Cat. 2 investigation report: simplified-format report, adapted to the circumstances of the occurrence and the investigation stakes.
In cruise flight, when flying over the Alps, the pilot detected the illumination of the MOT.LIM indicator light signalling the presence of chips in the engine. He decided to head towards Megève mountain airfield (Haute Savoie) which was located a few flight minutes away and he started descent. Around three minutes later, he heard an unusual noise, reduced the engine power to idle and initiated an autorotation. A witness saw the helicopter fly by in descent. He also heard a bang and saw smoke coming out of the engine. At the end of the final approach, during the flare, the pilot felt a jolt through the pedals and observed a yaw movement of the helicopter. He performed a running landing.
The separation of some of the laterite that had built up in the hollow shaft of the engine’s module 3 and that had not been detected during the previous years of engine operation, caused an imbalance on the engine’s rotating parts. This imbalance caused damage to the gas generator rear bearing and the generation of chips, which triggered illumination of the MOT.LIM indicator light.
Faced with this warning when flying over a hostile mountainous region, the pilot initially attempted to reach Megève mountain airfield located a few flight minutes away. With the engine’s condition rapidly deteriorating during the descent, the pilot reduced the engine power to spare the engine and initiated an autorotation to land in an area clear of obstacles in the vicinity of Col de Véry. The pilot pulled on the collective pitch lever when approaching the ground during the flare. This latter action resulted in a rapid decrease in the rotation speed of the main rotor. The nose-up action on the cyclic pitch stick during the flare caused the rotor disk to tilt backwards and to subsequently come into contact with and damage the top of the tail boom.