Definition
A specified distance, expressed in nautical miles, between aircraft on the same route, the same final approach course, or the same arrival/departure fix, used by air traffic control to manage traffic flow.
Plain English
A required gap, measured in miles, that controllers keep between one aircraft and the next when they're following the same path.
Context Anchor
You may see or hear Miles-in-Trail during traffic management for busy routes, arrival flows, or airports with more traffic than they can handle at once.
Derivation
Plain English phrase. 'In trail' means following one behind another, like cars on a single road. 'Miles-in-trail' simply states how many miles apart those following aircraft must be.
Why Pilots Care
Prevents congestion, maintains safe longitudinal spacing, and keeps arrivals moving efficiently into busy airports.
Intuition Check
“Trail” does not mean a smoke trail or a mark left behind. Here it means one aircraft following another in the same flow of traffic, with the gap measured in miles.
Example Sentence 1
Center advised us to expect 20 miles-in-trail behind the traffic ahead due to arrival delays at the destination.
Example Sentence 2
The pilot maintained 15 miles-in-trail behind the preceding heavy jet as instructed.