At Duncan Aviation, we’ve seen a couple of instances where the autopilot computer or yaw damper computer on a Learjet 35 would disengage when the aileron trim motor is actuated.
With a defective aileron trim motor, an unwanted induction spike caused by the motor field collapse is sent back down the wire bundle that shares wiring with the autopilot and yaw damper systems. It is suspected that internal spike suppression is incorporated into the motor, and when that circuitry fails, the unsuppressed spike trips the autopilot or yaw damper disconnect, possibly via the three axis disconnect box.
In both instances, even though the trim motor appeared to operate properly, replacing the motor cleared the autopilot or yaw damper discrepancy.
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