What happened
On 9 December 2022, a Pilatus PC-24, registration D-CMSL, was conducting a non-revenue positioning flight from Liverpool Airport to Birmingham Airport. While climbing through FL90 in instrument meteorological conditions, the aircraft experienced an uncommanded nose-down pitch, resulting in a descent of approximately 800 ft.
During this period, the flight crew found the elevator controls to be extremely stiff, requiring significant physical force to achieve even minimal control surface deflection. The autopilot disengaged due to the resistance. The pilot flying implemented the Quick Reference Handbook (QRH) procedure for jammed elevator controls, utilizing the pitch trim to stabilize the aircraft and return to the assigned altitude. As the flight progressed toward Birmingham, the control heaviness gradually diminished, and the crew eventually regained full authority over the elevators. The aircraft landed safely without further incident.
The investigation
The AAIB examined the aircraft's flight data, which confirmed periods where the elevator position remained nearly static despite large inputs on the control column. The investigation also reviewed the aircraft's maintenance history and the manufacturer's fleet data.
Technical inspections by the manufacturer and the AAIB found no evidence of mechanical defects, foreign object damage, or obstructions in the elevator control system. While the investigation noted that the elevator control cable tensions and the pitch servo bridle cable tension were below the required limits, the manufacturer stated that such findings are not uncommon and would not typically cause a restriction in movement. Furthermore, the aircraft's dual ice detectors did not trigger any alerts for airframe icing during the flight.
Findings
- The aircraft experienced an uncommanded descent and severe pitch control restriction.
- The flight crew successfully managed the event by following established QRH procedures.
- Ice formation within the elevator control system could not be ruled out as a potential cause, despite no external airframe icing being detected.
- No mechanical faults or structural obstructions were identified in the elevator or control cable systems during subsequent examinations.