Definition
An uncommanded, continuous movement of the trim system to a full nose-up or full nose-down position, usually caused by an electric trim motor or switch malfunction. The trim keeps running on its own without pilot input, progressively loading the flight controls and making the airplane increasingly difficult to hold in level flight.
Plain English
The trim system starts moving by itself and won't stop, pushing the nose up or down with growing force until the pilot stops it or counteracts it.
Context Anchor
Encountered inside the airplane when discussing electric trim switches, automatic flight systems, and emergency actions for unexpected trim movement.
Derivation
From 'trim' (the small surface that relieves control pressure) and 'run-away' (something moving on its own without control). The phrase describes a trim system that has effectively escaped pilot command.
Why Pilots Care
A trim runaway quickly creates large pitch changes and heavy control forces that can lead to loss of control unless power to the trim motor is cut immediately.
Analogy
It is like a power car seat that keeps moving after you let go of the switch. The switch action was small, but if it keeps going, the position can quickly become a problem.
Intuition Check
Trim-run-away does not mean the airplane itself is running away or speeding up. It means the trim setting is moving when you did not command it, or keeps moving after you stop commanding it.
Example Sentence 1
During cruise, the pilot felt the yoke steadily pushing forward and recognized a nose-up trim run-away, so he disconnected the autopilot and pulled the electric trim breaker.
Example Sentence 2
Following the checklist, the crew pulled the trim circuit breaker to stop the trim-run-away and regain normal pitch control.