Definition
The horizontal (left-right) displacement of the aircraft from the centerline of the intended course, as measured and displayed by navigation equipment such as a CDI, HSI, or FMS-driven flight display.
Plain English
How far the aircraft is sitting to the left or right of the line you are supposed to be flying along.
Context Anchor
Seen when using an FMS, GPS, or course display to follow a route, an instrument procedure, or a selected navigation path.
Derivation
Lateral' comes from the Latin 'latus' meaning 'side.' So lateral course deviation literally means 'sideways straying from the course.' This helps distinguish it from vertical deviation, which is up-or-down displacement from a glidepath.
Why Pilots Care
Large or uncorrected deviations can cause loss of required navigation performance, missed approach criteria, or airspace violations.
Analogy
It is like driving in a lane and noticing that the car has drifted left or right of the center of the lane. The issue is not speed or height; it is side-to-side position compared with the intended path.
Intuition Check
Do not read “deviation” as automatic failure; here it means a measured offset from the intended course. “Lateral” means side-to-side, not high or low.
Example Sentence 1
As the autopilot captured the final approach course, the lateral course deviation centered and the needle stopped drifting.
Example Sentence 2
During the RNAV arrival the crew kept lateral course deviation under 0.2 nautical miles to meet the required navigation performance.