What happened
On March 10, 1963, a pilot was conducting training flights from the Wangen-Lachen airfield using a Piper Cub J3C, registration HB-OBP, which was equipped with wooden skis for snow operations. Following a standard landing, the pilot attempted a go-around. During this maneuver, the aircraft crossed a depression in the snow cover caused by melting over a buried ditch.
While the takeoff appeared normal initially, the aircraft reached an altitude of approximately 15 to 20 meters before the pilot and observers noticed intense vibrations in the right ski. The tip of the right ski began oscillating upward, reaching near the cockpit level. The pilot immediately initiated a wide left turn to attempt a landing in the opposite direction. During the subsequent landing attempt, the right ski dug deep into the snow. This caused the ski and its associated landing gear components to be largely torn away. The aircraft continued sliding on the left ski, veered sharply to the right, and eventually came to rest on its left wing and top surface.
The investigation
The investigation focused on the structural integrity of the ski attachment system. The aircraft was fitted with a custom-built ski arrangement consisting of wooden skis, a rear wire cable, and a forward bungee cord with a secondary tether. Investigators examined the wreckage of the right-side attachment components and found that the rear tether had failed due to a tensile break. Furthermore, the forward attachment points had also failed; specifically, the carabiner hooks were found to be broken. The investigation also noted that the strength of these carabiner hooks was only about 30% of the breaking strength of the cables they were meant to secure.
Findings
- The primary cause of the accident was the failure of the right ski attachment components due to excessive stress.
- The initial failure of the rear wire cable was triggered by the aircraft crossing the uneven terrain (the snow-covered ditch) during the go-around.
- Subsequent aerodynamic forces acting on the aircraft after takeoff caused the remaining forward attachment elements to break.
- The investigation noted that the carabiner hooks used in the assembly possessed insufficient strength relative to the cables.