What happened
On March 23, 2025, a Pilatus PC6 B2H2, registration F-GEBS, was conducting skydiving operations departing from Saint-Galmier aerodrome. While descending through 4,000 ft after the skydivers had jumped, the pilot experienced significant friction in the control column and discovered that the elevator trim was jammed in the full nose-down position.
To counteract the nose-down tendency, the pilot applied substantial manual force to the controls and increased turbine power. By managing the aircraft's vertical profile through a combination of airspeed, power, and flap deflection, the pilot maintained level flight. After notifying air traffic control, the pilot diverted to Saint-Etienne Loire aerodrome, utilizing its longer paved runway to perform a precautionary landing. The aircraft sustained slight damage during the event.
The investigation
The investigation focused on the tail section of the aircraft, specifically the structural frame FR12A which holds the lower connecting piece of the elevator trim actuator. Physical examination revealed that the actuator was no longer correctly fastened to the frame; instead, it had shifted forward and downward, causing the control cable to slacken. This movement had caused the structural frame to deform and tear in several places.
Investigators determined that the lower connecting piece of the actuator had been installed in the wrong direction during a recent maintenance period. The aircraft had undergone a 100-hour overhaul and annual inspection between December 2024 and February 2025. During this inspection, the connecting piece had been removed for a crack inspection and subsequently reinstalled incorrectly. Because the part is a hinged connection, the actuator could still be attached even though the base component was reversed.
Findings
- The primary cause of the incident was the incorrect installation of the elevator trim actuator connecting piece during a recent maintenance inspection.
- This improper assembly subjected the structural frame to out-of-plane loads and adverse stress modes for which the frame was not designed, eventually leading to the structural failure and subsequent jamming of the trim control.
- The maintenance task was not classified as a critical task, meaning it was not subject to a specific secondary check.
- The existing maintenance documentation failed to provide a warning regarding the possibility of installing the component in the wrong direction.