Definition
An ARINC 424 path and terminator leg type in which the aircraft flies a specified track outbound from a defined fix and ends the leg upon reaching a specified DME distance from a referenced DME station. The leg is defined by three elements: a starting fix, a track (course) to fly, and a terminating DME range from a named DME facility.
Plain English
It is one of the standard 'leg types' used to code instrument procedures. The aircraft starts at a named point, flies a set heading or course outbound, and the leg ends when the aircraft is a specific distance from a particular DME station.
Context Anchor
Seen in RNAV and instrument procedure coding, especially when describing how a published procedure leg begins and what condition ends it.
Derivation
The two-letter code 'FD' follows the ARINC 424 convention where the first letter identifies the path origin (F = Fix) and the second letter identifies how the leg terminates (D = DME distance). Knowing this pattern makes the other leg types (CF, TF, VA, etc.) much easier to read.
Why Pilots Care
Different leg types behave differently in the FMS and on the autopilot. Recognizing an FD leg tells the pilot the leg will end based on a DME range, not a time, an altitude, or another fix, which affects when the next leg becomes active and where the turn occurs.
Grounding Statement
Picture crossing a named point, following the charted path, and watching the DME readout until it reaches the published distance that ends the segment.
Intuition Check
A fix is not a repair; it is a known position. Track is not the same as heading; track is the path over the ground. The leg ends at a DME distance, not necessarily at a visible point or named waypoint.
Example Sentence 1
After crossing the departure fix, the FMS sequenced the FD leg, which held the runway track outbound until the aircraft reached 8 DME from the airport.
Example Sentence 2
The flight director commanded a constant heading on the FD leg until the DME readout matched the published value.