What happened
During the final approach phase of flight, the first officer encountered difficulty trimming the aircraft. In an attempt to rectify the issue, the captain used both the manual trim handle and the switch located on his yoke, but these actions failed to restore proper function.
As a result, the crew executed a go-around. Following the maneuver, the crew performed the necessary checklists; however, control of the stabilizer trim remained unavailable. The crew attempted to troubleshoot the system by pulling circuit breakers H11 and H20 and subsequently resetting them, but the stabilizer trim remained inoperative. After the crew reset the trim cutout switches two times, normal operation of the stabilizer trim was successfully reestablished.
The investigation
Technical examination of the aircraft revealed that both the left and right stabilizer position transmitters were out of calibration. Regarding the left system, investigators found that the left stabilizer trim control module (STCM) exhibited higher than normal leakage. Additionally, a connector for the associated shut-off valve was shorting across multiple pins because of Skydrol incursion.
No faults were discovered within the right system, with the exception of the right-stabilizer-position transmitter. While the operator noted that the system should have continued to operate at half speed despite these discrepancies, it failed to do so during the event.