Event Summary
A corporate jet airplane lost elevator trim control due to a runaway trim condition, resulting in an uncommanded nose-low pitch attitude. The pilot reported that after the loss of elevator trim authority, the aircraft became extremely difficult to control, with very high elevator control forces. Despite maneuvering the airplane, the pilot eventually ditched it into a nearby marine cove. The pilot stated that the runaway trim was not immediately recognized, and by the time it was noted, control forces were so great that there was little time to troubleshoot the system. The pilot elected to continue on the established heading and ditch the aircraft.
Investigation Findings
Post-accident examination and functional testing of the aircraft's electric pitch trim printed circuit board (PCB) revealed a repeatable fault in the operation of the K6 relay, causing the relay contacts to remain closed. This condition would be representative of the autopilot pitch trim remaining engaged, providing a continuous electrical current that drove the elevator trim motor to a nose-down position. Pulling the circuit breaker, as called for by the checklist during a trim runaway, would have arrested the trim movement.
Maintenance and Design Issues
Examination of the aircraft's maintenance records showed that the PCB had been removed and replaced during the phase inspection prior to the accident. The replacement PCB had originally been installed in another aircraft that experienced an electric trim runaway on the ground. Following that incident, the PCB was removed and shipped to the manufacturer, who tested it and found no discrepancies. The unit was subsequently approved for return to service and installed on the accident aircraft.
Further investigation revealed a single-point failure mode of trim runaway due to the failed K6 relay and a latent system design anomaly in the autopilot/trim disconnect switch on the pitch trim PCB. This design prevented disengagement of the electric trim motor during autopilot operation.
Regulatory Actions
As a result of the investigation, the FAA issued three airworthiness directives (AD 2003-21-07, AD 2003-23-20, and AD 2004-14-20). Additionally, the pitch trim printed circuit board was redesigned and evaluated for compliance with safety requirements through a system safety assessment.