Figure 9-10. A portion of the New York area Tower En Route List. Figure 2-10. A portion of the New York area tower en route list (from the A/FD).
Figure 2-11. Center radar displays.
Figure 2-12. A center controller’s scope.
Figure 2-13. A/FD center frequencies listing.
A common clearance in these situations is “When able,
proceed direct to the Astoria VOR…” The words “when able”
mean to proceed to the waypoint, intersection, or NAVAID
when the pilot is able to navigate directly to that point using
onboard available systems providing proper guidance, usable
signal, etc. If provided such guidance while flying VFR, the
pilot remains responsible for terrain and obstacle clearance.
Using the standard climb gradient, an aircraft is 2 miles
from the departure end of the runway before it is safe to
turn (400 feet above ground level (AGL)). When a Center
controller issues a heading, a direct route, or says “direct
when able,” the controller becomes responsible for terrain
and obstruction clearance.
Another common Center clearance is “Leaving (altitude)
fly (heading) or proceed direct when able.” This keeps the
terrain/obstruction clearance responsibility in the flight deck
until above the minimum IFR altitude. A controller cannot
issue an IFR clearance until an aircraft is above the minimum
IFR altitude unless it is able to climb in VFR conditions.
On a Center controller’s scope, 1 NM is about 1⁄28 of an inch.
When a Center controller is providing Approach/Departure
control services at an airport many miles from the radar
antenna, estimating headings and distances is very difficult.
Controllers providing vectors to final must set the range on
their scopes to not more than 125 NM to provide the greatest
possible accuracy for intercept headings. Accordingly, at
locations more distant from a Center radar antenna, pilots
should expect a minimum of vectoring.
ATC radar systems cannot detect turbulence. Generally,
turbulence can be expected to occur as the rate of rainfall or
intensity of precipitation increases. Turbulence associated
with greater rates of rainfall/precipitation is normally more
severe than any associated with lesser rates of rainfall/
precipitation. Turbulence should be expected to occur near
convective activity, even in clear air. Thunderstorms are a
form of convective activity that implies severe or greater
turbulence. Operation within 20 miles of thunderstorms
should be approached with great caution, as the severity of
turbulence can be markedly greater than the precipitation
intensity might indicate.
Weather Avoidance Assistance
ATC’s first duty priority is to separate aircraft and issue
safety alerts. ATC provides additional services to the extent
possible, contingent upon higher priority duties and other
factors including limitations of radar, volume of traffic,
frequency congestion, and workload. Subject to the above
factors/limitations, controllers issue pertinent information
on weather or chaff areas; and if requested, assist pilots, to
the extent possible, in avoiding areas of precipitation. Pilots
should respond to a weather advisory by acknowledging the
advisory and, if desired, requesting an alternate course of
action, such as:
1. Request to deviate off course by stating the direction
and number of degrees or miles needed to deviate from
the original course;
2. Request a change of altitude; or
3. Request routing assistance to avoid the affected
area. Because ATC radar systems cannot detect the
presence or absence of clouds and turbulence, such
assistance conveys no guarantee that the pilot will not
encounter hazards associated with convective activity.
Pilots wishing to circumnavigate precipitation areas
by a specific distance should make their desires
clearly known to ATC at the time of the request for
services. Pilots must advise ATC when they can
resume normal navigation.
IFR pilots shall not deviate from their assigned course or
altitude without an ATC clearance. Plan ahead for possible
course deviations because hazardous convective conditions
can develop quite rapidly. This is important to consider
because the precipitation data displayed on ARTCC radar
scopes can be up to 6 minutes old, and thunderstorms can
develop at rates exceeding 6,000 feet per minute (fpm). When
encountering weather conditions that threaten the safety of
the aircraft, the pilot may exercise emergency authority as
ATC Inflight Weather Avoidance
Assistance
ATC Radar Weather Displays
ATC radar systems are able to display areas of precipitation
by sending out a beam of radio energy that is reflected back to
the radar antenna when it strikes an object or moisture, which
may be in the form of rain drops, hail, or snow. The larger
the object, or the denser its reflective surface, the stronger
the return. Radar weather processors indicate the intensity
of reflective returns in terms of decibels with respect to the
radar reflectively factor (dBZ).
ATC systems cannot detect the presence or absence of
clouds. ATC radar systems can often determine the intensity
of a precipitation area, but the specific character of that area
(snow, rain, hail, VIRGA, etc.) cannot be determined. For
this reason, ATC refers to all weather areas displayed on
ATC radar scopes as “precipitation.”
All ATC facilities using radar weather processors with the
ability to determine precipitation intensity describes the
intensity to pilots as:
1. “LIGHT” (< 30 dBZ)
2. “MODERATE” (30 to 40 dBZ)
3. “HEAVY” (>40 to 50 dBZ)
4. “EXTREME” (>50 dBZ)
ARTCC controllers do not use the term “LIGHT” because
their systems do not display “LIGHT” precipitation
intensities. ATC facilities that, due to equipment limitations,
cannot display the intensity levels of precipitation, describe
the location of the precipitation area by geographic position or
position relative to the aircraft. Since the intensity level is not
available, the controller states, “INTENSITY UNKNOWN.”
ARTCC facilities normally use a Weather and Radar
Processor (WARP) to display a mosaic of data obtained from
multiple NEXRAD sites. The WARP processor is only used
in ARTCC facilities.
There is a time delay between actual conditions and those
displayed to the controller. For example, the precipitation
data on the ARTCC controller’s display could be up to 6
minutes old. When the WARP is not available, a secondary
system, the narrowband ARSR is utilized. The ARSR system
can display two distinct levels of precipitation intensity that
is described to pilots as “MODERATE” (30 to 40 dBZ) and
“HEAVY to EXTREME” (>40 dBZ).
Figure 2-14. High-resolution ATC displays used in PRM.
stated in 14 CFR part 91, section 91.3 should an immediate
deviation from the assigned clearance be necessary and time
does not permit approval by ATC.
Generally, when weather disrupts the flow of air traffic,
greater workload demands are placed on the controller.
Requests for deviations from course and other services
should be made as far in advance as possible to better
assure the controller’s ability to approve these requests
promptly. When requesting approval to detour around
weather activity, include the following information to
facilitate the request:
1. The proposed point where detour commences;
2. The proposed route and extent of detour (direction
and distance);
3. The point where original route will be resumed;
4. Flight conditions (instrument meteorological
conditions (IMC) or visual meteorological conditions
(VMC);
5. Whether the aircraft is equipped with functioning
airborne radar; and
6. Any further deviation that may become necessary.
To a large degree, the assistance that might be rendered
by ATC depends upon the weather information available
to controllers. Due to the extremely transitory nature of
hazardous weather, the controller’s displayed precipitation
information may be of limited value.
Obtaining IFR clearance or approval to circumnavigate
hazardous weather can often be accommodated more readily
in the en route areas away from terminals because there
is usually less congestion and, therefore, greater freedom
of action. In terminal areas, the problem is more acute
because of traffic density, ATC coordination requirements,
complex departure and arrival routes, and adjacent airports.
As a consequence, controllers are less likely to be able to
accommodate all requests for weather detours in a terminal
area. Nevertheless, pilots should not hesitate to advise
controllers of any observed hazardous weather and should
specifically advise controllers if they desire circumnavigation
of observed weather.
Pilot reports (PIREPs) of flight conditions help define the
nature and extent of weather conditions in a particular area.
These reports are disseminated by radio and electronic means
to other pilots. Provide PIREP information to ATC regarding
pertinent flight conditions, such as:
1. Turbulence;
2. Visibility;
3. Cloud tops and bases; and
4. The presence of hazards such as ice, hail, and lightning.
Approach Control Facility
An approach control facility is a terminal ATC facility
that provides approach control service in the terminal area.
Services are provided for arriving and departing VFR and
IFR aircraft and, on occasion, en route aircraft. In addition,
for airports with parallel runways with ILS or LDA
approaches, the approach control facility provides monitoring
of the approaches.
Approach Control Advances
Precision Runway Monitor (PRM)
Over the past few years, a new technology has been installed
at airports that permits a decreased separation distance
between parallel runways. The system is called a Precision
Runway Monitor (PRM) and is comprised of high-update
radar, high-resolution ATC displays, and PRM-certified
controllers. [Figure 2-14]
PRM Radar
The PRM uses a Monopulse Secondary Surveillance Radar
(MSSR) that employs electronically-scanned antennas.
Because the PRM has no scan rate restrictions, it is capable
of providing a faster update rate (up to 1.0 second) over
conventional systems, thereby providing better target
presentation in terms of accuracy, resolution, and track
prediction. The system is designed to search, track, process,
and display SSR-equipped aircraft within airspace of over
30 miles in range and over 15,000 feet in elevation. Visual
and audible alerts are generated to warn controllers to take
corrective actions.
26L
8R
Intersection Alpha
NO TRANSGRESSION ZONE (NTZ)
NO TRANSGRESSION ZONE (NTZ)
26R
8L
Figure 9-XX. ATC displays used in PRM.
Intersection Beta
Figure 2-15. Aircraft management using PRM. (Note the no transgression zone (NTZ) and how the aircraft are separated.)
PRM Benefits
Typically, PRM is used with dual approaches with
centerlines separated less than 4,300 feet but not less
than 3,000 feet (under most conditions). [Figure 2-15]
Separating the two final approach courses is a No
Transgression Zone (NTZ) with surveillance of that zone
provided by two controllers, one for each active approach.
The system tracking software provides PRM monitor
controllers with aircraft identification, position, speed,
projected position, as well as visual and aural alerts.
Control Sequence
The IFR system is flexible and accommodating if pilots
do their homework, have as many frequencies as possible
written down before they are needed, and have an alternate
in mind if the flight cannot be completed as planned.
Pilots should familiarize themselves with all the facilities
and services available along the planned route of flight.
[Figure 2-16] Always know where the nearest VFR
conditions can be found, and be prepared to head in that
direction if the situation deteriorates.
A typical IFR flight, with departure and arrival at airports with
control towers, would use the ATC facilities and services in
the following sequence:
1. FSS: Obtain a weather briefing for a departure,
destination and alternate airports, and en route
conditions, and then file a flight plan by calling
1-800-WX-BRIEF.
2. ATIS: Preflight complete, listen for present conditions
and the approach in use.
3. Clearance Delivery: Prior to taxiing, obtain a
departure clearance.
Figure 2-16. ATC facilities, services, and radio call signs.
4. Ground Control: Noting that the flight is IFR, receive
taxi instructions.
5. Tower: Pre-takeoff checks complete, receive clearance
to takeoff.
6. Departure Control: Once the transponder “tags up”
with the ARTS, the tower controller instructs the pilot
to contact Departure to establish radar contact.
7. ARTCC: After departing the departure controller’s
airspace, aircraft is handed off to Center, who
coordinates the flight while en route. Pilots may
be in contact with multiple ARTCC facilities; they
coordinate the hand-offs.
8. EFAS/ Hazardous Inflight Weather Advisory Service
(HIWAS): Coordinate with ATC before leaving their
frequency to obtain inflight weather information.
9. ATIS: Coordinate with ATC before leaving their
frequency to obtain ATIS information.
10. Approach Control: Center hands off to approach
control where pilots receive additional information
and clearances.
11. Tower: Once cleared for the approach, pilots are
instructed to contact tower control; the flight plan is
canceled by the tower controller upon landing.
A typical IFR flight, with departure and arrival at airports
without operating control towers, would use the ATC
facilities and services in the following sequence:
1. FSS: Obtain a weather briefing for departure,
destination, and alternate airports, and en route
conditions, and then file a flight plan by calling
1-800-WX-BRIEF. Provide the latitude/longitude
description for small airports to ensure that Center is
able to locate departure and arrival locations.
2. FSS or UNICOM: ATC clearances can be filed and
received on the UNICOM frequency if the licensee
has made arrangements with the controlling ARTCC;
otherwise, file with FSS via telephone. Be sure all
preflight preparations are complete before filing. The
clearance includes a clearance void time. Pilots must
be airborne prior to the void time.
3. ARTCC: After takeoff, establish contact with Center.
During the flight, pilots may be in contact with multiple
ARTCC facilities; ATC coordinates the hand-offs.
4. EFAS/HIWAS: Coordinate with ATC before leaving
their frequency to obtain inflight weather information.
5. Approach Control: Center hands off to approach
control where pilots receive additional information
and clearances. If a landing under VMC is possible,
pilots may cancel their IFR clearance before landing.
Letters of Agreement (LOA)
The ATC system is indeed a system and very little happens by
chance. As a flight progresses, controllers in adjoining sectors
or adjoining Centers coordinate its handling by telephone
or by computer. Where there is a boundary between the
airspace controlled by different facilities, the location and
altitude for hand-off is determined by Letters of Agreement
(LOA) negotiated between the two facility managers. This
information is not available to pilots in any Federal Aviation
Administration (FAA) publication. For this reason, it is good
practice to note on the en route chart the points at which
hand-offs occur. Each time a flight is handed off to a different
facility, the controller knows the altitude and location—this
was part of the hand-off procedure.