Definition
The speed at which two aircraft are approaching each other, calculated as the combined rate at which the distance between them is decreasing. When two aircraft fly directly toward one another, the closure rate is the sum of their individual airspeeds; when one is overtaking the other on the same track, it is the difference between their airspeeds.
Plain English
How fast you and another aircraft are coming together. If you're both flying at 120 knots straight at each other, the gap between you is closing at 240 knots.
Context Anchor
Used in collision avoidance when judging how much time a pilot has to see, decide, and avoid another aircraft.
Derivation
From 'close,' meaning to bring together or shut a gap. The 'rate' is simply how quickly that gap is shrinking. The phrase emphasizes that what matters in a traffic conflict isn't either aircraft's speed alone, but how fast the space between them is disappearing.
Why Pilots Care
High closure rates reduce the time available to see, decide, and maneuver, directly affecting collision avoidance decisions.
Analogy
If two cars drive toward each other on the same road, the distance between them closes faster than either car is moving by itself. Aircraft work the same way, but with much less time to react.
Intuition Check
Do not think of closure rate as only your airplane’s speed. It is the speed at which the distance between your airplane and another aircraft is shrinking.
Example Sentence 1
Two aircraft approaching head-on at 130 knots each have a closure rate of 260 knots, leaving only seconds to react once the traffic is spotted.
Example Sentence 2
During formation practice the instructor emphasized keeping closure rates low to maintain safe spacing.