What happened
On June 9, 2015, a Cabri G2 helicopter, registration HB-ZOA, was conducting a flight training session near Semsales, Switzerland. The flight, involving an instructor and a student pilot, was intended to practice the precision of autorotation landings. After several successful maneuvers at different locations, the instructor decided to perform a final autorotation exercise at a field west of Semsales.
The instructor initiated the autorotation at approximately 1,000 feet above ground level with the engine at idle power. While the student pilot initially managed the flight parameters well, the aircraft's landing on the grassy terrain was unstable. Upon the first contact of the landing skids with the ground, the helicopter bounced, causing the aircraft to become destabilized. During this second impact, the main rotor blades flexed downward and struck the tail boom, severing the tail rotor drive shaft.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the flight parameters, the aircraft's condition, and the operational decisions made during the maneuver. The investigation utilized onboard collision warning system data to reconstruct the flight path.
Technical analysis confirmed that the aircraft was in good mechanical condition and within weight and balance limits. The investigation focused on the decision-making process regarding the landing technique and the timing of the instructor's intervention. It was noted that the specific plan to continue the autorotation all the way to the ground had not been established during the initial pre-flight briefing.
Findings
- The instructor decided to proceed with the autorotation to the ground while maintaining the engine at idle, a decision that carried increased risk due to the uneven nature of the grassy landing surface.
- The instructor's attempt to regain control of the aircraft occurred after the first contact with the ground, which was too late to prevent the subsequent instability.
- As the student pilot applied collective pitch to cushion the landing, the rotor RPM decreased significantly.
- The reduction in rotor speed diminished the centrifugal force required to keep the blades horizontal, allowing the vertical force of the second impact to drive the blades into the tail boom.
- There were no mechanical failures or technical defects identified as contributing to the accident.
Safety action
Following the accident, the operator, Swiss Helicopter AG, updated its operations manual to prohibit "full down" autorotation training. The manual now states that because such maneuvers generate high risks relative to their training effectiveness, they are not permitted on training flights unless specifically approved in writing by the Head of Training for specialized test flights.