Definition
A situation in which two aircraft come closer to each other than the minimum distance required by air traffic control rules, either horizontally or vertically. The required minimums depend on the airspace, type of flight (IFR or VFR), and the equipment in use. A loss of separation is a reportable event and triggers ATC and safety review.
Plain English
Two aircraft got closer to each other than the rules allow. They didn't necessarily collide or even come into sight of each other, but the safety buffer between them was broken.
Context Anchor
Seen in collision avoidance, traffic alert, and air traffic control discussions when aircraft are close enough that spacing has become a safety concern.
Derivation
‘Separation’ comes from the Latin separare, meaning ‘to set apart.’ In ATC, ‘separation’ is the protective distance kept between aircraft. ‘Loss of separation’ simply means that protective distance was lost.
Why Pilots Care
It signals an immediate risk of collision and may require prompt pilot action or controller intervention to restore safe spacing.
Intuition Check
Do not read separation here as just being apart. In aviation, separation means enough safe spacing between aircraft, not merely that they are not touching.
Example Sentence 1
ATC issued an immediate turn after detecting a loss of separation between the two IFR arrivals.
Example Sentence 2
Maintaining visual contact with traffic helps prevent a loss of separation during VFR approaches.