Definition
The perpendicular distance between an aircraft's actual position and the desired course line between two waypoints, expressed in nautical miles left or right of course.
Plain English
How far off to the side of your intended track you actually are. If the line you wanted to fly goes straight from A to B, cross-track error is how far sideways you've drifted from that line.
Context Anchor
Seen on navigation displays during area navigation, especially when following a planned route or an instrument procedure.
Derivation
Cross' here means 'across' or 'sideways from,' and 'track' is the path over the ground the aircraft is meant to follow. So cross-track error is literally the sideways error from the intended track.
Why Pilots Care
Maintaining zero cross-track error keeps the aircraft on the correct flight path, which is essential for obstacle clearance, airspace compliance, and safe arrival at waypoints under instrument conditions.
Grounding Statement
If the planned path is a line on the map, cross-track error is the sideways gap between the airplane and that line.
Intuition Check
Do not read error as an equipment failure here. It means the measured amount the airplane is off the desired path.
Example Sentence 1
A light tailwind shift pushed them slightly south, and the GPS showed a cross-track error of 0.3 nautical miles right of course.
Example Sentence 2
The pilot adjusted heading to reduce the cross-track error and rejoin the RNAV route.