Definition
A distance measurement, displayed by an area navigation (RNAV) or flight management system, showing how far the aircraft is from a specific waypoint measured along the programmed flight path rather than in a straight line.
Plain English
It is the distance to a waypoint measured along the route you are actually flying, not as the crow flies.
Context Anchor
Seen in navigation and route discussions, especially when distance is being measured along the planned ground path rather than off to the side of it.
Derivation
Along track' simply means 'following the course line,' and 'distance' is the measured length. Together the phrase emphasises that the measurement is taken along the path of flight rather than direct point-to-point.
Why Pilots Care
Allows accurate time, fuel, and speed planning on instrument routes and arrival procedures.
Intuition Check
Do not read ATD as just any distance between two points. It means distance measured along the route path, not distance measured sideways from the path.
Example Sentence 1
The chart showed a stepdown fix at 4.0 ATD from the final approach waypoint, so the pilot began the descent once the GPS indicated that distance.
Example Sentence 2
We reduced speed to cross the fix at the required along track distance.