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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, will
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).
9-12
Figure 9-14. 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 (IMC or 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 9-14]
Precision Runway Monitor (PRM) Radar
The PRM uses a Monopulse Secondary Surveillance Radar
(MSSR) that employs electronically scanned antennas.
　
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