What happened
On June 1, 1968, a flight instructor and a student pilot were conducting glacier landing training in a Piper PA-18 Super Cub, registration HB-OOV. The flight originated from Bex, with the initial intention of practicing landings at Wildhorn. However, due to fog obscuring the summit and the northern slope, the instructor diverted to the southern slope of Petersgrat.
Upon arriving at the site, the instructor observed a patch of clear terrain on the southern slope, below the ridge line. To assess the snow conditions, the pilot performed a reconnaissance pass, flying diagonally and nearly parallel to the ridge. During the turn to align for the landing, a sudden wind gust lifted the right wing of the aircraft. Despite the pilot's immediate application of counter-aileron, the left ski and wing caught in the snow, causing the aircraft to yaw violently around its vertical axis. The impact at relatively high ground speed resulted in the total destruction of the aircraft. Both occupants escaped the wreckage without injury.
The investigation
The investigation examined the weather conditions, the flight path, and the pilot's decision-making process. Investigators noted that while the instructor was highly experienced in glacier operations, the specific meteorological conditions at the time were significant. The investigation also reviewed the aircraft's configuration, noting it was equipped with Federal skis for snow operations, and confirmed that the weight and balance were within limits.
Findings
- The presence of fog on the northern slope and the ridge, while the southern slope remained clear, was a clear indicator of the Föhn effect. This phenomenon often produces sudden, gusty downdrafts along the slope.
- The instructor's decision to approach the landing site diagonally or parallel to the ridge—intended to avoid flying into the fog—unintentionally exposed the aircraft to the side-loading force of a gust.
- The primary cause of the accident was the combination of an inappropriate flight approach and the impact of a sudden wind gust.
Safety action
While the instructor's decision to attempt a landing was considered justifiable given the training objectives, the investigation highlighted the inherent risks of approaching a slope parallel to the ridge line when Föhn-induced turbulence is a possibility.