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 (manual and autopilot-controlled) in general aviation has led to a corresponding rise in dangerous "trim runaway" incidents.
Trim runaway occurs when the trim servo motor runs continuously due to various failures, causing control forces to rapidly escalate, making the aircraft extremely difficult or impossible to control if not immediately stopped.
Timely recognition and swift, decisive action to disable the trim system are critical for pilot recovery, as every second counts; however, many pilots are inadequately trained or aware of how to handle this severe emergency.

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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.