ATC weather deviation requests

ATC passes significant weather info and helps avoid weather areas, when asked (TC AIM MET 1.3.8). IFR flight must not deviate from assigned course or altitude without ATC clearance.

Request options (MET 1.3.9.2). In response to a weather advisory, request:

• Deviation off course: state estimated miles and direction
• New route avoiding the affected area
• Change of altitude
• Vectors around the affected area

Ask as early as possible: delay may delay or preclude approval, or add clearance restrictions.

Detour request: give ATC (MET 1.3.9.2):

• Proposed route and extent of detour (direction and distance)
• Flight conditions: IMC or VMC
• Whether cockpit weather radar is functioning

ATC limits:

• Primary function: safe separation between aircraft
• Weather avoidance: additional service, given only to the extent it does not detract from separation
• Weather raises controller separation workload
• Radar/surveillance limits and frequency congestion further limit ATC help

Trap: assuming a pilot may deviate around weather without clearance whenever prudent. Emergency deviation: see clearances-instructions.

Practice