What happened
On March 21, 2012, a commercial helicopter flight carrying four passengers and a mountain guide was conducting a heli-skiing operation near the Clariden Pass in Switzerland. The flight, operated by Heli-Linth AG, had departed from Tierfed to transport the group to the Claridenfirn glacier.
During the approach to the landing site, the pilot attempted to use a marker bag to establish a visual reference on the snow. While performing a second approach, a bank of fog moved in front of a rock face that the pilot had been using as a background reference. Due to the increasingly diffuse lighting and loss of visual contrast, the pilot decided to abort the landing and execute a go-around.
During the right-hand turn for the go-around, the front of the right skid made contact with the snow. The aircraft began to dig into the soft, powdery surface, causing the helicopter to tilt heavily. The rotor blades subsequently struck the snow, severing the tail boom and breaking parts of the structure. The cabin slid into the snow, resulting in the destruction of the Eurocopter AS 350 B3, registered HB-ZKK.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight path using data from a moving terrain device and passenger photographs. The investigation also reviewed the pilot's training records, the operator's flight manual, and statements from the crew and passengers.
Investigators noted that while the pilot had completed mountain landing training, he had relatively low flight experience in mountainous terrain. The investigation also looked into the safety procedures regarding passenger seatbelts and the decision-making process during the approach in deteriorating weather.
Findings
- The primary cause of the accident was the helicopter colliding with the snow layer following an aborted approach caused by diffuse lighting conditions.
- The pilot failed to interrupt the approach in a timely manner despite the deteriorating weather and light conditions.
- The pilot's low level of flight experience, particularly in mountain environments, contributed to the accident.
- The loss of a high-contrast background reference due to moving fog made it difficult to maintain visual altitude awareness.
- Several passengers and the mountain guide were not wearing seatbelts at the time of the impact.