Trim Runaway

Trim runaway is a serious—occasionally fatal—system malfunction. We look at what causes it and what you can do to handle it.

BTeter

Saturday, February 17, 2024 at 08:00 AM ET

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Most modern yoke-mounted electric trim switches are actually two switches that must be activated together via a split, two-piece knob. (Photo: Cygnet Aerospace)

Key Takeaways:

The increasing prevalence of electric pitch trim in general aviation cockpits, both manual and autopilot-controlled, is linked to a rise in "trim runaway" incidents and accidents.
A trim runaway occurs when the trim servo motor runs continuously without pilot input, quickly resulting in extremely high control forces that can make the aircraft difficult or impossible to control.
These runaways can stem from various causes, including stuck switches or autopilot malfunctions, and real-world incidents highlight that pilots need better awareness and practice of immediate actions to disable the trim system to ensure recovery.

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General aviation cockpits increasingly have electric pitch trim, both manual and autopilot-controlled, bringing a corresponding increase in incidents and accidents related to these systems. For manual electric trim, the trim servo-motor is activated by a switch on the control yoke. Manual electric trim aids flying as you needn’t break your visual concentration to reach for the trim wheel.

Newer, more sophisticated digital autopilots also have auto trim to complete the autopilot installation. Without the autopilot being able to trim the aircraft, the autopilot is only halfway flying the plane as you must manually adjust the trim. Configuration changes like dropping the gear and changing the flap settings are where auto trim shines.