What happened
On March 8, 2009, an Eurocopter AS350 B3, registration HB-ZEI, was conducting a commercial heli-skiing operation near the Wildhorn summit in the Swiss Alps. After an initial successful landing at the summit, the pilot attempted to land on a slope between the Wildhorn peak and the Audannes Glacier to drop off a group of passengers and a mountain guide.
During the final approach, the pilot encountered significant wind turbulence and shifting wind directions. While in a hover approximately two meters above the ground, the pilot rotated the aircraft 45 degrees to the right to facilitate the slope landing. As the aircraft made contact with the snow, a sudden wind gust caught the rear of the fuselage. The pilot attempted to correct the pitch using cyclic input, but the maneuver was ineffective. The helicopter pitched forward, causing the main rotor blades to strike the ground. The impact caused the aircraft to capsize on its right side and resulted in the breakage of the forward right skid.
The investigation
The investigation focused on the meteorological conditions, the aircraft's technical state, and the operational decisions made during the landing phase. Investigators examined the pilot's flight logs, the aircraft's maintenance records, and the performance charts provided in the flight manual.
Technical analysis confirmed that the HB-ZEI was in good working order with no mechanical failures identified. The investigation also looked into the aircraft's weight and balance, noting that the longitudinal center of gravity was near the forward limit. Meteorological data from MeteoSuisse indicated that the area was experiencing west-southwest winds with gusts reaching approximately 25 knots, alongside moderate turbulence caused by the local topography.
Findings
- The primary cause of the accident was a main rotor strike following a loss of control during a landing on a snowy slope under turbulent, crosswind conditions.
- The pilot's decision to rotate the aircraft 45 degrees to the right placed the fuselage and vertical stabilizer in a position where they were highly susceptible to wind pressure, increasing the demand on the anti-torque system and reducing stability.
- The aircraft's center of gravity was positioned near the forward limit, which restricted the available aft cyclic authority needed to counter the pitch change.
- The use of the cyclic control to correct the pitch was an inadequate response to the sudden gust; reducing collective pitch might have prevented the loss of control.
- The landing site was not ideal for passenger disembarkation given the prevailing wind and terrain conditions.