Definition
An uncommanded, continuous movement of an airplane's trim system to a position not selected by the pilot, typically caused by a malfunction in the electric trim circuit. The trim continues to drive in one direction (nose-up or nose-down) without pilot input, producing increasing control forces that the pilot must counter on the flight controls.
Plain English
The airplane's trim is moving on its own and won't stop. It keeps driving toward nose-up or nose-down, and the pilot has to physically push or pull against the controls to keep the airplane flying straight while shutting the system off.
Context Anchor
Encountered in discussions of electric trim, autopilot use, and abnormal cockpit procedures.
Derivation
"Run-away" here means a system that has gotten loose from its controls and keeps going on its own — like a runaway shopping trolley. Applied to trim, it means the trim has escaped pilot command and is driving itself.
Why Pilots Care
Left uncorrected it produces rapid pitch changes that can exceed control authority or lead to loss of aircraft control.
Grounding Statement
The key idea is not that the airplane is out of trim; it is that the trim is still moving when it should not be.
Intuition Check
Do not read “run-away” as the airplane literally flying away. Here it means the trim system is continuing on its own and is no longer under normal control.
Example Sentence 1
During cruise, the pilot felt the nose pitching up and recognized a trim-run-away condition, immediately disconnecting the autopilot and pulling the trim circuit breaker.
Example Sentence 2
Following the emergency checklist, the pilot pressed the trim interrupt switch to stop the runaway condition.