What happened
On September 30, 2021, a Guimbal Cabri G2, registration HB-ZVK, was conducting flight training maneuvers near Neuendorf, Switzerland. The flight, operated by a flight instructor and a student pilot, involved a series of simulated autorotations. During the first seven maneuvers, the instructor practiced reducing engine power by adjusting the throttle twist grip without prior notice, subsequently re-engaging the engine at approximately 200 feet above ground level to stabilize the aircraft.
During the eighth attempt, the student pilot initiated a flare to land near a road. As the pilot pulled the collective to reduce sink rate and stabilize the aircraft, the helicopter experienced a significant drop in both engine and main rotor RPM. The aircraft began to yaw uncontrolled to the right. Despite attempts by the instructor to correct the heading using the pedals, the helicopter struck the ground with significant force. The impact caused a main rotor blade to strike the tail boom, destroying the tail rotor assembly. Both occupants escaped the crash without injury.
The investigation
SUST examined the aircraft's flight data from a GPS-based navigation device and the cockpit's Electronic Pilot Monitor (EPM). While the EPM's integrated data recorder had been non-functional for two months, the investigation was able to reconstruct the flight path and the sequence of events.
Technical analysis of the wreckage revealed that the throttle correlation cam had broken. Investigators determined that this breakage was likely caused by the tail rotor control cable being torn away by a rotor blade during the impact. Crucially, the investigation found that the engine was not producing the necessary power to recover during the flare, contradicting the crew's recollection that the throttle had been opened.
Findings
- The primary cause of the accident was an unexpected drop in main rotor RPM during the flare, which forced a transition from a power-recovery autorotation to a forced landing without engine power.
- The investigation concluded that the throttle twist grip was not opened during the final maneuver, preventing the engine from providing the power required to stabilize the flight.
- High workload during the training session, involving multiple consecutive maneuvers and the need to correct a high approach, likely contributed to a cognitive error where the instructor failed to re-engage the throttle.
- The aircraft's weight and center of gravity were within permissible limits, and no pre-existing mechanical defects were found in the engine or airframe.