Aero-Tips!

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Aero-Tips 11.23.06

Many airplanes have electric trim systems. Electric trim reduces the workload of fine-tuning trim position. It is also an essential component of autopilots -- autopilots adjust flight attitude by adjusting the electric trim. Like any other system there's the remote chance of a failure. When a trim system fails aircraft attitude can change rapidly -- why we call the scenario a "trim runaway".

The scenario

It happens (I've had it happen to me twice). You're alone in the clouds, or at night, or even in good visual weather. Blissfully cruising along on autopilot, suddenly the airplane pitches up sharply, g-forces pushing you deeper into your seat, and the airspeed drops rapidly toward a stall. Perhaps the pitch excursion is downward, negative g's flexing the airplane structure as the airplane noses down, speed building rapidly into the yellow arc. Maybe it's an uncommanded roll that takes you off heading and toward an incipient spiral.

Or maybe you're hand-flying the airplane, and when you blip the electric trim one way or the other it continues to run in that direction, headed for the stops. Again, the plane will pitch, roll and/or yaw, ever diverging from safety.

If either happens close to the ground or during an instrument approach you may have very little room to recover. Whatever the scenario, you have to act FAST to prevent a dangerous situation.

The solution

Most airplanes with electric trim systems have a disconnect switch. In some types the switch is labeled "autopilot disconnect/trim interrupt" -- it will disengage the autopilot when hit, but interrupts power to the electric trim system only as long as you hold down the switch.

From where you sit you usually can't tell whether a trim runway is a trim servo failure or some faulty logic in your autopilot, telling the trim to keep running. Regardless, the solution is the same, adjusted as needed for the equipment in the airplane you're flying:

If your airplane has a TRIM DISCONNECT switch:

1. HIT AND HOLD the trim disconnect switch
2. Manually overpower the trim excursion to regain aircraft control
3. If able, manually trim off trim pressures (note: not all airplanes with electric trim have a manual trim capability)
4. Turn OFF the TRIM POWER switch, if equipped
5. Pull the TRIM POWER circuit breaker
6. Continue flight trimming manually and without use of the autopilot.

If your airplane does NOT have a TRIM DISCONNECT switch:

1. Manually overpower the trim using the control yoke
2. If able, manually trim off trim pressures (note: not all airplanes with electric trim have a manual trim capability)
3. Turn OFF the TRIM POWER switch, if equipped
4. Pull the TRIM POWER circuit breaker
5. Continue flight trimming manually and without use of the autopilot.

If you're flying one of the new-generation airplanes with an electric trim system and no manual control, you'll have to manually overpower the out-of-trim airplane all the way to landing -- even more incentive to catch a trim runway FAST before it progresses too far.

FMI: Aero-Tips