Learn what causes runaway trim, the signs pilots recognise, and the recovery priorities for electric trim and stabiliser systems.
Runaway trim is an uncommanded, sustained movement of an aircraft's pitch-trim system, usually caused by a stuck switch, electrical fault, trim motor or relay failure, or faulty automatic-trim command. Pilots hold the required attitude, disconnect automation, stop electrical trim using the aircraft's approved cut-out procedure, retrim manually if available, and land.
In real-world aviation, exact memory actions and switch names vary considerably between aircraft. The approved POH, AFM or QRH for the type always takes precedence over a generic sequence. Runaway trim usually means elevator or stabiliser trim; uncommanded rudder or aileron trim requires a different type-specific response.
What causes runaway trim?
Runaway trim starts when a powered trim system continues moving without the pilot's intended command, or moves in the opposite direction to that commanded.
- Stuck cockpit switch: A yoke, control-wheel or centre-console trim switch can remain closed mechanically or electrically after being released.
- Wiring or relay fault: A short circuit, failed relay or contactor can keep supplying power to the trim motor.
- Trim motor or control-unit failure: Faults in a servo, amplifier, clutch or electronic controller may generate continuous movement.
- Automatic-trim malfunction: An autopilot, speed-trim or stability system may command inappropriate trim because of a computer fault, erroneous sensor input or another flight-control problem.
- Trapped or prolonged input: A switch held by an object, a pilot pressing the wrong control, or an input maintained during high workload can resemble a system failure.
The consequences depend on the design. A light aircraft may move an elevator trim tab, while many transport aircraft alter the angle of the entire horizontal stabiliser. A moving stabiliser can produce forces that become extremely difficult to oppose, particularly as airspeed rises.
How do pilots recognise a trim runaway?
The clearest sign is trim movement that continues after the command has been released, usually accompanied by an increasing need to hold forward or backward control pressure.
| Condition | Typical clue | Key distinction |
|---|---|---|
| True runaway | Trim wheel, indicator, motor or aircraft response continues changing | Movement persists without an intended command |
| Incorrect initial trim | Heavy but broadly constant control force | The setting is wrong but is not continuing to move |
| Trim jam | Trim does not respond to a valid command | No movement rather than continuous movement |
| Normal automatic trim | Trim moves while the autopilot or another approved system is controlling pitch | Movement is expected, limited and stops when the system is disconnected |
An incorrectly set take-off trim can be just as serious during rotation, but it is a fixed mistrim rather than a runaway. Our guidance on setting and checking take-off trim explains the normal baseline.
An autopilot can also conceal a developing mistrim by holding the commanded attitude with elevator while the stabiliser moves. The problem may become obvious only when the autopilot disconnects and the pilot suddenly feels a large control force.
How do pilots recover from runaway trim?
Pilots recover by controlling pitch first, stopping the source of powered trim, and then reducing the remaining control force without surrendering control of the aircraft.
- Hold the required attitude. Use the primary flight controls, level the wings and prevent an overspeed or stall. Significant force may be necessary, and the pilot must not release that force merely to reach a switch.
- Disconnect the automation. Use the normal autopilot disconnect and any trim-interrupt control specified for the aircraft. If the trim movement stops, leave the suspect automation disengaged.
- Apply the approved trim response. Some systems allow an opposite electric-trim input to interrupt the runaway; others require immediate isolation. Pilots use only the action taught for that aircraft rather than fighting two electrical commands indefinitely.
- Cut power to the failed trim system. Operate the designated trim cut-out, trim master or equivalent controls exactly as the memory item directs. Randomly pulling or resetting circuit breakers can disable unrelated equipment or restart the fault, so it is done only when the checklist specifies it.
- Retrim manually if available. Once powered movement has stopped, use the manual trim wheel or other approved backup. If aerodynamic load makes it difficult to move, the type-specific procedure may call for carefully adjusting elevator load, power or airspeed before trimming; improvised large pitch changes can make the situation worse.
- Run the checklist and land. Inform ATC, obtain the QRH or checklist, keep the failed system isolated and plan a landing at a suitable aerodrome. Difficult aircraft control warrants an emergency declaration. Do not re-engage the failed automation unless the approved procedure explicitly permits it.
Can a pilot simply overpower runaway trim?
A pilot may be able to oppose the trim initially, but control force can increase rapidly as the trim travels farther or airspeed rises.
That is why stopping the movement early matters. Holding opposite elevator alone is not a complete recovery: the trim motor may continue towards its limit, and the remaining elevator authority may eventually be inadequate. If the system has reached an extreme position, reducing aerodynamic load can help, but speed must remain safely above the stall and within the checklist's limits.
Can the aircraft continue flying after the trim stops?
An aircraft may remain controllable after the runaway is stopped, but it should be treated as a serious flight-control malfunction.
The crew may have no electric trim, no autopilot and unusually high control forces. Power, flap, gear and speed changes can alter those forces sharply, so the approach should be stabilised early and configuration changes made deliberately. The safest destination depends on weather, runway length, aircraft handling and the applicable checklist.
Why does runaway trim happen in a flight simulator?
Apparent runaway trim in a simulator is more often caused by duplicate bindings, a noisy trim axis, a stuck controller button, assistance features or autopilot logic than by a simulated electrical failure.
Check every connected controller for overlapping pitch-trim assignments and watch for axis jitter before blaming the aircraft. Our trim-axis sensitivity and binding checks for MSFS and X-Plane cover that diagnosis. If the trim problem appears alongside automation dropping offline, also work through the common causes of repeated simulator autopilot disconnects.