What happened
On the morning of November 12, 1960, a Piper Super Cub HB-OPM was performing commercial transport duties near the Ar du Tsan plateau in the Val de Réchy. The flight involved transporting passengers to the high-altitude plateau for instrument inspections.
Earlier that morning, the aircraft had successfully completed a flight from Sion to the plateau and back. However, during the return leg in the afternoon, the situation changed. After picking up a passenger at 12:16, the pilot attempted a takeoff at 12:30. Unlike the morning departure, the aircraft failed to lift off during the sloped portion of the runway. Instead, it continued sliding across approximately 100 meters of fresh, undisturbed snow on the horizontal section of the plateau.
As the aircraft approached the edge of a cliff, it briefly left the ground but immediately sank. The left ski struck a snow mound, and the right ski violently hit a rock. The left wing subsequently struck a small larch tree, causing the aircraft to fall approximately 30 to 40 meters and become lodged in the trees at the base of the cliff. The impact resulted in the destruction of the aircraft and left two occupants with serious injuries.
The investigation
The investigation examined the flight history, the mechanical state of the aircraft, and the environmental conditions at the time of the accident. The aircraft, a Piper Super Cub HB-OPM, was noted to have slightly lower performance capabilities compared to other aircraft in the fleet due to its specific equipment.
Investigators compared the afternoon accident to a successful takeoff performed by a different aircraft, an HB-OPE, earlier that same day. They also analyzed the temperature fluctuations in the region, noting that temperatures had risen significantly between the morning and afternoon flights.
Findings
- The primary cause of the accident was insufficient airspeed at the end of the runway, as the aircraft's acceleration was significantly hindered by the increased friction of the snow.
- A rise in temperature during the day caused the snow surface to soften and become heavier, creating much higher resistance than the firm snow present during the morning flight.
- The pilot was unable to reach the necessary takeoff speed on the sloped portion of the runway and was forced to rely on the horizontal section, where the heavy snow further impeded progress.
- The pilot faced a difficult decision: attempting to extend the takeoff run risked overshooting the plateau and sliding down the cliff, leading to the decision to continue the maneuver on the available surface.