Definition
A path terminator used in RNAV procedures that defines a flight path from the aircraft's present position directly to a specified fix. The starting point is wherever the aircraft happens to be when the leg becomes active; the ending point is the named fix. The course is not fixed in advance — it is computed by the flight management system based on the aircraft's current location.
Plain English
A leg in an RNAV procedure that tells the aircraft to fly straight from wherever it is right now to a named point ahead.
Context Anchor
Seen in GPS-based instrument procedures and flight plan leg descriptions, especially when the panel navigator is showing how it will guide the aircraft to the next named point.
Derivation
Direct' comes from the Latin directus, meaning 'straight' or 'made straight.' A 'fix' in aviation is a defined geographic point. So a DF leg is literally a 'straight line to a defined point' — with the starting end left open until the aircraft gets there.
Why Pilots Care
Allows clean transitions onto an approach or missed approach without requiring a specific course from the last fix.
Intuition Check
Do not read “fix” as a repair. In this term, a fix is a named navigation point, and “direct” means the system is guiding from the leg’s starting position straight toward that point.
Example Sentence 1
After the initial climb, the SID begins with a DF leg to WYNNE, so the aircraft will turn from its current position straight toward the fix.
Example Sentence 2
On the missed approach the DF leg takes the aircraft straight to the holding fix.