Accident Overview
On 3 May 1970, at 1327 hours local time, a Pilatus Porter aircraft registered as HB-FDU departed from Sion, Switzerland, with 7 skiers aboard. The flight was conducted under non-scheduled air transport operations by Air Glaciers S.A., based in Sion. The destination was the Glacier de Tsanfleuron in the canton of Valais. While attempting to land on the glacier, the pilot reported strong vibrations just before touchdown. The aircraft briefly pitched up, then from a height of about 3 meters fell onto its left ski, causing the left landing gear leg to break. The airplane then slid to the left, coming to a stop approximately 150 meters from the initial touchdown point at coordinates 583.060-130.180, elevation 2820 meters. At that time, the turbine was still running, but the propeller and its reduction gear were missing. During the landing, the pilot observed pieces of the propeller detaching.
Aircraft and Pilot Information
The pilot, born in 1935, held a valid pilot license and was authorized for professional mountain landings. His total flight experience was 3600 hours, including 1900 hours on the Pilatus Porter. In the preceding 90 days, he had flown 95 hours, 25 of which were on the Porter. There was no indication that the pilot's health was impaired at the time of the accident. However, records from the Federal Air Office noted several incidents and violations of legal regulations.
The aircraft, a Pilatus Porter HB-FDU, possessed valid certificates of airworthiness and registration. The investigation found no evidence of any pre-existing defects in the propeller or aircraft during the flight prior to the accident. Several propeller parts were recovered three months later. All three propeller blades showed similar deformation patterns and marks consistent with the breaking of control gears in the blade roots. Deformations on the engine attachments and the firewall supported the hypothesis of inadvertent contact between the propeller and the glacier snow. The tracks left by the aircraft were obliterated by rescue team movements and could not be reconstructed.
Meteorological Conditions
The weather at the time was characterized by 2/8 cloud cover with a base at 1600 meters and tops at 2500 meters, and 5/8 cloud cover with base at 4000-5000 meters. Visibility was greater than 20 km, but contrast was low over snow-covered surfaces due to the high sun. Wind at the foot of Becca d'Audon was from 340°-360° or 060°-080° at 20 knots, gusting to 30 knots. Turbulence was irregular, occasionally strong. Temperature was -2°C, relative humidity 80%. No airframe icing was reported. Atmospheric pressure was 1021 mb (QNH). Sun elevation was 55°, azimuth 210°.
Damage and Injuries
The pilot and all seven passengers were uninjured. The aircraft sustained severe damage. No other damage occurred.
Conclusion
The federal commission of inquiry concluded unanimously that the accident was due to a pilot error of distance estimation caused by diffuse lighting.