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Instrument Flying Handbook
The Air Traffic Control System
ATC In-Flight Weather Avoidance Assistance

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Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

ATC In-Flight 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 raindrops, hail, or snow. The larger
the object, or the denser its reflective surface, the stronger the
return will be. 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 radarscopes 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, 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

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).

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 clean 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
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.