What happened
On 26 September 2019, an Airbus AS350 B2 helicopter, registration 3A-MLC, was performing a cross-country flight from Carros to Villers le lac. While flying over the Alps, the pilot observed the MOT.LIM indicator light, signaling the presence of metallic chips in the engine oil system. The pilot attempted to divert to Megève mountain airfield to perform a landing.
During the descent, the engine began to deteriorate rapidly. The pilot heard an unusual noise, observed smoke from the engine, and subsequently reduced power to idle to initiate an autorotation. While performing the flare during the final approach to a field at Col de Véry, the pilot experienced a sudden jolt and yaw. The helicopter completed a running landing, but the impact and rotor dynamics caused significant damage to the airframe.
The investigation
The BEA examined the wreckage at the accident site, located at an altitude of 2,000 meters. Investigators found that the tail rotor drive shaft had fractured, causing the tail rotor to strike the pitch-change rod. This mechanical failure was linked to the main rotor blade making contact with the top of the tail boom during the landing flare.
Engine analysis revealed significant erosion on the compressor blades and the presence of a large quantity of laterite (a type of soil) inside the engine's hollow shaft. The investigation also reviewed the maintenance history of the engine, which had previously operated in Madagascar and Guyana. Investigators found that while standard inspections were performed, the specific cleaning requirements for the hollow shaft—mandated by a Service Bulletin for engines operated in dusty environments—had not been implemented.
Findings
- The engine failure was caused by the separation of accumulated laterite within the engine's hollow shaft, which created an imbalance in the rotating parts.
- This imbalance led to the failure of the gas generator rear bearing, generating the metallic chips that triggered the initial warning light.
- The accumulation of soil was due to an incorrect assessment by the maintenance management organizations (CAMOs), which believed the aircraft had not been exposed to sufficiently dusty environments to require specialized cleaning.
- During the landing flare, the combination of low rotor speed and a significant aft cyclic input caused the rotor disk to tilt and strike the tail boom, resulting in the breakage of the tail drive shaft.