Background
On the morning of Saturday, 12 November 1960, a Piper Super Cub PA-18, registration HB-OPM, operated by the Valais Section of the Aé.C.S., was used for a commercial flight to transport a passenger to the snow-covered plateau of Ar du Tsan in Val de Réchy, at an elevation of approximately 2200 m. The aircraft was equipped with skis and had a maximum allowable weight of 794 kg; at the time of the accident, its weight was about 760 kg, within limits.
Flight History
The pilot, born in 1920, held a restricted professional pilot license valid until 17 April 1961 and had logged 802 total flight hours, including 1625 glacier landings. The passenger, born in 1916, was an engineer for a hydroelectric construction company with no aviation background.
Two Piper Super Cubs, HB-OPM and HB-OPE, departed Sion at 0925 hours for Ar du Tsan. HB-OPM, known to have slightly inferior performance compared to HB-OPE, arrived a few minutes later and landed at 0940 in the tracks of the other aircraft. After dropping off their passengers, both aircraft returned to Sion at 0943, taking off downhill and covering about 160–180 m before becoming airborne.
At 1205 hours, both aircraft again departed Sion to retrieve their passengers. They landed at Ar du Tsan at 1216. The HB-OPM pilot did not stop the engine; after turning around, he applied full power, and the engine responded normally.
Accident
The aircraft began rolling in the landing tracks on the inclined part of the runway but did not become airborne as it had in the morning. It continued into fresh snow on the horizontal portion, sliding for at least 100 m before lifting off just before the cliff. Immediately after becoming airborne, it sank slightly and the left ski struck a small snow-covered bump. About 20 m further, the right ski violently hit a rock, then the left wing struck a small larch tree. The aircraft then fell approximately 30–40 m down the wooded cliff, ending up lodged in trees.
Damage and Injuries
The Piper Super Cub HB-OPM was destroyed by the impact. Both occupants were seriously injured: the pilot was hospitalized for five months, the passenger for four months, with full recovery expected during 1961.
Investigation
The investigation revealed that the morning's takeoff required about 160–180 m, but the afternoon departure of the lighter HB-OPE (pilot only, weight about 650 kg) needed about 200 m – indicating significantly degraded snow conditions. The HB-OPM, at full load with inferior performance, could not reach takeoff speed on the inclined section. It then entered the fresh snow on the horizontal part, where acceleration was even poorer. Although it finally lifted off, it could not sustain flight and contacted the ground again, leading to the inevitable collision.
Probable Cause
The probable cause was insufficient speed at the end of the runway because normal acceleration was impeded by heavy snow.