What happened
On May 15, 2004, a Pilatus PC-6 B2-H2, registered F-GOYE, was performing a skydiving mission near Agen, France. During the sixth flight of the day, while climbing through approximately 300 feet, the pilot began retracting the flaps. After adjusting the pitch and completing the flap retraction, the aircraft suddenly entered a steep dive. The pilot attempted to pull back on the control column with significant force and manipulated the trim control; however, the trim moved from the full nose-down to the full nose-up position with very little resistance.
Upon inspection, the pilot found that while the trim control chain was still in place, the cable had become slack. To manage the situation, the pilot partially extended the flaps and continued the climb to complete the jump for the three remaining passengers. During the subsequent landing approach, while at an altitude of approximately 300 feet, a loud noise was heard, and the aircraft entered another sharp dive as the flaps unexpectedly retracted from the full extended position to the fully retracted position. The pilot re-extended the flaps, and the aircraft landed safely without further damage.
The investigation
Investigators examined the aircraft's trim system and discovered that the frame supporting the actuator, which controls the adjustable horizontal stabilizer, had been torn away. Metallurgical analysis determined that the frame had failed due to fatigue caused by horizontal, fore-and-aft stresses transmitted by the actuator. While the frame is designed to withstand vertical loads, these unintended horizontal forces led to the structural failure.
Dimensional analysis of the frame revealed that the component did not conform to the manufacturer's original specifications in several areas. These deviations likely altered the angle between the actuator and the frame, introducing the horizontal force component that caused the fatigue.
Findings
- The primary cause of the trim ineffectiveness was a structural failure of the stabilizer actuator support frame due to fatigue.
- The failure was driven by horizontal stresses caused by a non-conforming component that deviated from the manufacturer's design specifications.
- The fatigue crack appeared to be an old defect, though the exact date of its formation could not be determined.
- The defect was not detected during previous maintenance inspections, despite the requirement to inspect this specific area during 100-hour maintenance checks.