Definition
A specialized flight service that provides en route aircraft with timely weather information tailored to the pilot's type of flight, route, and altitude. Historically reached on the common frequency 122.0 MHz under the call sign 'Flight Watch,' EFAS was designed specifically to support in-flight weather decision-making and to gather pilot reports (PIREPs) from aircraft already aloft. (Note: The FAA discontinued the dedicated Flight Watch service in 2015; en route weather advisories are now handled by regular Flight Service on standard FSS frequencies. The term still appears in the Instrument Flying Handbook and on the knowledge test.)
Plain English
A radio service pilots could call while flying to get weather updates that actually mattered for where they were and where they were headed, instead of generic area forecasts.
Context Anchor
Seen in older FAA instrument training material when discussing Flight Service Stations and the weather help available to pilots while en route.
Derivation
En route' is French for 'on the way' — meaning the service was for aircraft already in flight, not on the ground. 'Advisory' signals that the information is to help you decide, not a clearance or instruction you must follow.
Why Pilots Care
Gives pilots real-time weather information needed to avoid hazards and adjust routing during flight.
Intuition Check
Do not read “advisory” as “clearance.” EFAS could help a pilot understand weather and conditions ahead, but it did not give legal permission to fly a route or control the flight.
Example Sentence 1
Halfway through the cross-country, the pilot contacted Flight Service for an en route weather advisory and learned that thunderstorms had developed along the planned route.
Example Sentence 2
EFAS reported moderate turbulence ahead and suggested an altitude change.