Event Description
During cruise flight, the horizontal stabilizer jammed in the (-1)-degree trim position. The aircraft landed without further incident.
Examination
Following the landing, the elevator trim actuator was removed and sent to the manufacturer for examination under the supervision of a Safety Board investigator. The actuator contained a primary and a backup clutch gear, each with a respective torque nut and lock washer.
Findings
Examination revealed that the tabs on the lock washers had not been bent over into the locked position. There was no evidence that the tabs had ever been bent into that position. Additionally, the backup clutch torque nut had backed off and moved down the shaft, coming to rest against the clutch gear cluster bearing outer race. The backed-off nut exhibited a scratch around the outer edge of its face. The scratch was consistent with fouling against the clutch gear cluster bearing outer race, which subsequently jammed the unit.
Production Error
The manufacturer identified the isolated error in the production process as the cause of the lock washer tabs not being bent.