What happened
While operating in the cruise phase of flight, the aircraft's horizontal stabilizer became jammed in the (-1)-degree trim position. The aircraft subsequently completed an uneventful landing.
The investigation
Following the incident, the elevator trim actuator was sent to the manufacturer for a detailed examination conducted under the supervision of a Safety Board investigator. This component contained both a primary and a back-up clutch gear, with each gear equipped with its own torque nut and lock washer.
Upon inspection, investigators discovered that the tabs on the lock washers had not been bent over into the locked position. Furthermore, there was no evidence to suggest that these tabs had ever been properly secured. As a result, the back-up clutch torque nut had moved off its position and slid down the shaft until it came to rest against the outer race of the clutch gear cluster bearing. The face of this backed-off nut showed a scratch around its outer edge, which was consistent with fouling against the bearing's outer race. This mechanical interference is what caused the unit to jam.