History of the Flight
The student-pilot, accompanied by an instructor, took off from a dirt track for a flight to the helicopter landing site at Marnaz. After entering a hover, the helicopter was positioned into the northerly wind and began forward climb. At approximately 15 m above ground, a clacking noise was heard, and the helicopter abruptly jolted and yawed. The instructor reduced collective pitch to relieve engine stress and took control. He planned to return to land on the dirt track, executed a left U-turn, and continued south. To avoid a high descent rate, he intended to fly beyond the dirt track and make another U-turn before landing at the departure point into the wind. Near the dirt track, heading south at about 5 m height and just below 30 kt, engine power decreased and a continuous aural signal sounded. The instructor continued descent and decided to land in a field opposite. The front of the skids contacted a tarpaulin covering the field and sank into loose soil. The helicopter tipped forward, damaging the main rotor and breaking the tail boom. The helicopter came to rest on its left side.
Additional Information
Meteorological Information
Météo-France estimated wind from 360° at 14 to 16 kt with gusts up to 30 kt, visibility greater than 10 km, clear sky to few clouds, temperature 12°C, dew point 2°C, QNH 1018 hPa.
Personnel Information
The 27-year-old student-pilot, training for a Private Pilot Licence - Helicopters (PPL(H)), had 69 flight hours, 33 on the Cabri G2. He had made four prior instruction flights that day with the same instructor, totaling 2 hours 36 minutes.
The 36-year-old instructor held a Commercial Pilot Licence - Helicopters (CPL(H)) obtained in April 2017 and an instructor rating from October 2017. He had type ratings for the Airbus AS350/EC130 (April 2018) and Guimbal Cabri G2 (December 2018). He had 1,693 total flight hours, with 157 in the previous three months (86 on the Cabri G2).
Aircraft Information
The Guimbal Cabri G2, registration F-HPTI, entered service in December 2018 and had 651 flight hours at the time of the accident. The engine was a Lycoming O-360-J2A flat-four delivering 145 hp. The last 100-hour maintenance inspection was on 11 March 2021 at 605 flight hours. Approximately 45 flight hours had accumulated since that inspection.
Examination of the Helicopter
Examination revealed that one main rotor blade struck the tail boom, separating it forward of the horizontal stabilizer at a skew angle, indicating low rotor speed. The main gearbox, rotor head, and Fenestron tail rotor showed no anomalies. The engine was externally clean; oil and fuel filters were clean with no metal particles. Cylinder compression measurements showed no pressure rise in cylinder No. 1. Endoscopic examination of cylinder combustion chambers revealed no anomalies. A slight gap was detected at the two valve seats of cylinder No. 1, but after removing valve springs, no friction points or abnormal friction were found. After reinstalling springs, pressure in cylinder No. 1 was similar to other cylinders. A complete engine tear-down revealed marked carbon deposits on the two valve seats and piston of cylinder No. 1, damage to exhaust valves, and coking of intake valves on all cylinders. The workshop estimated valves had not seized and carbon deposits could not have caused the malfunction.
Conclusions
The instructor, suspecting a minor failure after take-off, turned back. During the turn, engine power reduced, possibly due to a sealing fault at the valves of one of the cylinders. An immediate landing was made with a tailwind of about 15 kt. During the running landing in loose soil with grooves and a tarpaulin, the helicopter tipped forward and then onto its side.