Definition
An uncommanded, continuous movement of the trim system in which the trim continues to run in one direction without pilot input, typically caused by an electrical or mechanical failure in the trim motor, switch, or control circuit. It produces increasing control forces that the pilot must counter, and if not promptly identified and stopped it can drive the aircraft out of trim to the point of loss of control.
Plain English
The trim has started moving on its own and won't stop. The airplane keeps trying to pitch up or down, and the pilot has to physically hold the controls against it while quickly shutting off whatever is driving the trim.
Context Anchor
A pilot may encounter runaway trim during flight when the airplane suddenly requires unusual forward or back pressure on the control wheel or stick, especially in an airplane with electric trim.
Derivation
"Runaway" describes something that has gotten loose and is moving on its own without control — like a runaway car or runaway horse. Applied to trim, it captures the idea that the trim system is moving without permission and won't respond to normal input.
Why Pilots Care
It can produce sudden, severe pitch changes that may lead to loss of control unless the pilot immediately disconnects the system.
Grounding Statement
If the trim keeps moving nose-up or nose-down after the pilot lets go of the trim switch, the airplane may quickly become harder to hold in the desired attitude.
Intuition Check
Runaway trim does not mean the airplane itself is out of control. It means the trim system is moving without the pilot’s intended command, and that movement can make the airplane difficult to control if it is not stopped.
Example Sentence 1
During cruise the pilot felt the nose pitching down and recognized a runaway trim, immediately disconnecting the autopilot and pulling the trim circuit breaker.
Example Sentence 2
During the flight review the instructor simulated runaway trim so the pilot could practice maintaining control and securing the system.