What happened
On June 26, 2001, a Pilatus PC-6/B2-H2 Turbo Porter, registration F-GUAS, was conducting a skydiving operation at Annemasse Aerodrome. The aircraft was carrying the pilot and nine skydivers. After completing the drop at 12,500 feet, the aircraft began a descent, increasing speed from approximately 70 knots to over 110 knots.
During this descent, the pilot experienced intense vibrations and observed the outer right aileron separating from the wing. To maintain level flight, the pilot had to apply significant physical effort to hold the control column fully to the right. The pilot managed to navigate the aircraft back to the aerodrome, performing several 3-60-degree turns using the rudder to maintain direction. The aircraft was landed on the grass adjacent to the paved runway 30. During the heavy landing, the right main landing gear collapsed, causing the gear leg to puncture the fuselage.
The investigation
The investigation focused on the structural failure of the control surfaces and the maintenance history of the aircraft. Investigators examined the wreckage and found that the outer right aileron and its associated balance mass were missing. The remaining ailerons showed signs of friction, and the control linkages for the right ailerons were severed. Specifically, the central aileron control rod was broken, and the right aileron control rods were snapped, with evidence of impact against the low travel stops.
Investigators also reviewed maintenance records, noting that the aircraft had undergone a 100-hour inspection in May 2001, which had found no anomalies. Crucially, the investigation uncovered that three days prior to the accident, a crack had been observed at the base of the outer right aileron balance mass. The pilot had contacted the manufacturer, Pilatus, the day before the accident, and had been advised to ground the aircraft until a service team arrived. However, the flight was continued.
Findings
- The loss of the outer right aileron was likely caused by the propagation of a previously identified crack at the base of the aileron balance mass.
- The loss of the balance mass likely triggered an aeroelastic resonance phenomenon, causing the aileron to exceed its operational limits and eventually detach.
- The structural failure of the aileron caused the breakage of the control linkages and significant deformation of the wing structure.
- The decision to continue flight operations despite the known crack and the manufacturer's advice to cease flying was a primary contributing factor.
- The decision to continue flying may have been influenced by the potential financial impact of grounding the aircraft.