Definition
A course displacement expressed as a fixed distance from the desired track, typically in nautical miles, regardless of how far the aircraft is from the navigation reference. In RNAV systems, the course deviation indicator (CDI) shows lateral error as a constant distance per dot rather than as an angle.
Plain English
Your sideways error from the planned path is shown as a set distance off track — for example, one nautical mile per dot — and that distance stays the same whether you are near or far from the station.
Context Anchor
Seen in VOR/DME RNAV discussions, where the navigation equipment creates a course to or from a selected point and shows how far the aircraft is from that course.
Derivation
Linear comes from the Latin linea, meaning a line or measured length. Here it signals that the deviation is measured as a straight-line distance, not as an angle that widens with range.
Why Pilots Care
It provides constant distance-based guidance that does not grow larger with distance from the station, allowing precise track-keeping during RNAV legs.
Grounding Statement
Picture the desired course as a line drawn on a map; linear deviation is the sideways gap between the aircraft and that line.
Intuition Check
Do not read deviation here as a vague “error” or “difference.” In this context, it means a measured left-or-right distance from the selected course line.
Example Sentence 1
On this RNAV leg the CDI is set to linear deviation, so each dot represents one nautical mile off course no matter how far we are from the next waypoint.
Example Sentence 2
The pilot corrected to center the needle after noting the linear deviation had grown during the turn.