AIM

4/20/23

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Meteorology

acknowledging the advisory or by acknowledging the advisory and requesting an alternative course of action as
follows:

(a)

Request to deviate off course by stating

 

a heading or degrees, direction of deviation, and approximate

number of miles. In this case, when the requested deviation is approved, navigation is at the pilot’s prerogative,
but must maintain the altitude assigned, and remain within the lateral restrictions issued by ATC.

(b)

An approval for lateral deviation authorizes the pilot to maneuver left or right within the lateral limits

specified in the clearance.

NOTE

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1.

It is often necessary for ATC to restrict the amount of lateral deviation (“twenty degrees right,” “up to fifteen degrees

left,” “up to ten degrees left or right of course”).

2.

The term “when able, proceed direct,” in an ATC weather deviation clearance, refers to the pilot’s ability to remain clear

of the weather when returning to course/route.

(c)

Request a new route to avoid the affected area.

(d)

Request a change of altitude.

(e)

Request radar vectors around the affected areas.

2.

For obvious reasons of safety, an IFR pilot must not deviate from the course or altitude or flight level

without a proper ATC clearance. When weather conditions encountered are so severe that an immediate deviation
is determined to be necessary and time will not permit approval by ATC, the pilot’s emergency authority may
be exercised.

3.

When the pilot requests clearance for a route deviation or for an ATC radar vector, the controller must

evaluate the air traffic picture in the affected area, and coordinate with other controllers (if ATC jurisdictional
boundaries may be crossed) before replying to the request.

4.

It should be remembered that the controller’s primary function is to provide safe separation between

aircraft. Any additional service, such as weather avoidance assistance, can only be provided to the extent that
it does not derogate the primary function. It’s also worth noting that the separation workload is generally greater
than normal when weather disrupts the usual flow of traffic. ATC radar limitations and frequency congestion may
also be a factor in limiting the controller’s capability to provide additional service.

5.

It is very important, therefore, that the request for deviation or radar vector be forwarded to ATC as far

in advance as possible. Delay in submitting it may delay or even preclude ATC approval or require that additional
restrictions be placed on the clearance. Insofar as possible the following information should be furnished to ATC
when requesting clearance to detour around weather activity:

(a)

Proposed point where detour will commence.

(b)

Proposed route and extent of detour (direction and distance).

(c)

Point where original route will be resumed.

(d)

Flight conditions (IFR or VFR).

(e)

Any further deviation that may become necessary as the flight progresses.

(f)

Advise if the aircraft is equipped with functioning airborne radar.

6.

To a large degree, the assistance that might be rendered by ATC will depend upon the weather information

available to controllers. Due to the extremely transitory nature of severe weather situations, the controller’s
weather information may be of only limited value if based on weather observed on radar only. Frequent updates
by pilots giving specific information as to the area affected, altitudes, intensity and nature of the severe weather
can be of considerable value. Such reports are relayed by radio or phone to other pilots and controllers and also
receive widespread teletypewriter dissemination.

7.

Obtaining IFR clearance or an ATC radar vector to circumnavigate severe weather can often be

accommodated more readily in the en route areas away from terminals because there is usually less congestion