Definition
An RNAV path-terminator leg that begins at a defined fix and continues on a specified course or track until the pilot manually terminates it, typically in response to ATC instructions such as a heading assignment, vector, or 'fly runway heading' clearance after departure.
Plain English
A flight path that starts at a known point and keeps going in a set direction until the pilot ends it, usually because ATC tells them to turn or do something else.
Context Anchor
Seen in instrument procedure coding, especially in discussions of path-and-terminator legs used by GPS and flight management systems.
Derivation
In ARINC 424 path-terminator coding, every leg type is named by two letters: the first letter is the path (how you fly it), and the second letter is the terminator (what ends it). 'F' means the path starts from a fix; 'M' means the leg ends at a manual termination. So 'FM' literally reads as 'from a fix, ending when the pilot manually ends it.'
Why Pilots Care
FM legs are common on RNAV departures off the end of a runway. The procedure database can only take you so far -- at some point ATC has to give you a heading or vector. Knowing the leg is FM tells you to expect that intervention rather than waiting for the aircraft to turn on its own.
Analogy
It is like directions that say, “From the bridge, keep going on this road until I tell you to turn.” The bridge is the starting point, but the turn is not tied to another named landmark.
Intuition Check
Manual termination does not mean the procedure is optional or that the airplane must be hand-flown. It means the navigation system will not automatically end that leg; the pilot must take the next action when appropriate.
Example Sentence 1
After crossing the departure fix, the FMS displayed an FM leg, so the crew expected ATC to issue a vector before the aircraft would turn on course.
Example Sentence 2
The procedure specified an FM leg from the initial fix so the pilot could maintain the published course until the runway environment was in sight.