Definition
A path and terminator (path-terminator) leg type used in RNAV and RNP procedure coding that defines a direct flight path from the aircraft's present position to a specified fix. The DF leg has no defined starting point — it begins wherever the aircraft happens to be when the leg becomes active — and ends at the named fix. The flight management system computes the track required to reach that fix from the current position.
Plain English
A coded instruction in a published procedure that tells the aircraft's navigation system: 'from wherever you are right now, fly straight to this point.' The starting place isn't fixed — only the destination point is.
Context Anchor
Seen in GPS and area-navigation instrument procedures, especially in the way a panel navigator sequences the legs of an approach, departure, or arrival.
Derivation
From 'Direct to a Fix.' The two-letter code DF comes from the ARINC 424 navigation database standard, where each leg type has a two-letter identifier. 'D' stands for Direct, 'F' for Fix. Knowing this helps decode related leg types like TF (Track to Fix) and CF (Course to Fix).
Why Pilots Care
It creates a precise straight-line segment the autopilot or GPS can follow without ambiguity.
Grounding Statement
The key idea is: current position first, named fix second, with the path drawn directly between them.
Intuition Check
Do not assume a DF leg always follows a pre-drawn course from the previous fix. It means the aircraft goes directly from its present position to the specified fix.
Example Sentence 1
After the radar vector ended, the FMS sequenced the DF leg and turned the aircraft directly toward the initial approach fix.
Example Sentence 2
On the approach, the aircraft joined the DF leg and tracked straight to the initial approach fix.