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AIM

10/12/17

7−1−37

Meteorology

(b) An approval for lateral deviation autho-

rizes the pilot to maneuver left or right within the

limits specified in the clearance.

NOTE−

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 com-

mence.

(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 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

Airspace.

1. When the pilot initiates communications with

ATC, rapid response may be obtained by stating

“WEATHER DEVIATION REQUIRED” to indicate