Definition
A failure of the landing gear to support the aircraft's weight, in which the gear folds, breaks, or fails to remain in the down-and-locked position during landing or ground operations, causing the aircraft to settle onto the runway surface.
Plain English
The wheels and their support structure give way when the aircraft is on the ground, so the airplane drops down onto the runway instead of rolling on its wheels.
Context Anchor
Seen in accident discussions, emergency scenarios, hard-landing reports, and training about abnormal landings.
Derivation
Collapse comes from the Latin collabi, meaning to fall together or fall in. The image is of a structure giving way under load — exactly what happens when the gear leg can no longer hold the aircraft up.
Why Pilots Care
It increases the risk of aircraft damage, loss of directional control, propeller strike, or fire on touchdown.
Grounding Statement
Picture an aircraft settling lower than it should because one or more of its wheel supports has given way.
Intuition Check
Collapsed does not mean the same thing as retracted. Retracted gear is pulled up on purpose; collapsed landing gear has failed or folded when it should be holding the aircraft up.
Example Sentence 1
The pilot reported a collapsed landing gear after the right main strut failed during the rollout.
Example Sentence 2
The student practiced the checklist for a collapsed landing gear scenario during the lesson.