Aeronautical Information Manual (AIM), page 479

Index   478 -- Page 479 -- 480


(b) An approval for lateral deviation author-
izes the pilot to maneuver left or right within the
limits specified in the clearance.
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

(c) Request a new route to avoid the affected

(d) Request a change of altitude.

(e) Request radar vectors around the affected


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

(b) Proposed route and extent of detour

(direction and distance).

(c) Point where original route will be
(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 and, therefore, offer greater freedom of
action. In terminal areas, the problem is more acute
because of traffic density, ATC coordination
requirements, complex departure and arrival routes,
adjacent airports, etc. As a consequence, controllers
are less likely to be able to accommodate all requests
for weather detours in a terminal area or be in a
position to volunteer such routing to the pilot.
Nevertheless, pilots should not hesitate to advise
controllers of any observed severe weather and
should specifically advise controllers if they desire
circumnavigation of observed weather.

c. Procedures for Weather Deviations and
Other Contingencies in Oceanic Controlled
1. When the pilot initiates communications with
ATC, rapid response may be obtained by stating

Meteorology 7-1-35

Page 479 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   478 -- Page 479 -- 480