What happened
On May 18, 2015, a Hugues 369 helicopter, registration OH-HNR, was conducting a low-altitude aerial reconnaissance mission near Bocognano, France. The flight, operated by Heliwest, was intended to inspect an electrical line for upcoming tree trimming work. The crew consisted of the pilot and an operator.
Approximately five minutes after takeoff, while the pilot was reducing airspeed, the aircraft experienced vibrations from the rear section, unusual engine noises, and fluctuations in rotor RPM. A "LOW RPM" alarm sounded. Due to the proximity of power lines and other obstacles, an emergency autorotation was not feasible. The pilot attempted to maneuver away from the lines, but the aircraft lost altitude abruptly and performed a hard landing in a residential garden.
Upon impact, the left skid broke, and the helicopter settled on its side. During the evacuation, the operator was struck by a still-rotating main rotor blade. Shortly after the crew exited the aircraft, the helicopter caught fire, which ultimately destroyed the airframe.
The investigation
The investigation focused on the engine's performance and the cause of the power loss. While the fire prevented a comprehensive examination of the wreckage, specialized analysis of the Rolls-Royce 250-C20B engine was conducted by the manufacturer.
Investigators identified damage to the first and second-stage turbine disks, including missing blade tips and signs of melting. This damage was caused by excessive airflow temperatures. The manufacturer determined that this thermal damage was not caused by the post-impact fire, but rather by either high-temperature engine operation during flight or operation with reduced airflow after the crash. It could not be definitively determined when this temperature spike occurred.
Technical records showed that the engine's governor had been replaced just days before the accident following reports of slow reaction. Additionally, the investigation reviewed airworthiness directives regarding fuel regulation piping, noting that the operator had applied the relevant service letters following the governor replacement.
Findings
- The primary cause of the accident was a loss of engine power while the aircraft was flying at a low altitude.
- The presence of obstacles and the low altitude of the flight prevented the pilot from executing a safe autorotation.
- The engine sustained damage due to excessive temperatures, which led to the power reduction, though the exact timing of this thermal event remains unconfirmed.