Definition
A discontinued Flight Service radio service, formerly available on 122.0 MHz below 18,000 feet, that provided pilots already in flight with real-time weather information tailored to their route, altitude, and aircraft type. EFAS, commonly called Flight Watch, was retired by the FAA in 2015, and its functions were absorbed into standard Flight Service frequencies.
Plain English
EFAS was a dedicated radio service pilots used in flight to get up-to-date weather along their route. It is no longer in operation, but the same kind of weather help is now provided through regular Flight Service.
Context Anchor
Seen in discussions of in-flight weather services, especially near HIWAS and other hazardous-weather advisory material.
Derivation
"En Route" means while flying along the route (as opposed to before takeoff). "Flight Advisory Service" describes its purpose: giving advice about weather to pilots already airborne.
Why Pilots Care
Gives pilots access to updated weather details in flight so they can adjust routing or altitude to avoid hazards such as thunderstorms or icing.
Intuition Check
Do not read EFAS as just another name for HIWAS. EFAS was a pilot-request weather advisory service; HIWAS was a broadcast of hazardous weather information.
Example Sentence 1
The instructor explained that EFAS used to be the go-to service for in-flight weather updates before it was retired in 2015.
Example Sentence 2
EFAS advised of a developing line of thunderstorms twenty miles ahead of the aircraft.