What happened
On August 30, 2016, an SA 315 B helicopter, registration HB-XEO, was conducting a commercial transport mission near Salvan, Switzerland. The flight, operated by Air Glaciers S.A., was tasked with transporting workers to a remote location near a power line pylon.
The crew approached a small, rocky landing area characterized by a steep 40-degree slope. Due to recent electrical line installations, the pilot had to utilize a different approach technique than usual, flying parallel to the lines. While performing a hover to disembark the passengers, the aircraft's weight shifted as workers exited the cabin. During this period of instability, the pilot attempted to reposition the aircraft. However, the main rotor blades struck the rocky terrain, causing the helicopter to enter a resonance state and lose control. The aircraft subsequently rolled onto its right side and came to rest approximately 17 meters downslope. There were no fatalities and no injuries were reported among the two crew members or the passengers.
The investigation
Sust examined the wreckage and the accident site, noting that the landing area was extremely narrow, making it impossible to land both skids simultaneously. The investigation focused on the operational challenges presented by the terrain and the lighting conditions. Investigators found that the accident occurred roughly 21 minutes after sunrise under fragmented cloud cover, which resulted in poor lighting and low visual contrast between the rotor disk and the rocky ground. The investigation also reviewed the aircraft's technical state and the pilot's flight parameters, including altitude, load, and temperature.
Findings
- The primary cause of the accident was a loss of control during a hover resulting from the main rotor blades striking the terrain.
- The pilot misjudged the distance between the rotor blades and the steep slope.
- The landing site was highly constrained, requiring the pilot to maintain a hover or touch only one skid to the ground.
- Poor lighting conditions and low visual contrast between the rotor and the rocks hindered the pilot's ability to accurately assess clearance.
- The shifting weight of passengers exiting the aircraft contributed to the aircraft's instability during the maneuver.
- No technical failures or mechanical defects were identified in the aircraft.