What happened
On April 23, 1972, a Pilatus Porter (registration HB-FAD) departed from Sion as part of an irregular air transport operation for Air Glaciers S.A. The flight was carrying seven passengers, all of whom were skiers, to the Trient Glacier. Following two preceding aircraft that had already completed their drops, the pilot approached the landing zone by flying along the slope from north to south before executing a 90-degree right turn to align with the terrain.
During the short final phase of the approach, the pilot struggled to accurately judge the distance to the ground due to a lack of visual contrast in the terrain. This resulted in an undershot landing, where the aircraft performed the flare at too high an altitude before sinking into the snow. The impact was severe enough to cause the right landing gear to collapse, bringing the aircraft to a halt at an elevation of 3,360 meters.
The investigation
The investigation confirmed that the pilot was a professionally licensed aviator with extensive mountain flying experience, including 1,180 mountain landings and significant recent time in the Pilatus Porter. There were no indications of health issues or prior regulatory infractions. The aircraft, HB-FAD, was found to have valid airworthiness certificates, and the weight and balance were within prescribed limits.
Meteorological conditions at the time of the accident involved variable cloud cover and a light barrier effect on the southern side of the Alps. Notably, the Trient Plateau was in shadow during the approach, which contributed to the difficult lighting conditions.
Findings
- The primary cause of the accident was a pilot error in distance estimation caused by diffuse lighting and a lack of terrain contrast.
- The landing gear failure was a direct result of the heavy impact during the landing in the snow.