Definition
A distance measured along the programmed flight path of an RNAV or GPS procedure, from the aircraft's present position to a specified waypoint, or from one waypoint to another along the route. ATD is computed by the navigation system and follows the curved or straight path of the route rather than a straight line through the air.
Plain English
How far it is to a point measured along the route you are actually flying, not in a straight line through the air.
Context Anchor
Seen in instrument procedure, navigation, and flight management discussions where distance is measured along the route being flown.
Derivation
Along-track' simply means 'following the track' — the path the aircraft is programmed to fly. The phrase makes clear the distance is measured along that path, not as the crow flies.
Why Pilots Care
It supports accurate timing, descent planning, and position awareness on published procedures.
Intuition Check
Do not read along-track distance as the shortest straight-line distance between two places. It means distance measured along the route or path you are actually expected to fly.
Example Sentence 1
The chart depicts a step-down fix at 4 NM ATD from the final approach fix.
Example Sentence 2
Pilots monitor ATD to start the descent at the correct point on the approach.