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AIM 

6/17/21 

(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. 

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13.  Runway Visual Range (RVR) 

There are currently two configurations of RVR in the 
NAS commonly identified as Taskers and New 
Generation RVR. The Taskers are the existing 
configuration which uses transmissometer 

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Meteorology