Definition
In RNAV (area navigation) operations, the allowable distance error measured along the intended flight path between the aircraft's computed position and a designated waypoint or fix. It defines how far ahead of or behind the published point the aircraft's navigation system may indicate position while still meeting the required navigation accuracy standard.
Plain English
How much the aircraft's position can be off — measured forward or backward along its route — and still be considered accurate enough for navigation.
Context Anchor
Seen in area navigation, GPS navigation, instrument procedure design, and discussions of waypoint or fix accuracy.
Derivation
‘Along-track’ describes a measurement taken in the direction of the flight path, as opposed to ‘cross-track’ which measures sideways from it. ‘Tolerance’ here means the permitted margin of error — borrowed from engineering, where it refers to how much a measurement may deviate from the target value while still being acceptable.
Why Pilots Care
Determines whether a waypoint is considered flown, which affects sequencing to the next leg or initiation of a missed approach.
Analogy
Think of driving toward a mile marker on a road. If you are a little before or a little past that marker, that is an along-the-road error, not a side-of-the-road error.
Intuition Check
Do not read “along-track” as meaning the width of the route. Along-track tolerance is forward or backward along the path; side-to-side error is a different kind of error.
Example Sentence 1
The procedure required an along-track tolerance of plus or minus 0.3 nautical miles, which the GPS easily met.
Example Sentence 2
Strong headwinds pushed the arrival time outside the along-track tolerance, requiring a speed adjustment.