Event Description
While in cruise flight, the flight crew reported an intermittent sonalert indicating trim in motion. However, the trim indicator and actual aircraft trim were unaffected at that time. Several minutes later, the elevator trim began to move without any input from the crew. Both the captain and co-pilot attempted to deactivate the trim using their control yoke switches, but the trim would not stop. It continued until the left side essential bus was selected off, at which point the trim deactivated. The airplane landed without further incident.
Investigation Findings
Post-flight inspection of the elevator trim system identified two electrical anomalies. The captain's side trim power wire was found to be shorted directly to the left side essential bus. Additionally, the jumper wire at the captain's yoke trim switch (on the ground side) was shorted to the yoke. These shorts allowed electrical power to bypass normal switching and energize the trim motor independently of crew commands.
Resolution
After the affected wires were replaced, the trim system was tested and operated normally, confirming the repair resolved the issue.