What happened
While operating in cruise flight, the flight crew observed an intermittent sonalert indicating that the trim was in motion. During this initial period, neither the aircraft trim nor the trim indicator showed any actual movement.
Several minutes later, the elevator trim began to move without any input from the crew. Both the captain and the co-pilot attempted to deactivate the trim using their respective control yoke switches, but these efforts were unsuccessful. The crew was only able to stop the movement by selecting the left side essential bus off. Following this procedure, the aircraft landed without further incident.
The investigation
An inspection of the electrical system was conducted following the event. Investigators discovered that the power wire for the captain's side trim was shorted directly to the left side essential bus. Additionally, the jumper wire located at the ground side of the captain's yoke trim switch had shorted to the yoke.