What happened
On April 27, 2002, an AS350B2 helicopter, registration HB-XUZ, was performing commercial VFR operations to transport a group of skiers to the Sustenhorn area in the Swiss Alps. After successfully completing several shuttle flights between the Steingletscher and Sustenlimi, the pilot attempted a return flight following a fourth landing. During this phase, weather conditions deteriorated rapidly as fog banks moved into the area.
While attempting to navigate back to the Steingletscher, the pilot encountered sudden fog that obscured the landscape. The aircraft struck a snow-covered surface with the front of its skids, causing the helicopter to overturn and resulting in the destruction of the cabin. The pilot sustained one serious injury (a broken leg, facial lacerations, and frostbite), while the passengers involved in the flight were not on board during the final impact.
The investigation
SUST examined the wreckage and the operational circumstances of the flight. Technical inspections of the AS3 350B2 revealed no mechanical failures or anomalies in the flight controls, engine, or fuel systems. The aircraft's weight and balance were within manufacturer limits, and the engine was found to be in the "flight" position with fuel valves open. The investigation noted that the rear drive shaft was heavily twisted, indicating high energy was present in the rotor system at the moment of impact.
Investigators also reviewed meteorological data from MeteoSchweiz and observations from a rescue helicopter. The data confirmed that a cold front had passed through the region, leading to shifting cloud bases and the formation of fog banks. The investigation also noted that the pilot had a valid commercial license with mountain landing endorsements and had performed regular training on the accident pattern.
Findings
- The primary cause of the accident was a collision with a snow-covered surface resulting from the loss of visual references.
- Rapidly changing weather conditions and the formation of fog banks significantly reduced visibility.
- The presence of diffuse light conditions, caused by the lack of contrasting objects against the snow, made it impossible for the pilot to maintain an accurate altitude or position relative to the ground.
- The impact occurred with the front of the skids, leading to the immediate destruction of the cabin structure.