Absolute accuracy. The ability to determine present position
in space independently, and is most often used by pilots.
Absolute altitude. The actual distance between an aircraft
and the terrain over which it is flying.
Absolute pressure. Pressure measured from the reference
of zero pressure, or a vacuum.
A.C. Alternating current.
Acceleration error. A magnetic compass error apparent when
the aircraft accelerates while flying on an easterly or westerly
heading, causing the compass card to rotate toward North.
Accelerometer. A part of an inertial navigation system
(INS) that accurately measures the force of acceleration in
one direction.
ADF. See automatic direction finder.
ADI. See attitude director indicator.
ADM. See aeronautical decision-making.
ADS–B. See automatic dependent surveillance–broadcast.
Adverse yaw. A flight condition at the beginning of a turn in
which the nose of the aircraft starts to move in the direction
opposite the direction the turn is being made, caused by the
induced drag produced by the downward-deflected aileron
holding back the wing as it begins to rise.
Aeronautical decision-making (ADM). A systematic
approach to the mental process used by pilots to consistently
determine the best course of action in response to a given
set of circumstances.
A/FD. See Airport/Facility Directory.
Glossary
Agonic line. An irregular imaginary line across the surface of
the Earth along which the magnetic and geographic poles are in
alignment, and along which there is no magnetic variation.
Aircraft approach category. A performance grouping of
aircraft based on a speed of 1.3 times the stall speed in the
landing configuration at maximum gross landing weight.
Air data computer (ADC). An aircraft computer that
receives and processes pitot pressure, static pressure, and
temperature to calculate very precise altitude, indicated
airspeed, true airspeed, and air temperature.
AIRMET. Inflight weather advisory issued as an amendment
to the area forecast, concerning weather phenomena of
operational interest to all aircraft and that is potentially
hazardous to aircraft with limited capability due to lack of
equipment, instrumentation, or pilot qualifications.
Airport diagram. The section of an instrument approach
procedure chart that shows a detailed diagram of the
airport. This diagram includes surface features and airport
configuration information.
Airport/Facility Directory (A/FD). An FAA publication
containing information on all airports, communications,
and NAVAIDs.
Airport surface detection equipment (ASDE). Radar
equipment specifically designed to detect all principal
features and traffic on the surface of an airport, presenting the
entire image on the control tower console; used to augment
visual observation by tower personnel of aircraft and/or
vehicular movements on runways and taxiways.
Airport surveillance radar (ASR). Approach control
radar used to detect and display an aircraft’s position in the
terminal area.
Airport surveillance radar approach. An instrument
approach in which ATC issues instructions for pilot
compliance based on aircraft position in relation to the final
approach course and the distance from the end of the runway
as displayed on the controller’s radar scope.
Air route surveillance radar (ARSR). Air route traffic
control center (ARTCC) radar used primarily to detect
and display an aircraft’s position while en route between
terminal areas.
Air route traffic control center (ARTCC). Provides ATC
service to aircraft operating on IFR flight plans within
controlled airspace and principally during the en route phase
of flight.
Airspeed indicator. A differential pressure gauge that
measures the dynamic pressure of the air through which the
aircraft is flying. Displays the craft’s airspeed, typically in
knots, to the pilot.
Air traffic control radar beacon system (ATCRBS).
Sometimes called secondary surveillance radar (SSR), which
utilizes a transponder in the aircraft. The ground equipment is
an interrogating unit, in which the beacon antenna is mounted
so it rotates with the surveillance antenna. The interrogating
unit transmits a coded pulse sequence that actuates the aircraft
transponder. The transponder answers the coded sequence by
transmitting a preselected coded sequence back to the ground
equipment, providing a strong return signal and positive
aircraft identification, as well as other special data.
Airway. An airway is based on a centerline that extends from
one navigation aid or intersection to another navigation aid
(or through several navigation aids or intersections); used
to establish a known route for en route procedures between
terminal areas.
Alert area. An area in which there is a high volume of pilot
training or an unusual type of aeronautical activity.
Almanac data. Information the global positioning system
(GPS) receiver can obtain from one satellite which describes
the approximate orbital positioning of all satellites in the
constellation. This information is necessary for the GPS
receiver to know what satellites to look for in the sky at a
given time.
ALS. See approach lighting system.
Alternate airport. An airport designated in an IFR flight
plan, providing a suitable destination if a landing at the
intended airport becomes inadvisable.
Alternate static source valve. A valve in the instrument static
air system that supplies reference air pressure to the altimeter,
airspeed indicator, and vertical speed indicator if the normal
static pickup should become clogged or iced over.
Altimeter setting. Station pressure (the barometric pressure
at the location the reading is taken) which has been corrected
for the height of the station above sea level.
AME. See aviation medical examiner.
Amendment status. The circulation date and revision
number of an instrument approach procedure, printed above
the procedure identification.
Ammeter. An instrument installed in series with an electrical
load used to measure the amount of current flowing through
the load.
Aneroid. The sensitive component in an altimeter or
barometer that measures the absolute pressure of the air.
It is a sealed, flat capsule made of thin disks of corrugated
metal soldered together and evacuated by pumping all of
the air out of it.
Aneroid barometer. An instrument that measures the
absolute pressure of the atmosphere by balancing the weight
of the air above it against the spring action of the aneroid.
Angle of attack. The acute angle formed between the
chord line of an airfoil and the direction of the air striking
the airfoil.
Anti-ice. Preventing the accumulation of ice on an aircraft
structure via a system designed for that purpose.
Approach lighting system (ALS). Provides lights that will
penetrate the atmosphere far enough from touchdown to give
directional, distance, and glide path information for safe
transition from instrument to visual flight.
Area chart. Part of the low-altitude en route chart series,
this chart furnishes terminal data at a larger scale for
congested areas.
Area navigation (RNAV). Allows a pilot to fly a selected
course to a predetermined point without the need to overfly
ground-based navigation facilities, by using waypoints.
ARSR. See air route surveillance radar.
ARTCC. See air route traffic control center.
ASDE. See airport surface detection equipment.
ASOS. See automated surface observing station.
ASR. See airport surveillance radar.
ATC. Air Traffic Control.
ATCRBS. See air traffic control radar beacon system.
ATIS. See automatic terminal information service.
Atmospheric propagation delay. A bending of the
electromagnetic (EM) wave from the satellite that creates
an error in the GPS system.
Attitude and heading reference systems (AHRS). System
composed of three-axis sensors that provide heading, attitude,
and yaw information for aircraft. AHRS are designed to
replace traditional mechanical gyroscopic flight instruments
and provide superior reliability and accuracy.
Attitude director indicator (ADI). An aircraft attitude
indicator that incorporates flight command bars to provide
pitch and roll commands.
Attitude indicator. The foundation for all instrument flight,
this instrument reflects the airplane’s attitude in relation to
the horizon.
Attitude instrument flying. Controlling the aircraft by
reference to the instruments rather than by outside visual
cues.
Autokinesis. Nighttime visual illusion that a stationary light
is moving, which becomes apparent after several seconds of
staring at the light.
Automated Weather Observing System (AWOS).
Automated weather reporting system consisting of various
sensors, a processor, a computer-generated voice subsystem,
and a transmitter to broadcast weather data.
Automated Surface Observing Station (ASOS). Weather
reporting system which provides surface observations every
minute via digitized voice broadcasts and printed reports.
Automatic dependent surveillance–broadcast (ADS-B). A
device used in aircraft that repeatedly broadcasts a message
that includes position (such as latitude, longitude, and
altitude), velocity, and possibly other information.
Automatic direction finder (ADF). Electronic navigation
equipment that operates in the low- and medium-frequency
bands. Used in conjunction with the ground-based
nondirectional beacon (NDB), the instrument displays the
number of degrees clockwise from the nose of the aircraft
to the station being received.
Automatic terminal information service (ATIS). The
continuous broadcast of recorded non-control information in
selected terminal areas. Its purpose is to improve controller
effectiveness and relieve frequency congestion by automating
repetitive transmission of essential but routine information.
Aviation medical examiner (AME). A physician with
training in aviation medicine designated by the Civil
Aerospace Medical Institute (CAMI).
AWOS. See automated weather observing system.
Azimuth card. A card that may be set, gyroscopically
controlled, or driven by a remote compass.
Back course (BC). The reciprocal of the localizer course
for an ILS. When flying a back-course approach, an aircraft
approaches the instrument runway from the end at which the
localizer antennas are installed.
Baro-aiding. A method of augmenting the GPS integrity
solution by using a non-satellite input source. To ensure that
baro-aiding is available, the current altimeter setting must
be entered as described in the operating manual.
Barometric scale. A scale on the dial of an altimeter to which
the pilot sets the barometric pressure level from which the
altitude shown by the pointers is measured.
BC. See back course.
Block altitude. A block of altitudes assigned by ATC to
allow altitude deviations; for example, “Maintain block
altitude 9 to 11 thousand.”
Cage. The black markings on the ball instrument indicating
its neutral position.
Calibrated. The instrument indication compared with a
standard value to determine the accuracy of the instrument.
Calibrated orifice. A hole of specific diameter used to delay
the pressure change in the case of a vertical speed indicator.
Calibrated airspeed. The speed at which the aircraft
is moving through the air, found by correcting IAS for
instrument and position errors.
CAS. Calibrated airspeed.
CDI. Course deviation indicator.
Changeover point (COP). A point along the route or
airway segment between two adjacent navigation facilities
or waypoints where changeover in navigation guidance
should occur.
Circling approach. A maneuver initiated by the pilot to
align the aircraft with a runway for landing when a straight-
in landing from an instrument approach is not possible or is
not desirable.
Class A airspace. Airspace from 18,000 feet MSL up to and
including FL 600, including the airspace overlying the waters
within 12 NM of the coast of the 48 contiguous states and
Alaska; and designated international airspace beyond 12 NM
of the coast of the 48 contiguous states and Alaska within areas
of domestic radio navigational signal or ATC radar coverage,
and within which domestic procedures are applied.
Class B airspace. Airspace from the surface to 10,000 feet
MSL surrounding the nation’s busiest airports in terms of
IFR operations or passenger numbers. The configuration of
each Class B airspace is individually tailored and consists
of a surface area and two or more layers, and is designed to
contain all published instrument procedures once an aircraft
enters the airspace. For all aircraft, an ATC clearance is
required to operate in the area, and aircraft so cleared receive
separation services within the airspace.
Class C airspace. Airspace from the surface to 4,000 feet
above the airport elevation (charted in MSL) surrounding
those airports having an operational control tower, serviced
by radar approach control, and having a certain number of IFR
operations or passenger numbers. Although the configuration
of each Class C airspace area is individually tailored, the
airspace usually consists of a 5 NM radius core surface area
that extends from the surface up to 4,000 feet above the airport
elevation, and a 10 NM radius shelf area that extends from
1,200 feet to 4,000 feet above the airport elevation.
Class D airspace. Airspace from the surface to 2,500 feet
above the airport elevation (charted in MSL) surrounding
those airports that have an operational control tower. The
configuration of each Class D airspace area is individually
tailored, and when instrument procedures are published, the
airspace is normally designed to contain the procedures.
Class E airspace. Airspace that is not Class A, Class B, Class
C, or Class D, and is controlled airspace.
Class G airspace. Airspace that is uncontrolled, except
when associated with a temporary control tower, and has
not been designated as Class A, Class B, Class C, Class D,
or Class E airspace.
Clean configuration. A configuration in which all flight
control surfaces have been placed to create minimum drag.
In most aircraft this means flaps and gear retracted.
Clearance. ATC permission for an aircraft to proceed under
specified traffic conditions within controlled airspace, for the
purpose of providing separation between known aircraft.
Clearance delivery. Control tower position responsible for
transmitting departure clearances to IFR flights.
Clearance limit. The fix, point, or location to which an
aircraft is cleared when issued an air traffic clearance.
Clearance on request. An IFR clearance not yet received
after filing a flight plan.
Clearance void time. Used by ATC, the time at which the
departure clearance is automatically canceled if takeoff has
not been made. The pilot must obtain a new clearance or
cancel the IFR flight plan if not off by the specified time.
Clear ice. Glossy, clear, or translucent ice formed by the
relatively slow freezing of large, supercooled water droplets.
Compass course. A true course corrected for variation and
deviation errors.
Compass locator. A low-power, low- or medium-frequency
(L/MF) radio beacon installed at the site of the outer or middle
marker of an ILS.
Compass rose. A small circle graduated in 360° increments,
printed on navigational charts to show the amount of
compass variation at different locations, or on instruments
to indicate direction.
Computer navigation fix. A point used to define a
navigation track for an airborne computer system such as
GPS or FMS.
Concentric rings. Dashed-line circles depicted in the plan
view of IAP charts, outside of the reference circle, that show
en route and feeder facilities.
Cone of confusion. A cone-shaped volume of airspace
directly above a VOR station where no signal is received,
causing the CDI to fluctuate.
Control and performance. A method of attitude instrument
flying in which one instrument is used for making attitude
changes, and the other instruments are used to monitor the
progress of the change.
Control display unit. A display interfaced with the master
computer, providing the pilot with a single control point
for all navigations systems, thereby reducing the number of
required flight deck panels.
Controlled airspace. An airspace of defined dimensions
within which ATC service is provided to IFR and VFR flights
in accordance with the airspace classification. It includes
Class A, Class B, Class C, Class D, and Class E airspace.
Control pressures. The amount of physical exertion on the
control column necessary to achieve the desired attitude.
Convective weather. Unstable, rising air found in
cumiliform clouds.
Convective SIGMET. Weather advisory concerning
convective weather significant to the safety of all aircraft,
including thunderstorms, hail, and tornadoes.
Coordinated flight. Flight with a minimum disturbance of
the forces maintaining equilibrium, established via effective
control use.
COP. See changeover point.
Coriolis illusion. The illusion of rotation or movement in an
entirely different axis, caused by an abrupt head movement,
while in a prolonged constant rate turn that has ceased
stimulating the brain’s motion sensing system.
Crew resource management (CRM). The effective
use of all available resources—human, hardware, and
information.
Critical areas. Areas where disturbances to the ILS localizer
and glide slope courses may occur when surface vehicles or
aircraft operate near the localizer or glide slope antennas.
CRM. See crew resource management.
Cross-check. The first fundamental skill of instrument flight,
also known as “scan,” the continuous and logical observation
of instruments for attitude and performance information.
Cruise clearance. An ATC clearance issued to allow a
pilot to conduct flight at any altitude from the minimum
IFR altitude up to and including the altitude specified in the
clearance. Also authorizes a pilot to proceed to and make an
approach at the destination airport.
Current induction. An electrical current being induced into,
or generated in, any conductor that is crossed by lines of flux
from any magnet.
DA. See decision altitude.
D.C. Direct current.
Dark adaptation. Physical and chemical adjustments of the
eye that make vision possible in relative darkness.
Deceleration error. A magnetic compass error that occurs
when the aircraft decelerates while flying on an easterly
or westerly heading, causing the compass card to rotate
toward South.
Decision altitude (DA). A specified altitude in the precision
approach, charted in feet MSL, at which a missed approach
must be initiated if the required visual reference to continue
the approach has not been established.
Decision height (DH). A specified altitude in the precision
approach, charted in height above threshold elevation,
at which a decision must be made either to continue the
approach or to execute a missed approach.
Deice. The act of removing ice accumulation from an
aircraft structure.
Density altitude. Pressure altitude corrected for nonstandard
temperature. Density altitude is used in computing the
performance of an aircraft and its engines.
Departure procedure (DP). Preplanned IFR ATC departure,
published for pilot use, in textual and graphic format.
Deviation. A magnetic compass error caused by local
magnetic fields within the aircraft. Deviation error is different
on each heading.
DGPS. Differential global positioning system.
DH. See decision height.
Differential Global Positioning System (DGPS). A system
that improves the accuracy of Global Navigation Satellite
Systems (GNSS) by measuring changes in variables to
provide satellite positioning corrections.
Direct indication. The true and instantaneous reflection of
aircraft pitch-and-bank attitude by the miniature aircraft,
relative to the horizon bar of the attitude indicator.
Direct User Access Terminal System (DUATS). A system
that provides current FAA weather and flight plan filing
services to certified civil pilots, via personal computer,
modem, or telephone access to the system. Pilots can request
specific types of weather briefings and other pertinent data
for planned flights.
Distance circle. See reference circle.
Distance measuring equipment (DME). A pulse-type
electronic navigation system that shows the pilot, by an
instrument-panel indication, the number of nautical miles
between the aircraft and a ground station or waypoint.
DME. See distance measuring equipment.
DME arc. A flight track that is a constant distance from the
station or waypoint.
DOD. Department of Defense.
Doghouse. A turn-and-slip indicator dial mark in the shape
of a doghouse.
Domestic Reduced Vertical Separation Minimum
(DRVSM). Additional flight levels between FL 290 and FL
410 to provide operational, traffic, and airspace efficiency.
Double gimbal. A type of mount used for the gyro in an
attitude instrument. The axes of the two gimbals are at right
angles to the spin axis of the gyro, allowing free motion in
two planes around the gyro.
DP. See departure procedure.
Drag. The net aerodynamic force parallel to the relative
wind, usually the sum of two components: induced drag
and parasite drag.
Drag curve. The curve created when plotting induced drag
and parasite drag.
DUATS. See direct user access terminal system.
Duplex. Transmitting on one frequency and receiving on a
separate frequency.
Eddy currents. Current induced in a metal cup or disc when
it is crossed by lines of flux from a moving magnet.
EFAS. See En Route Flight Advisory Service.
EFC. See expect-further-clearance.
Electronic flight display (EFD). For the purpose of
standardization, any flight instrument display that uses
LCD or other image-producing system (Cathode Ray Tube
[CRT], etc.)
Elevator illusion. The sensation of being in a climb or
descent, caused by the kind of abrupt vertical accelerations
that result from up- or downdrafts.
Emergency. A distress or urgent condition.
Emphasis error. The result of giving too much attention
to a particular instrument during the cross-check, instead of
relying on a combination of instruments necessary for attitude
and performance information.
EM wave. Electromagnetic wave.
Encoding altimeter. A special type of pressure altimeter
used to send a signal to the air traffic controller on the ground,
showing the pressure altitude the aircraft is flying.
En route facilities ring. Depicted in the plan view of IAP
charts, a circle which designates NAVAIDs, fixes, and
intersections that are part of the en route low altitude airway
structure.
En Route Flight Advisory Service (EFAS). An en route
weather-only AFSS service.
En route high-altitude charts. Aeronautical charts for en
route instrument navigation at or above 18,000 feet MSL.
En route low-altitude charts. Aeronautical charts for en
route IFR navigation below 18,000 feet MSL.
Equivalent airspeed. Airspeed equivalent to CAS in
standard atmosphere at sea level. As the airspeed and pressure
altitude increase, the CAS becomes higher than it should be,
and a correction for compression must be subtracted from
the CAS.
Expect-further-clearance (EFC). The time a pilot can
expect to receive clearance beyond a clearance limit.
FAA. Federal Aviation Administration.
FAF. See final approach fix.
False horizon. Inaccurate visual information for aligning the
aircraft, caused by various natural and geometric formations
that disorient the pilot from the actual horizon.
Federal airways. Class E airspace areas that extend upward
from 1,200 feet to, but not including, 18,000 feet MSL, unless
otherwise specified.
Feeder facilities. Used by ATC to direct aircraft to
intervening fixes between the en route structure and the
initial approach fix.
Final approach. Part of an instrument approach
procedure in which alignment and descent for landing are
accomplished.
Final approach fix (FAF). The fix from which the IFR
final approach to an airport is executed, and which identifies
the beginning of the final approach segment. An FAF is
designated on government charts by a Maltese cross symbol
for nonprecision approaches, and a lightning bolt symbol for
precision approaches.
Fixating. Staring at a single instrument, thereby interrupting
the cross-check process.
FL. See flight level.
Flight configurations. Adjusting the aircraft control surfaces
(including flaps and landing gear) in a manner that will
achieve a specified attitude.
Flight director indicator (FDI). One of the major components
of a flight director system, it provides steering commands that
the pilot (or the autopilot, if coupled) follows.
Flight level (FL). A measure of altitude (in hundreds of feet)
used by aircraft flying above 18,000 feet with the altimeter
set at 29.92" Hg.
Flight management system (FMS). Provides pilot and crew
with highly accurate and automatic long-range navigation
capability, blending available inputs from long- and short-
range sensors.
Flightpath. The line, course, or track along which an aircraft
is flying or is intended to be flown.
Flight patterns. Basic maneuvers, flown by reference to the
instruments rather than outside visual cues, for the purpose
of practicing basic attitude flying. The patterns simulate
maneuvers encountered on instrument flights such as holding
patterns, procedure turns, and approaches.
Flight strips. Paper strips containing instrument flight
information, used by ATC when processing flight plans.
FMS. See flight management system.
Form drag. The drag created because of the shape of a
component or the aircraft.
Fundamental skills. Pilot skills of instrument cross-check,
instrument interpretation, and aircraft control.
Glideslope (GS). Part of the ILS that projects a radio beam
upward at an angle of approximately 3° from the approach
end of an instrument runway. The glideslope provides
vertical guidance to aircraft on the final approach course for
the aircraft to follow when making an ILS approach along
the localizer path.
Glideslope intercept altitude. The minimum altitude of an
intermediate approach segment prescribed for a precision
approach that ensures obstacle clearance.
Global landing system (GLS). An instrument approach with
lateral and vertical guidance with integrity limits (similar to
barometric vertical navigation (BRO VNAV).
Global navigation satellite systems (GNSS). Satellite
navigation systems that provide autonomous geo-spatial
positioning with global coverage. It allows small electronic
receivers to determine their location (longitude, latitude, and
altitude) to within a few meters using time signals transmitted
along a line of sight by radio from satellites.
GNSS. See global navigation satellite systems.
Global positioning system (GPS). Navigation system
that uses satellite rather than ground-based transmitters for
location information.