What happened
On September 7, 2009, an SA 315B helicopter, registration I-NERY, was engaged in aerial work near the Punta Helbronner group at the foot of Mont Blanc. The mission involved installing power line cables along a mountain ridge. The aircraft was in the process of picking up a technical assistant who had been working on the ground to support the installation.
As the aircraft approached the ridge at an altitude of approximately 3,130 meters, the technical operator on board held the left cabin door open to facilitate the boarding of the ground crew member. During this maneuver, the main rotor blades struck nearby rocks. The impact rendered the helicopter uncontrollable, causing it to roll down the western slope of the ridge toward the Toula glacier. During the descent, the tail boom separated from the fuselage. The two technical operators on board were ejected from the cabin and sustained fatal injuries, while the pilot sustained critical injuries but was recovered alive.
The investigation
The ANSV investigation utilized a sequence of five photographs taken by a tourist at the scene to reconstruct the accident dynamics. Because the wreckage settled in an unstable position on a steep rock face, a physical inspection of the debris was not possible due to the high risk of falling rocks. However, technical documentation for the aircraft and its engine showed no maintenance deficiencies or mechanical anomalies.
Investigators examined the interaction between the aircraft and the safety equipment on the ground. It was noted that a safety rope, used by the ground crew for stability on the ridge, was present at the site. The investigation also noted that a construction worker nearby sustained minor injuries when a nylon pulling rope, used for the cable installation, was suddenly tensioned by the accident.
Findings
- The primary cause of the accident was the accidental contact of the main rotor blades with rocks.
- The incident was likely triggered by the accidental interference of a safety rope with one of the helicopter's skids.
- It is hypothesized that as the ground crew member prepared to board, the safety rope became caught under the helicopter's skids.
- When the pilot applied flight controls to depart the ridge, the rope likely became snagged on the right skid, causing the aircraft to tilt sharply to the right.
- A sudden corrective maneuver by the pilot, combined with the rope slipping off the skid, caused a rapid tilt to the left, leading to the fatal rotor strike against the terrain.