What happened
On September 14, 2011, at approximately 9:35 a.m., an Eurocopter AS350 B3, registration F-HFBI, departed from the Vallorcine power station for an aerial work mission. The crew, consisting of the pilot and an assistant, was accompanied by four passengers. The flight mission involved transporting two technicians to a pylon along a cableway line connecting Barberine to the Emosson dam, followed by a reconnaissance flight over a forest path for the remaining passengers.
During the return leg to the power station, the helicopter was on final approach when it struck one of the cables belonging to the cableway system. The impact caused the rotor mast to fail and forced the fuselage into the ground, where it subsequently caught fire. The accident resulted in four fatalities and the total destruction of the aircraft.
The investigation
Investigators examined the wreckage and performed a spectral analysis of audio from a video recording. The technical examination revealed no mechanical anomalies; the engine was producing power at the moment of ground impact.
Regarding the flight path, the pilot had selected a trajectory that passed between a high-voltage power line—situated more than 200 meters above the ground—and the four cableway wires, which were positioned approximately 50 meters above the terrain. This specific path was likely chosen to avoid a steep descent angle that would have been required to clear the power line while approaching the landing area.
Environmental factors were also scrutinized. At the time of the accident, the sun was positioned low over the terrain (7 degrees above the horizon) and was shining almost directly toward the pilot. The investigation noted that in the seconds immediately preceding the collision, the helicopter transitioned from a shaded area into direct sunlight. Additionally, the pilot was not wearing sunglasses.
Findings
- The primary cause of the accident was a pilot error in judging the position of the obstacles he had intended to navigate.
- The pilot, an experienced mountain guide with over 9,000 flight hours, was highly familiar with the area and the specific site.
- A loss of vigilance may have been contributed to by a sense of routine or fatigue, following seven hours of flight in the same area the previous day.
- The sudden transition from shade to bright sunlight, combined with the pilot's failure to wear sunglasses, likely impaired his visual detection capabilities.