What happened
On March 12, 2015, an instructor and a student were conducting flight training maneuvers near the Plateau de la Matheysine in France. The flight began at Grenoble Le Versoud aerodrome, with the crew heading toward a mountain pass at an altitude of 5,400 ft to perform ground-effect hovering exercises.
During the maneuvers, the instructor performed two approaches. The first approach was made into the wind, involving a brief touchdown of one skid on the snow. Following a turnaround, a second approach was attempted from the opposite direction. As the pilot increased collective pitch to climb out of a hover, the Guimbal Cabri G2, registered F-HOLA, suddenly entered a rapid leftward yaw rotation.
Despite the pilot's attempts to counter the rotation using the pedals, the aircraft continued to spin, completing approximately five to seven rotations. The pilot then increased power and collective to gain altitude, which triggered a "LOW NR" alarm. To mitigate the rotation and avoid further uncontrolled flight, the pilot eventually reduced power and collective, resulting in a hard landing in the snow. The aircraft sustained significant damage.
The investigation
The investigation focused on the mechanical state of the aircraft and the environmental conditions at the time of the accident. Investigators examined the wreckage and found that the airframe was broken at the attachment point to the rear landing gear skid, with the fuselage shifted approximately 15 degrees to the left. The tail boom was partially ruptured at the fenestron junction, and the rear drive shaft showed signs of a sudden failure due to combined bending and torsion loads caused by the hard impact.
No defects were found in the flight controls, and all remained continuous. The investigation also reviewed the meteorological data, noting that while winds were light, a temperature inversion at 5,500 ft was likely generating turbulence in the area. The aircraft's weight and center of gravity were within manufacturer limits, but the power required for a hover in a headwind/tailwind condition was near the aircraft's performance limits.
Findings
- The sudden leftward yaw occurred as the pilot increased collective pitch to exit a hover.
- The second approach likely involved a tailwind component, which significantly increased the power required to maintain a hover.
- The loss of control was likely caused by a loss of rotor RPM (NR) during the hover in turbulent conditions while the aircraft was operating near its performance limits.
- The pilot's decision to increase collective pitch to gain altitude increased the torque generated by the main rotor, which exacerbated the leftward yaw.
- The pilot's subsequent reduction of power and collective before impact helped decrease the rotation rate, though it could not prevent the hard landing.