation of helicopter s when flying in the offshore environment, it is
recommended that helicopter operators flying in a particular area establish a cooperatively developed Standard
Operating Procedure (SOP) for VFR operating altitudes. An example of such an SOP is contained in this
example.
2. Recommended Practice Example
(a) Field Operations. Without compromising minimum safe operating altitudes, helicopters working
within an offshore field “constituting a cluster” should use altitudes not to exceed 500 feet.
(b) En Route Operations
(1) Helicopters operating below 750’ AGL should avoid transitioning through offshore fields.
(2) Helicopters en route to and from offshore locations, below 3,000 feet, weather permitting, should
use en route altitudes as outlined in TBL 10−2−1.
Special Operations 10−2−5
AIM 2/20/25
TBL 10−2−1
Magnetic Heading Altitude
0 to 179 750’
1750’
2750’
180 to 359 1250’
2250’
(c) Area Agreements. See HSAC Area Agreement Maps for operating procedures for onshore high
density traffic locations.
NOTE−
Pilots of helicopters operating VFR above 3,000 feet above the surface should refer to the current Federal Aviation
Regulations (14 CFR part 91), and paragraph 3−1−4, Basic VFR Weather Minimums, of the AIM.
(d) Landing Lights. Aircraft landing lights should be on to enhance aircraft identification:
(1) During takeoff and landings;
(2) In congested helicopter or fixed wing traffic areas;
(3) During reduced visibility; or,
(4) Anytime safety could be enhanced.
k. Offshore Helidecks/Landing Communications
1. Background. To enhance safety, and provide appropriate time to prepare for helicopter operations, the
following is recommended when anticipating a landing on an offshore helideck.
2. Recommended Practices
(a) Before landing on an offshore helideck, pilots are encouraged to establish communications with the
company owning or operating the helideck if frequencies exist for that purpose.
(b) When impracticable, or if frequencies do not exist, pilots or operations personnel should attempt to
contact the company owning or operating the helideck by telephone. Contact should be made before the pilot
departs home base/point of departure to advise of intentions and obtain landing permission if necessary.
NOTE−
It is recommended that communications be established a minimum of 10 minutes prior to planned arrival time. This practice
may be a requirement of some offshore owner/operators.
NOTE−
1. See subparagraph 10−2−1d for Tanker Operations.
2. Private use Heliport. Offshore heliports are privately owned/operated facilities and their use is limited to persons having
prior authorization to utilize the facility.
l. Two (2) Helicopter Operations on Offshore Helidecks
1. Background. Standardized procedures can enhance the safety of operating a second helicopter on an
offshore helideck, enabling pilots to determine/maintain minimum operational parameters. Orientation of the
parked helicopter on the helideck, wind and other factors may prohibit multi −helicopter operations. More
conservative Rotor Diameter (RD) clearances may be required under differing condition, i.e., temperature, wet
deck, wind (velocity/direction/gusts), obstacles, approach/departure angles, etc. Operations are at the pilot’s
discretion.
2. Recommended Practice. Helideck size, structural weight capability, and type of main rotor on the
parked and operating helicopter will aid in determining accessibility by a second helicopter. Pilots should
determine that multi−helicopter deck operations are permitted by the helideck owner/operator.
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3. Recommended Criteria
(a) Minimum one−third rotor diameter clearance ( 1/3 RD). The landing helicopter maintains a
minimum 1/3 RD clearance between the tips of its turning rotor and the closest part of a parked and secured
helicopter (rotors stopped and tied down).
(b) Three foot parking distance from deck edge (3’). Helicopters operating on an offshore helideck
land or park the helicopter with a skid/wheel assembly no closer than 3 feet from helideck edge.
(c) Tiedowns. Main rotors on all helicopters that are shut down be properly secured (tied down) to
prevent the rotor blades from turning.
(d) Medium (transport) and larger helicopters should not land on any offshore helideck where a light
helicopter is parked unless the light helicopter is property secured to the helideck and has main rotor tied down.
(e) Helideck owners/operators should ensure that the helideck has a serviceable anti−skid surface.
4. Weight and limitations markings on helideck. The helideck weight limitations should be displayed
by markings visible to the pilot (see State of Louisiana “Offshore Heliport Design Guide” and FAA AC
150/5390−2A, Heliport Design Guide).
NOTE−
Some offshore helideck owners/operators have restrictions on the number of helicopters allowed on a helideck. When
helideck size permits, multiple (more than two) helicopter operations are permitted by some operators.
m. Helicopter Rapid Refueling Procedures (HRR)
1. Background. Helicopter Rapid Refueling (HRR), engine(s)/rotors operating, can be conducted safely
when utilizing trained personnel and observing safe practices. This recommended practice provides minimum
guidance for HRR as outlined in National Fire Protection Association (NFPA) and industry practices. For
detailed guidance, please refer to National Fire Protection Association (NFPA) Document 407, “Standard for
Aircraft Fuel Servicing,” 1990 edition, including 1993 HRR Amendment.
NOTE−
Certain operators prohibit HRR, or “hot refueling,” or may have specific procedures for certain aircraft or refueling
locations. See the General Operations Manual and/or Operations Specifications to determine the applicable procedures or
limitations.
2. Recommended Practices
(a) Only turbine−engine helicopters fueled with JET A or JET A−1 with fueling ports located below any
engine exhausts may be fueled while an onboard engine(s) is (are) operating.
(b) Helicopter fueling while an onboard engine(s) is (are) operating should only be conducted under the
following conditions:
(1) A properly certificated and current pilot is at the controls and a trained refueler attending the fuel
nozzle during the entire fuel servicing process. The pilot monitors the fuel quantity and signals the refueler when
quantity is reached.
(2) No electrical storms (thunderstorms) are present within 10 nautical miles. Lightning can travel
great distances beyond the actual thunderstorm.
(3) Passengers disembark the helicopter and move to a safe location prior to HRR operations. When
the pilot−in−command deems it necessary for passenger safety that they remain onboard, passengers should be
briefed on the evacuation route to follow to clear the area.
(4) Passengers not board or disembark during HRR operations nor should cargo be loaded or unloaded.
(5) Only designated personnel, trained in HRR operations should conduct HRR written authorization
to include safe handling of the fuel and equipment. (See your Company Operations/Safety Manual for detailed
instructions.)
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AIM 2/20/25
(6) All doors, windows, and access points allowing entry to the interior of the helicopter that are
adjacent to or in the immediate vicinity of the fuel inlet ports kept closed during HRR operations.
(7) Pilots ensure that appropriate electrical/electronic equipment is placed in standby−off position, to
preclude the possibility of electrical discharge or other fire hazard, such as [i.e., weather radar is on standby and
no radio transmissions are made (keying of the microphone/transmitter)]. Remember, in addition to
communications radios, radio transmissions are also emitted by aircraft radar, transponders, ADS−B equipment,
radar altimeters, DME equipment, and ELTs.
(8) Smoking be prohibited in and around the helicopter during all HRR operations.
The HRR procedures are critical and present associated hazards requiring attention to detail regarding quality
control, weather conditions, static electricity, bonding, and spill/fires potential.
Any activity associated with rotors turning (i.e.; refueling embarking/disembarking, loading/unloading
baggage/freight; etc.) personnel should only approach the aircraft when authorized to do so. Approach should
be made via safe approach path/walkway or “arc”− remain clear of all rotors.
NOTE−
1. Marine vessels, barges etc.: Vessel motion presents additional potential hazards to helicopter operations (blade flex,
aircraft movement).
2. See National Fire Protection Association (NFP A) Document 407, “Standard for Aircraft Fuel Servicing” for
specifics regarding non−HRR (routine refueling operations).
10−2−2. Helicopter Night VFR Operations
a. Effect of Lighting on Seeing Conditions in Night VFR Helicopter Operations
NOTE−
This guidance was developed to support safe night VFR helicopter emergency medical services (HEMS) operations. The
principles of lighting and seeing conditions are useful in any night VFR operation.
While ceiling and visibility significantly affect safety in night VFR operations, lighting conditions also have a
profound effect on safety. Even in conditions in which visibility and ceiling are determined to be visual
meteorological conditions, the ability to discern unlighted or low contrast objects and terrain at night may be
compromised. The ability to discern these objects and terrain is the seeing condition, and is related to the amount
of natural and man made lighting available, and the contrast, reflectivity, and texture of surface terrain and
obstruction features. In order to conduct operations safely, seeing conditions must be accounted for in the
planning and execution of night VFR operations.
Night VFR seeing conditions can be described by identifying “high lighting conditions” and “low lighting
conditions.”
1. High lighting conditions exist when one of two sets of conditions are present:
(a) The sky cover is less than broken (less than 5/8 cloud cover), the time is between the local Moon rise
and Moon set, and the lunar disk is at least 50% illuminated; or
(b) The aircraft is operated over surface lighting which, at least, provides for the lighting of prominent
obstacles, the identification of terrain features (shorelines, valleys, hills, mountains, slopes) and a horizontal
reference by which the pilot may control the helicopter. For example, this surface lighting may be the result of:
(1) Extensive cultural lighting (man−made, such as a built−up area of a city),
(2) Significant reflected cultural lighting (such as the illumination caused by the reflection of a major
metropolitan area’s lighting reflecting off a cloud ceiling), or
(3) Limited cultural lighting combined with a high level of natural reflectivity of celestial illumination,
such as that provided by a surface covered by snow or a desert surface.
2. Low lighting conditions are those that do not meet the high lighting conditions requirements.
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3. Some areas may be considered a high lighting environment only in specific circumstances. For example,
some surfaces, such as a forest with limited cultural lighting, normally have little reflectivity, requiring
dependence on significant moonlight to achieve a high lighting condition. However, when that same forest is
covered with snow, its reflectivity may support a high lighting condition based only on starlight. Similarly, a
desolate area, with little cultural lighting, such as a desert, may have such inherent natural reflectivity that it may
be considered a high lighting conditions area regardless of season, provided the cloud cover does not prevent
starlight from being reflected from the surface. Other surfaces, such as areas of open water, may never have
enough reflectivity or cultural lighting to ever be characterized as a high lighting area.
4. Through the accumulation of night flying experience in a particular area, the operator will develop the
ability to determine, prior to departure, which areas can be considered supporting high or low lighting conditions.
Without that operational experience, low lighting considerations should be applied by operators for both
pre−flight planning and operations until high lighting conditions are observed or determined to be regularly
available.
b. Astronomical Definitions and Background Information for Night Operations
1. Definitions
(a) Horizon. Wherever one is located on or near the Earth’s surface, the Earth is perceived as essentially
flat and, therefore, as a plane. If there are no visual obstructions, the apparent intersection of the sky with the
Earth’s (plane) surface is the horizon, which appears as a circle centered at the observer. For rise/set
computations, the observer’s eye is considered to be on the surface of the Earth, so that the horizon is
geometrically exactly 90 degrees from the local vertical direction.
(b) Rise, Set. During the course of a day the Earth rotates once on its axis causing the phenomena of
rising and setting. All celestial bodies, the Sun, Moon, stars and planets, seem to appear in the sky at the horizon
to the East of any particular place, then to cross the sky and again disappear at the horizon to the West. Because
the Sun and Moon appear as circular disks and not as points of light, a definition of rise or set must be very
specific, because not all of either body is seen to rise or set at once.
(c) Sunrise and sunset refer to the times when the upper edge of the disk of the Sun is on the horizon,
considered unobstructed relative to the location of interest. Atmospheric conditions are assumed to be average,
and the location is in a level region on the Earth’s surface.
(d) Moonrise and moonset times are computed for exactly the same circumstances as for sunrise and
sunset. However, moonrise and moonset may occur at any time during a 24 hour period and, consequently, it is
often possible for the Moon to be seen during daylight, and to have moonless nights. It is also possible that a
moonrise or moonset does not occur relative to a specific place on a given date.
(e) Transit. The transit time of a celestial body refers to the instant that its center crosses an imaginary
line in the sky − the observer’s meridian − running from north to south.
(f) Twilight. Before sunrise and again after sunset there are intervals of time, known as “twilight,” during
which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects
part of it toward the Earth’s surface.
(g) Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun
is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under
good weather conditions, for terrestrial objects to be clearly distinguished.
2. Title 14 of the Code of Federal Regulations applies these concepts and definitions in addressing the
definition of night (section 1.1), the requirement for aircraft lighting (section 91.209) and pilot recency of night
experience (section 61.67).
c. Information on Moon Phases and Changes in the Percentage of the Moon Illuminated
From any location on the Earth, the Moon appears to be a circular disk which, at any specific time, is illuminated
to some degree by direct sunlight. During each lunar orbit (a lunar month), we see the Moon’s appearance change
Special Operations 10−2−9
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from not visibly illuminated through partially illuminated to fully illuminated, then back through partially
illuminated to not illuminated again. There are eight distinct, traditionally recognized stages, called phases. The
phases designate both the degree to which the Moon is illuminated and the geometric appearance of the
illuminated part. These phases of the Moon, in the sequence of their occurrence (starting from New Moon), are
listed in FIG 10−2−3.
FIG 10−2−3
Phases of the Moon
New Moon − The Moon’s unilluminated side is facing the Earth. The Moon is not visible
(except during a solar eclipse).
Waxing Crescent − The Moon appears to be partly but less than one−half illuminated by
direct sunlight. The fraction of the Moon’s disk that is illuminated is increasing.
First Quarter − One−half of the Moon appears to be illuminated by direct sunlight. The
fraction of the Moon’s disk that is illuminated is increasing.
Waxing Gibbous − The Moon appears to be more than one−half but not fully illuminated by
direct sunlight. The fraction of the Moon’s disk that is illuminated is increasing.
Full Moon − The Moon’s illuminated side is facing the Earth. The Moon appears to be
completely illuminated by direct sunlight.
Waning Gibbous − The Moon appears to be more than one −half but not fully
illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is
decreasing.
Last Quarter − One−half of the Moon appears to be illuminated by direct sunlight. The
fraction of the Moon’s disk that is illuminated is decreasing.
Waning Crescent − The Moon appears to be partly but less than one−half illuminated by
direct sunlight. The fraction of the Moon’s disk that is illuminated is decreasing.
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1. The percent of the Moon’s surface illuminated is a more refined, quantitative description of the Moon’s
appearance than is the phase. Considering the Moon as a circular disk, at New Moon the percent illuminated is
0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent
illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%.
2. For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of
the location on the Earth from where the Moon is observed. That is, all the phases occur at the same time
regardless of the observer’s position.
3. For more detailed information, refer to the United States Naval Observatory site referenced below.
d. Access to Astronomical Data for Determination of Moon Rise, Moon Set, and Percentage of Lunar
Disk Illuminated
1. Astronomical data for the determination of Moon rise and set and Moon phase may be obtained from the
United States Naval Observatory using an interactive query available at: http://aa.usno.navy.mil/
2. Click on “Data Services,” and then on “Complete Sun and Moon Data for One Day.”
3. You can obtain the times of sunrise, sunset, moonrise, moonset, transits of the Sun and Moon, and the
beginning and end of civil twilight, along with information on the Moon’s phase by specifying the date and
location in one of the two forms on this web page and clicking on the “Get data” button at the end of the form.
Form “A” is used for cities or towns in the U.S. or its territories. Form “B” for all other locations. An example
of the data available from this site is shown in TBL 10−2−2.
4. Additionally, a yearly table may be constructed for a particular location by using the “Table of
Sunrise/Sunset, Moonrise/Moonset, or Twilight Times for an Entire Year” selection.
TBL 10−2−2
Sample of Astronomical Data Available from the Naval Observatory
The following information is provided for New Orleans,
Orleans Parish, Louisiana
(longitude W90.1, latitude N30.0)
Tuesday
29 May 2007
Central Daylight Time
SUN
Begin civil twilight 5:34 a.m.
Sunrise 6:01 a.m.
Sun transit 12:58 p.m.
Sunset 7:55 p.m.
End civil twilight 8:22 p.m.
MOON
Moonrise 5:10 p.m. on preceding day
Moonset 4:07 a.m.
Moonrise 6:06 p.m.
Moon transit 11:26 p.m.
Moonset 4:41 a.m. on following day
Phase of the Moon on 29 May: waxing gibbous with 95% of the
Moon’s visible disk illuminated.
Full Moon on 31 May 2007 at 8:04 p.m. Central Daylight Time.
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10−2−3. Landing Zone Safety
a. This information is provided for use by helicopter emergency medical services (HEMS) pilots, program
managers, medical personnel, law enforcement, fire, and rescue personnel to further their understanding of the
safety issues concerning Landing Zones (LZs). It is recommended that HEMS operators establish working
relationships with the ground responder organizations they may come in contact with in their flight operations
and share this information in order to establish a common frame of reference for LZ selection, operations, and
safety.
b. The information provided is largely based on the booklet, LZ − Preparing the Landing Zone, issued by
National Emergency Medical Services Pilots Association (NEMSPA), and the guidance developed by the
University of Tennessee Medical Center’s LIFESTAR program, and is used with their permission. For additional
information, go to http://www.nemspa.org/.
c. Information concerning the estimation of wind velocity is based on the Beaufort Scale. See
http://www.spc.noaa.gov/faq/tornado/beaufort.html for more information.
d. Selecting a Scene LZ
1. If the situation requires the use of a helicopter, first check to see if there is an area large enough to land
a helicopter safely.
FIG 10−2−4
Recommended Minimum Landing Zone Dimensions
2. For the purposes of FIG 10−2−4 the following are provided as examples of relative helicopter size:
(a) Small Helicopter: Bell 20 6/407, Eurocopter AS−350/355, BO−105, BK−117.
(b) Medium Helicopter: Bell UH− 1 (Huey) and derivatives (Bell 212/412), Bell 222/230/430 Sikorsky
S−76, Eurocopter SA−365.
(c) Large Helicopter: Boeing Chinook, Eurocopter Puma, Sikorsky H−60 series (Blackhawk), SK−92.
3. The LZ should be level, firm and free of loose debris that could possibly blow up into the rotor system.
4. The LZ should be clear of people, vehicles and obstructions such as trees, poles and wires. Remember
that wires are difficult to see from the air. The LZ must also be free of stumps, brush, post and large rocks. See
FIG 10−2−5.
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FIG 10−2−5
Landing Zone Hazards
5. Keep spectators back at least 200 feet. Keep emergency vehicles 100 feet away and have fire equipment
(if available) standing by. Ground personnel should wear eye protection, if available, during landing and takeoff
operations. To avoid loose objects being blown around in the LZ, hats should be removed; if helmets are worn,
chin straps must be securely fastened.
6. Fire fighters (if available) should wet down the LZ if it is extremely dusty.
e. Helping the Flightcrew Locate the Scene
1. If the LZ coordinator has access to a GPS unit, the exact latitude and longitude of the LZ should be relayed
to the HEMS pilot. If unable to contact the pilot directly, relay the information to the HEMS ground
communications specialist for relaying to the pilot, so that they may locate your scene more efficiently.
Recognize that the aircraft may approach from a direction different than the direct path from the takeoff point
to the scene, as the pilot may have to detour around terrain, obstructions or weather en route.
2. Especially in daylight hours, mountainous and densely populated areas can make sighting a scene from
the air difficult. Often, the LZ coordinator on the ground will be asked if she or he can see or hear the helicopter.
3. Flightcrews use a clock reference method for directing one another’s attention to a certain direction from
the aircraft. The nose of the aircraft is always 12 o’clock, the right side is 3 o’clock, etc. When the LZ coordinator
sees the aircraft, he/she should use this method to assist the flightcrew by indicating the scene’s clock reference
position from the nose of the aircraft. For example, “Accident scene is located at your 2 o’clock position.” See
FIG 10−2−6.
FIG 10−2−6
“Clock” System for Identifying Positions Relative to the Nose of the Aircraft
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4. When the helicopter approaches the scene, it will normally orbit at least one time as the flight crew
observes the wind direction and obstacles that could interfere with the landing. This is often referred to as the
“high reconnaissance” maneuver.
f. Wind Direction and Touchdown Area
1. Determine from which direction the wind is blowing. Helicopters normally land and takeoff into the
wind.
2. If contact can be established with the pilot, either directly or indirectly through the HEMS ground
communications specialist, describe the wind in terms of the direction the wind is from and the speed.
3. Common natural sources of wind direction information are smoke, dust, vegetation movement, water
streaks and waves. Flags, pennants, streamers can also be used. When describing the direction, use the compass
direction from which the wind is blowing (example: from the North−West).
4. Wind speed can be measured by small hand−held measurement devices, or an observer’s estimate can
be used to provide velocity information. The wind value should be reported in knots (nautical miles per hour).
If unable to numerically measure wind speed, use TBL 10 −2−3 to estimate velocity. Also, report if the wind
conditions are gusty, or if the wind direction or velocity is variable or has changed recently.
5. If any obstacle(s) exist, ensure their description, position and approximate height are communicated to
the pilot on the initial radio call.
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TBL 10−2−3
Table of Common References for Estimating Wind Velocity
Wind
(Knots)
Wind
Classification
Appearance of Wind Effects
On the Water On Land
Less than 1 Calm Sea surface smooth and mirror−like Calm, smoke rises vertically
1−3 Light Air Scaly ripples, no foam crests Smoke drift indicates wind direction,
wind vanes are still
4−6 Light Breeze Small wavelets, crests glassy, no
breaking
Wind felt on face, leaves rustle, vanes
begin to move
7−10 Gentle Breeze Large wavelets, crests begin to break,
scattered whitecaps
Leaves and small twigs constantly
moving, light flags extended
11−16 Moderate Breeze Small waves 1−4 ft. becoming longer,
numerous whitecaps
Dust, leaves, and loose paper lifted,
small tree branches move
17−21 Fresh Breeze Moderate waves 4−8 ft. taking longer
form, many whitecaps, some spray
Small trees in leaf begin to sway
22−27 Strong Breeze Larger waves 8−13 ft., whitecaps
common, more spray
Larger tree branches moving, whistling
in wires
28−33 Near Gale Sea heaps up, waves 13−20 ft., white
foam streaks off breakers
Whole trees moving, resistance felt
walking against wind
34−40 Gale Moderately high (13−20 ft.) waves of
greater length, edges of crests begin to
break into spindrift, foam blown in
streaks
Whole trees in motion, resistance felt
walking against wind
41−47 Strong Gale High waves (20 ft.), sea begins to roll,
dense streaks of foam, spray may reduce
visibility
Slight structural damage occurs, slate
blows off roofs
48−55 Storm Very high waves (20−30 ft.) with
overhanging crests, sea white with
densely blown foam, heavy rolling,
lowered visibility
Seldom experienced on land, trees
broken or uprooted, “considerable
structural damage”
56−63 Violent Storm Exceptionally high (30−45 ft.) waves,
foam patches cover sea, visibility more
reduced
64+ Hurricane Air filled with foam, waves over 45 ft.,
sea completely white with driving spray,
visibility greatly reduced
EXAMPLE−
Wind from the South−East, estimated speed 15 knots. Wind shifted from North−East about fifteen minutes ago, and is gusty.
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g. Night LZs
1. There are several ways to light a night LZ:
(a) Mark the touchdown area with five lights or road flares, one in each corner and one indicating the
direction of the wind. See FIG 10−2−7.
FIG 10−2−7
Recommended Lighting for Landing Zone Operations at Night
NOTE−
Road flares are an intense source of ignition and may be unsuitable or dangerous in certain conditions. In any case, they
must be closely managed and firefighting equipment should be present when used. Other light sources are preferred, if
available.
(b) If chemical light sticks may be used, care should be taken to assure they are adequately secured against
being dislodged by the helicopter’s rotor wash.
(c) Another method of marking a LZ uses four emergency vehicles with their low beam headlights aimed
toward the intended landing area.
(d) A third method for marking a LZ uses two vehicles. Have the vehicles direct their headlight beams
into the wind, crossing at the center of the LZ. (If fire/rescue personnel are available, the reflective stripes on
their bunker gear will assist the pilot greatly.)
2. At night, spotlights, flood lights and hand lights used to define the LZ are not to be pointed at the
helicopter. However, they are helpful when pointed toward utility poles, trees or other hazards to the landing
aircraft. White lights such as spotlights, flashbulbs and hi −beam headlights ruin the pilot’s night vision and
temporarily blind him. Red lights, however, are very helpful in finding accident locations and do not affect the
pilot’s night vision as significantly.
3. As in Day LZ operations, ensure radio contact is accomplished between ground and air, if possible.
h. Ground Guide
1. When the helicopter is in sight, one person should assist the LZ Coordinator by guiding the helicopter
into a safe landing area. In selecting an LZ Coordinator, recognize that medical personnel usually are very busy
with the patient at this time. It is recommended that the LZ Coordinator be someone other than a medical
responder, if possible. Eye protection should be worn. The ground guide should stand with his/her back to the
wind and his/her arms raised over his/her head (flashlights in each hand for night operations.)
2. The pilot will confirm the LZ sighting by radio. If possible, once the pilot has identified the LZ, the
ground guide should move out of the LZ.
3. As the helicopter turns into the wind and begins a descent, the LZ coordinator should provide assistance
by means of radio contact, or utilize the “unsafe signal” to wave off the helicopter if the LZ is not safe (see
FIG 10−2−8). The LZ Coordinator should be far enough from the touchdown area that he/she can still maintain
visual contact with the pilot.
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i. Assisting the Crew
1. After the helicopter has landed, do not approach the helicopter. The crew will approach you.
2. Be prepared to assist the crew by providing security for the helicopter. If asked to provide security, allow
no one but the crew to approach the aircraft.
3. Once the patient is prepared and ready to load, allow the crew to open the doors to the helicopter and guide
the loading of the patient.
4. When approaching or departing the helicopter, always be aware of the tail rotor and always follow the
directions of the crew. Working around a running helicopter can be potentially dangerous. The environment is
very noisy and, with exhaust gases and rotor wash, often windy. In scene operations, the surface may be uneven,
soft, or slippery which can lead to tripping. Be very careful of your footing in this environment.
5. The tail rotor poses a special threat to working around a running helicopter. The tail rotor turns many
times faster than the main rotor, and is often invisible even at idle engine power. Avoid walking towards the tail
of a helicopter beyond the end of the cabin, unless specifically directed by the crew.
NOTE−
Helicopters typically have doors on the sides of the cabin, but many use aft mounted “clamshell” type doors for loading and
unloading patients on litters or stretchers. When using these doors, it is important to avoid moving any further aft than
necessary to operate the doors and load/unload the patient. Again, always comply with the crew’ s instructions.
j. General Rules
1. When working around helicopters, always approach and depart from the front, never from the rear.
Approaching from the rear can increase your risk of being struck by the tail rotor, which, when at operating engine
speed, is nearly invisible.
2. To prevent injury or damage from the main rotor, never raise anything over your head.
3. If the helicopter landed on a slope, approach and depart from the down slope side only.
4. When the helicopter is loaded and ready for take off, keep the departure path free of vehicles and
spectators. In an emergency, this area is needed to execute a landing.
k. Hazardous Chemicals and Gases
1. Responding to accidents involving hazardous materials requires special handling by fire/rescue units on
the ground. Equally important are the preparations and considerations for helicopter operations in these areas.
2. Hazardous materials of concern are those which are toxic, poisonous, flammable, explosive, irritating,
or radioactive in nature. Helicopter ambulance crews normally don’t carry protective suits or breathing
apparatuses to protect them from hazardous materials.
3. The helicopter ambulance crew must be told of hazardous materials on the scene in order to avoid the
contamination of the crew. Patients/victims contaminated by hazardous materials may require special
precautions in packaging before loading on the aircraft for the medical crew’s protection, or may be transported
by other means.
4. Hazardous chemicals and gases may be fatal to the unprotected person if inhaled or absorbed through
the skin.
5. Upon initial radio contact, the helicopter crew must be made aware of any hazardous gases in the area.
Never assume that the crew has already been informed. If the aircraft were to fly through the hazardous gases,
the crew could be poisoned and/or the engines could develop mechanical problems.
6. Poisonous or irritating gases may cling to a victim’s clothing and go unnoticed until the patient is loaded
and the doors of the helicopter are closed. To avoid possible compromise of the crew, all of these patients must
be decontaminated prior to loading.
Special Operations 10−2−17
AIM 2/20/25
l. Hand Signals
1. If unable to make radio contact with the HEMS pilot, use the following signals:
FIG 10−2−8
Recommended Landing Zone Ground Signals
m. Emergency Situations
1. In the event of a helicopter accident in the vicinity of the LZ, consider the following:
(a) Emergency Exits:
(1) Doors and emergency exits are typically prominently marked. If possible, operators should
familiarize ground responders with the door system on their helicopter in preparation for an emergency event.
(2) In the event of an accident during the LZ operation, be cautious of hazards such as sharp and jagged
metal, plastic windows, glass, any rotating components, such as the rotors, and fire sources, such as the fuel
tank(s) and the engine.
(b) Fire Suppression:
Helicopters used in HEMS operations are usually powered by turboshaft engines, which use jet fuel. Civil HEMS
aircraft typically carry between 50 and 250 gallons of fuel, depending upon the size of the helicopter, and planned
flight duration, and the fuel remaining after flying to the scene. Use water to control heat and use foam over fuel
to keep vapors from ignition sources.
10−2−4. Emergency Medical Service (EMS) Multiple Helicopter Operations
a. Background. EMS helicopter operators often overlap other EMS operator areas. Standardized procedures
can enhance the safety of operating multiple helicopters to landing zones (LZs) and to hospital heliports.
Communication is the key to successful operations and in maintaining organization between helicopters, ground
units and communication centers. EMS helicopter operators which operate in the same areas should establish
joint operating procedures and provide them to related agencies.
b. Recommended Procedures.
1. Landing Zone Operations. The first helicopter to arrive on −scene should establish communications
with the ground unit at least 10 NMs from the LZ to receive a LZ briefing and to provide ground control the
10−2−18 Special Operations
2/20/25 AIM
number of helicopters that can be expected. An attempt should be made to contact other helicopters on 123.025
to pass on to them pertinent LZ information and the ground unit’s frequency. Subsequent helicopters arriving
on scene should establish communications on 123.025 at least 10 NMs from the LZ. After establishing contact
on 123.025, they should contact the ground unit for additional information. All helicopters should monitor
123.025 at all times.
(a) If the landing zone is not established by the ground unit when the first helicopter arrives, then the first
helicopter should establish altitude and orbit location requirements for the other arriving helicopters.
Recommended altitude separation between helicopters is 500 feet (weather and airspace permitting). Helicopters
can orbit on cardinal headings from the scene coordinates. (See FIG 10−2−9.)
(b) Upon landing in the LZ, the first helicopter should update the other helicopters on the LZ conditions,
i.e., space, hazards and terrain.
(c) Before initiating any helicopter movement to leave the LZ, all operators should attempt to contact
other helicopters on 123.025, and state their position and route of flight intentions for departing the LZ.
2. Hospital Operations. Because many hospitals require landing permission and have established
procedures (frequencies to monitor, primary and secondary routes for approaches and departures, and orbiting
areas if the heliport is occupied) pilots should al ways receive a briefing from the appropriate facility
(communication center, flight following, etc.) before proceeding to the hospital.
(a) In the event of multiple helicopters coming into the hospital heliport, the helicopter nearest to the
heliport should contact other inbound helicopters on 123.025 and establish intentions. Follow the guidelines
established in the LZ operations.
(b) To facilitate approach times, the pilot−in−command of the helicopter occupying the hospital heliport
should advise any other operators whether the patient will be off loaded with the rotor blades turning or stopped,
and the approximate time to do so.
(c) Before making any helicopter movement to leave the hospital heliport, all operators should attempt
to contact other helicopters on 123.025 and state their position and route of flight intentions for departing the
heliport.
Special Operations 10−2−19