Definition
A path and terminator leg type used in RNAV/RNP procedure coding that defines a curved flight path along a constant-radius arc around a designated navigation reference (typically a DME station or waypoint), terminating at a specified fix. The arc is defined by its center, radius, and the terminating fix.
Plain English
A curved segment of a published procedure where the aircraft flies along a circle of fixed distance around a reference point until it reaches a named fix that ends the segment.
Context Anchor
Seen in RNAV and instrument procedure descriptions, especially when reviewing how a published approach or departure path is built in the navigation system.
Derivation
In the ARINC 424 leg-type coding used by avionics databases, each leg type has a two-letter identifier. 'AF' stands for 'Arc to a Fix' — 'A' for arc, 'F' for the fix that terminates the leg. Knowing the code helps pilots interpret how the FMS will fly each segment of a procedure.
Why Pilots Care
It enables precise curved routing that aligns the aircraft with the final approach course while respecting terrain or airspace constraints.
Analogy
It is like driving around part of a traffic circle and exiting at a specific road. You are not just turning generally; you are following a defined curve to a defined exit point.
Grounding Statement
Picture flying a constant 15 NM from a VOR until you reach a named waypoint — your track on the map is a smooth curve, not a straight line, and it ends precisely at that fix.
Intuition Check
Do not read fix as repair. In this term, a fix is a named point used for navigation. Do not read arc as any loose curve. Here, it means a defined curved path with a specific shape.
Example Sentence 1
The transition from the en route structure to the final approach course is coded as an AF leg, so the FMS flies the 12 NM arc around the VOR until reaching the initial approach fix.
Example Sentence 2
After the initial fix, the AF leg maintained a 4-nautical-mile radius until reaching the intermediate fix.