What happened
On November 11, 2020, a Cabri G2 helicopter, registration HB-ZVN, was performing a training flight at Sion Airport. During the initial takeoff phase, the aircraft immediately entered an uncontrolled leftward rotation around the yaw axis. As the helicopter began to climb while spinning, the instructor attempted to correct the movement by applying heavy pressure to the right rudder pedal. Simultaneously, the student pilot increased collective pitch, which exacerbated the leftward yaw.
The instructor's intense physical effort on the pedals caused a mechanical failure in the pedal positioning mechanism, accompanied by a loud noise. Realizing the situation was untenable, the instructor instructed the student to release all controls. The helicopter then entered a rapid descent, striking the tarmac with significant force. The impact caused the landing skids to detach and the main rotor blades to strike a nearby stationary helicopter. Both the instructor and the student sustained minor injuries, but no fire broke out, allowing them to exit the cabin safely.
The investigation
Investigators from the SUST examined the wreckage and the flight sequence using webcam footage and crew statements. The technical examination of the Cabri G2 confirmed that the flight controls, including the cyclic and collective, were functioning correctly. The investigation found no mechanical defects in the tail rotor drive or the primary control systems that could have initiated the accident.
Regarding the pedal mechanism, investigators discovered that the cam in the instructor's side pedal positioning mechanism had been severely damaged. This damage was attributed to the extreme force applied by the instructor during the attempt to regain directional control. The aircraft's weight and center of gravity were within approved limits at the time of the incident.
Findings
- The primary cause of the accident was the ineffective corrective action taken by the instructor, whose attempt to counteract the yaw failed to stabilize the aircraft while it was in an ascending, spinning state.
- The student pilot's decision to increase collective pitch during the yaw event contributed to the intensification of the leftward rotation.
- The instructor's extreme physical pressure on the right pedal caused a sudden mechanical shift in the pedal mechanism, which may have contributed to the perception of degraded control.
- The anti-crash protection in the seats prevented more serious injuries to the occupants.