What happened
On July 20, 2010, a Centrair ASW 20 F glider, registration F-CFLP, was participating in a regional speed competition at the Buno-Bonnevaux airfield. During the fifth flight of the day, the pilot was returning to the airfield at approximately 16:50 local time. While flying at an altitude of 700 meters with a glide ratio of 25, the pilot encountered an updraft and applied forward pressure on the control stick to maintain altitude.
At this moment, the pilot felt the wings begin to vibrate violently. Upon deploying the airbrakes, the vibration ceased, but the pilot discovered that the ailerons were no longer responsive to lateral control inputs. Unable to use roll controls, the pilot managed to steer the aircraft using the rudder and elevator. The pilot opted for an emergency landing in a field located before the threshold of runway 28. During the landing sequence, the aircraft experienced a nose-up bounce (a "cheval de bois"), resulting in heavy damage to the glider.
The investigation
The investigation focused on the cause of the vibration and the subsequent failure of the control system. Investigators identified the vibration as aileron flutter. Post-accident inspection of the control linkage revealed that the end cap of the bellcrank connecting the control stick to the aileron transmission mechanism had failed. The failure was determined to be the result of buckling due to an overload.
Analysis showed that the forces required to cause such a rupture were consistent with the pilot holding the control stick firmly against the movement of the ailerons during the flutter event. While the aircraft's mass and balance were within limits, and the airbrakes and flaps were properly configured, the specific trigger for the flutter could not be definitively identified. The investigation also noted that the sealing tapes intended to close the gap between the wing and the control surfaces were intact and had been recently replaced.
Findings
- The primary cause of the accident was the onset of aileron flutter.
- The pilot's instinctive reaction to hold the control stick firm during the vibration led to a structural overload.
- This overload caused the failure of the aileron control linkage via buckling.
- The investigation could not determine the exact cause that triggered the flutter phenomenon in this specific instance.