Definition
A spacing requirement, expressed in minutes, that air traffic control applies between successive aircraft along the same route or fix. It is used as a traffic management tool to meter the flow of aircraft into a sector, airport, or airspace fix.
Plain English
A rule that says aircraft following each other on the same path must be separated by a set number of minutes. If the spacing is five minutes-in-trail, each aircraft must pass a given point at least five minutes after the one ahead.
Context Anchor
Seen in air traffic control traffic management, especially when weather, congestion, or limited airport capacity requires controllers to slow the flow of aircraft into an area.
Derivation
"In trail" comes from older usage meaning "following behind in a line," the same sense as a trail of footprints. Combined with "minutes," it simply means "following behind by this many minutes."
Why Pilots Care
It maintains safe spacing and smooth traffic flow when distance-based separation is impractical.
Grounding Statement
If a 5-minute Minutes-in-Trail requirement is in effect, the next aircraft should pass the named point at least 5 minutes after the aircraft ahead.
Intuition Check
Do not read “trail” as a path on the ground; here it means one aircraft following another in the traffic stream. The “minutes” are the required time gap between aircraft, not the total delay for the flight.
Example Sentence 1
Center advised the crew to expect 10 minutes-in-trail into the arrival fix due to weather rerouting.
Example Sentence 2
Due to congestion, the flow manager increased the minutes-in-trail spacing to fifteen minutes.