Definition
A flight path segment defined by a fixed-radius curve around a specified center fix, used in RNAV procedures to guide the aircraft along a precise circular ground track between two waypoints. In ARINC 424 path and terminator coding, it is identified as the RF (Radius-to-Fix) leg type and is the only curved RNP leg flown with the same accuracy and repeatability as a straight segment.
Plain English
A curved part of a flight path where the aircraft flies in a perfect arc at a set distance from a chosen point, ending at a specific waypoint. The curve is the same shape every time, so every aircraft flies it the same way.
Context Anchor
Seen in instrument procedure descriptions when a published route includes a smooth curved turn between two points instead of a sharp turn at a single point.
Derivation
‘Radius’ comes from the Latin word for the spoke of a wheel — the line from the center to the edge. A constant radius arc keeps that spoke length unchanged as the aircraft sweeps around the center, producing a true circular path.
Why Pilots Care
Maintains the aircraft inside protected airspace and on the exact path required by the procedure.
Analogy
Think of drawing part of a circle with a compass. The pencil makes a curved line while staying the same distance from the compass point; that curved line is an arc with a constant radius.
Grounding Statement
Picture the airplane following a smooth, fixed-size curve around a point on the map.
Intuition Check
Do not read “constant” as meaning constant heading or constant bank angle. Here it means the curve itself keeps the same radius from its center point.
Example Sentence 1
The approach included a constant radius arc around the mountain, allowing the aircraft to follow a curved path between the two waypoints without overflying high terrain.
Example Sentence 2
The pilot adjusted bank angle to hold the constant radius arc at the published speed.