Definition
The distance value displayed by an Along-Track Distance function on an IFR-certified GPS or FMS, showing how far the aircraft is from a specific waypoint along its programmed flight path. In holding pattern operations, the ATD readout is used to determine when to begin the outbound turn so the inbound leg meets the required timing or distance.
Plain English
A number on the GPS that tells you how far you are from a chosen point, measured along the path you are flying. Pilots watch this number to know when to turn in a holding pattern.
Context Anchor
Seen in instrument procedure and holding-pattern discussions when distance along a route or holding course is used to identify where the aircraft is in relation to a fix or turn point.
Derivation
ATD stands for Along-Track Distance. "Along-track" means measured along the line of flight, not in a straight line through the air. "Readout" simply means the value shown on the instrument display. Together: the distance-along-the-route number you read off the GPS.
Why Pilots Care
Allows pilots to accurately determine their position in a holding pattern using distance instead of elapsed time.
Intuition Check
Do not read ATD as direct distance across open space. ATD is distance along the selected path shown by the navigation system.
Example Sentence 1
Approaching the holding fix, the pilot watched the ATD readout and began the outbound turn as it reached one mile past the fix.
Example Sentence 2
Pilots use the ATD readout to time the outbound leg of the hold precisely.