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, has led to a rise in hazardous "trim runaway" incidents.
Trim runaways, where the trim servo motor runs continuously without pilot input, can be caused by issues like stuck manual switches, autopilot malfunctions, or artificial-feel system failures.
These incidents are extremely dangerous due to rapidly escalating control forces, demanding immediate recognition and disabling action, a critical skill many pilots are not adequately trained to handle.

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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.