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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.
9-11
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 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 radar scopes as “precipitation.”
All ATC facilities using radar weather processors with the
ability to determine precipitation intensity describes the
intensity to pilots as:
　
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