Definition
A course deviation display that shows the aircraft's lateral distance from the desired track in actual units of distance (typically nautical miles or tenths of a nautical mile), rather than as an angular offset from a station. Each dot on the course deviation indicator (CDI) represents a fixed distance off track, regardless of how far along the route the aircraft is.
Plain English
It's a way of showing how far you are off course in real distance — like 'half a mile left of track' — instead of as an angle. The needle tells you the same distance-per-dot whether you're near the start of the leg or near the end.
Context Anchor
Seen in VOR/DME RNAV discussions and on course deviation displays used to follow an RNAV course.
Derivation
Linear means 'along a straight line, measured in distance.' Cross-track means 'across the intended track' — sideways from the route. Deviation means 'how far off you are.' Together: how far sideways, in distance, you are from the line you're supposed to be flying.
Why Pilots Care
It tells the pilot exactly how far off course the aircraft is in distance, allowing precise corrections to stay within required navigation performance limits.
Analogy
Think of walking beside a painted line on the ground. Linear cross-track deviation is not how much you are pointed away from the line; it is how many feet left or right of the line you are.
Intuition Check
Do not read “linear” as just meaning “straight ahead.” Here it means the display is based on actual sideways distance from the course, not an angle. Do not read “deviation” as a vague mistake. Here it is a measured left-or-right displacement from the intended path.
Example Sentence 1
On the RNAV approach, full-scale deflection represented one nautical mile of linear cross-track deviation, so a one-dot offset meant the aircraft was a fifth of a mile left of track.
Example Sentence 2
The pilot turned to intercept and reduce the linear cross-track deviation before reaching the next waypoint.