What happened
On June 15, 2022, a Guimbal Cabri G2, registration HB-ZDQ, was performing a private flight departing from Gruyère Aerodrome (LSGT). After an initial takeoff and a 180-degree right turn, the pilot attempted a left turn at an altitude of approximately 50 meters to fly over the grass runway. During this maneuver, the aircraft experienced a sudden and uncontrolled rotation around the yaw axis to the left.
The pilot, flying at an indicated airspeed between 40 and 50 knots, was unable to stop the leftward rotation. Video evidence showed the helicopter spinning several times with a nose-low attitude. Although the pilot eventually managed to stop the rotation by applying significant collective input, the aircraft continued at a very low altitude. The helicopter performed a second left turn and struck a field, causing the landing skids to break. The impact caused the aircraft to briefly bounce before coming to a rest. While the pilot escaped without injury, the passenger sustained serious injuries to the back and required hospitalization.
The investigation
Investigators from the SUST examined the wreckage and analyzed flight data from the onboard Garmin GTN 650 and Floice 1 systems. The physical inspection of HB-ZDQ revealed that the landing gear was dislocated and the aircraft was resting on its fuselage. The investigation confirmed that the tail rotor drive shaft broke as a direct result of the impact with the ground. No mechanical failures in the flight controls, engine, or power systems were identified.
The investigation also reviewed the pilot's training history and manufacturer service letters. It was noted that the pilot was not aware of a specific manufacturer service letter (No. 12-001) regarding yaw control risks in the Cabri G2. Furthermore, weather conditions at the time included a west wind with gusts up to 25 knots, which contributed to a significant rear-right wind component during the turn.
Findings
- The accident was caused by a loss of control in yaw at low speed and low altitude while operating in gusty, rear-quarter winds.
- A contributing factor was the pilot's lack of specific knowledge regarding the unique handling characteristics of the fenestron-type anti-torque rotor during such maneuvers.