What happened
On April 29, 2018, a Pilatus PC-12/45, registration HB-FPC, was conducting a private flight from Prague to Buochs. While descending through 7,500 feet near Dündorf, the aircraft experienced sudden, uncommanded oscillations around its vertical axis. The crew, perceiving a rudder trim runaway, disconnected the autopilot.
In an attempt to stabilize the aircraft, the second pilot activated the trim interrupt switch. However, during this process, the pilot also inadvertently engaged the flap interrupt switch. This action was irreversible in flight and prevented the deployment of flaps during the subsequent approach. The crew declared a 'mayday' and requested an immediate visual approach to runway 10 at Zurich Airport. Despite the difficulty in maintaining a straight course and the increased approach speed caused by the lack of flaps, the aircraft landed safely at approximately 15:30 UTC.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the mechanical cause of the oscillations and the crew's emergency response. Investigators examined the aircraft's flight control systems, including the yaw damper and rudder trim actuators. Maintenance records and technical inspections were reviewed to determine the source of the uncommanded movement.
Findings
Technical analysis revealed that a faulty yaw servo was providing a continuous, erroneous signal to the rudder trim actuator, causing the aircraft to yaw to the right. While disconnecting the autopilot successfully released the faulty servo from the control loop, the crew's subsequent actions complicated the landing.
Key contributing factors included:
- The crew's decision to prioritize an immediate landing over completing the established Quick Reference Handbook (QRH) checklist, which created unnecessary time pressure.
- The inadvertent activation of the flap interrupt switch, which prevented the use of flaps and resulted in a higher-than-normal approach speed.
- The crew's failure to fully utilize manual trim capabilities because the trim interrupt switch had disabled all trim systems.
Safety action
It was noted that the aircraft manufacturer had previously issued a Safety Information Letter (SIL-003) highlighting the risks of trim runaway and the potential for pilots to erroneously activate the flap interrupt switch. The investigation concluded that while the mechanical failure was the primary trigger, the crew's handling of the emergency increased the operational risks during the final approach.