Definition
The longitudinal distance, expressed in time or distance, maintained between an aircraft and another aircraft directly ahead of it on the same flight path or departure track. Used by ATC and pilots to ensure safe separation, particularly to avoid wake turbulence from a preceding aircraft during takeoff, approach, or while following traffic in sequence.
Plain English
How far you stay behind the aircraft in front of you when you're following the same path. It's measured in either distance or time, and it keeps you clear of their wake turbulence and gives you safe room to operate.
Context Anchor
Pilots encounter this during taxi, takeoff sequencing, airport traffic flow, and air traffic control instructions that keep aircraft separated along the same path.
Derivation
"In-trail" comes from the idea of one aircraft being in the trail (the path) left behind by another, like following in someone's footsteps. "Spacing" is the gap kept between them. Together it simply means "the gap behind the one ahead of you."
Why Pilots Care
Adequate spacing reduces wake turbulence risk and supports orderly, safe operations during the highest-risk phases of flight.
Analogy
It is like leaving enough following distance behind a car in the same lane. The issue is not how far away the car is sideways; it is how much room you have because you are following the same path.
Intuition Check
Do not read “spacing” here as general distance in any direction. In-trail spacing specifically means the gap between aircraft following one behind another along the same path.
Example Sentence 1
Tower instructed us to extend our downwind to maintain in-trail spacing behind the 737 on final.
Example Sentence 2
During pattern work the tower requested tighter in-trail spacing to keep the flow moving efficiently.