Definition
A curved flight path defined by a fixed radius from a center point, terminating at a named fix. The aircraft flies a precise circular arc of a specified radius, beginning at one waypoint and ending at another, with the center of the arc and the radius value published as part of the procedure. RF legs are flown by RNAV-equipped aircraft with the required navigation accuracy and authorization.
Plain English
A curved segment of a flight path that follows a perfect arc of a set distance from a center point, starting at one waypoint and ending at another. Instead of flying straight from point to point, the aircraft tracks a smooth curve of known size.
Context Anchor
Seen on RNAV instrument procedures, especially where the published path uses a precise curved turn between procedure points.
Derivation
RF stands for 'Radius to Fix.' 'Radius' comes from Latin meaning 'spoke of a wheel' — the straight line from the center of a circle to its edge. That image fits exactly: the aircraft flies along the rim of a circle while the radius (the spoke) holds a constant length to the center.
Why Pilots Care
It permits tight, repeatable curved paths that keep the aircraft inside protected airspace or clear of terrain where a straight leg would not work.
Analogy
Think of walking around a fountain while holding a rope tied to its center. If the rope length stays the same, your path is a constant radius arc.
Grounding Statement
Picture a string tied to a post in the middle of a field. You walk holding the string tight — your path traces a perfect curve at a fixed distance from the post. That curve, ending at a marked spot, is an RF leg.
Intuition Check
Constant radius does not mean constant speed, constant bank angle, or a normal hand-flown turn. It means the path stays the same distance from a defined center point all the way around the curve.
Example Sentence 1
The approach uses an RF leg to curve the aircraft around the ridge before lining up with the runway.
Example Sentence 2
The autopilot remained in LNAV and tracked the constant-radius arc without deviation until reaching the final approach fix.