Definition
A curved segment of an instrument flight route that follows a fixed-radius arc around a navigation aid (typically a VOR or DME station), used to transition between two straight route segments while maintaining a constant distance from the station. The arc is flown by referencing DME distance to keep the aircraft on the prescribed radius.
Plain English
A curved part of a flight route that wraps around a navigation station at a set distance, smoothly connecting two straight legs of the route.
Context Anchor
Seen in instrument procedure and route-design discussions when a published path curves smoothly into another segment.
Derivation
From Latin tangere, meaning 'to touch.' A tangent line touches a circle at one point without crossing it. A tangential arc is the curved portion between two such touching lines — the route 'touches' the arc at each end and follows the curve between them.
Why Pilots Care
Flying a tangential arc requires holding a constant DME distance from the station while continuously adjusting heading. Misreading it as a straight segment leads to course deviation and possible terrain or airspace conflicts.
Grounding Statement
Picture a curved road ramp merging smoothly onto a straight highway; the ramp touches the highway and lines up with it as you join.
Intuition Check
Tangential does not mean “barely related” here. It means the curved path touches another path and lines up with it at the point where they meet.
Example Sentence 1
The arrival procedure included a tangential arc at 15 DME from the VOR, joining the inbound radial to the final approach course.
Example Sentence 2
Procedure designers use a tangential arc to join the DME arc to the inbound leg without an abrupt turn.