Definition
An FD leg is an RNAV path-and-terminator leg type that defines a track beginning at a specified fix and ending when the aircraft reaches a defined DME (Distance Measuring Equipment) slant-range distance from a designated navigation aid. The leg has a published course, and the terminator is the moment the aircraft crosses that DME distance.
Plain English
A coded flight path that starts at a named point and ends when you are a set number of miles from a navigation station, measured by onboard distance equipment.
Context Anchor
Seen in instrument procedure design, navigation database coding, and FMS leg descriptions when a procedure segment ends at a DME distance instead of at another named point.
Derivation
In ARINC 424 path-terminator coding, the two-letter code names the path type and how it ends. 'F' marks a leg that starts at a Fix; 'D' marks a terminator at a DME Distance. So 'FD' literally reads as 'Fix to DME Distance.'
Why Pilots Care
It lets procedure designers end a leg at a precise distance rather than requiring a physical fix, supporting obstacle clearance and flexible routing.
Grounding Statement
Picture leaving a named point and flying the assigned line until a distance display from a ground station reaches the required number.
Intuition Check
Do not read track as “watching” something, and do not read DME distance as distance along the route. Here, track means the path over the ground, and the FD leg ends when the aircraft reaches the published distance from the referenced DME station.
Example Sentence 1
The departure procedure includes an FD leg from WILMA on a 090° track, ending at 15 DME from the field.
Example Sentence 2
The missed approach procedure uses an FD leg that ends when the aircraft is 8 miles from the reference facility.