Narrative:

I was notified of an air return on aircraft X due to a failed horizontal stabilizer autopilot actuator. The flight crew was able to control the stabilizer manually and returned to ZZZ from which it originated. Upon investigation in the ZZZ hangar; it was determined that a bent pin on the connector of the autopilot servo was the cause of failure. The servo actuator was replaced and the entire horizontal stabilizer system was inspected and tested with no defects found. The aircraft was then returned to service. Later; at the hangar; we performed a scheduled change of the jack screw gear box. The jack screw was changed and the existing primary trim and autopilot actuators were reinstalled; as no paperwork was issued to replace them. The jack screw was removed and replaced in accordance with task card. The installation went normally with no problems encountered. The system was tested per the task card with no abnormalities found. The aircraft was then released for service. The aircraft did receive a crosscheck later. Also; a line routine overnight event in which the crew reported a possible slow stabilizer movement. The system was tested per maintenance manual and no discrepancies were found. A time span of approximately 6 weeks elapsed between jack screw installation and servo failure. At what point the connector became bent is unknown. I suggest a step be added to the task card to inspect for weak or deformed connector pins; prior to installation; if the original trim servos are to be revised.

Google
 

Original NASA ASRS Text

Title: Mechanic was informed of an air return of a B737-500 caused by a bent pin on the connector of the autopilot servo. Incident happened six weeks after he replaced the horizontal stabilizer jackscrew.

Narrative: I was notified of an air return on Aircraft X due to a failed horizontal stabilizer autopilot actuator. The flight crew was able to control the stabilizer manually and returned to ZZZ from which it originated. Upon investigation in the ZZZ hangar; it was determined that a bent pin on the connector of the autopilot servo was the cause of failure. The servo actuator was replaced and the entire horizontal stabilizer system was inspected and tested with no defects found. The aircraft was then returned to service. Later; at the hangar; we performed a scheduled change of the jack screw gear box. The jack screw was changed and the existing primary trim and autopilot actuators were reinstalled; as no paperwork was issued to replace them. The jack screw was removed and replaced in accordance with task card. The installation went normally with no problems encountered. The system was tested per the task card with no abnormalities found. The aircraft was then released for service. The aircraft did receive a crosscheck later. Also; a line routine overnight event in which the crew reported a possible slow stabilizer movement. The system was tested per Maintenance Manual and no discrepancies were found. A time span of approximately 6 weeks elapsed between jack screw installation and servo failure. At what point the connector became bent is unknown. I suggest a step be added to the task card to inspect for weak or deformed connector pins; prior to installation; if the original trim servos are to be revised.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.