Summary
On Sunday, 10 March 1963, a Piper Cub J3C (registration HB-OBP), equipped with skis, was conducting practice flights from Wangen-Lachen airfield. During a go-around following a normal landing, the right ski became detached. The pilot initiated an emergency landing, during which the right ski dug into the snow, causing substantial damage to the aircraft. The pilot was uninjured.
Aircraft and Ski Equipment
The aircraft was a Piper Cub J3C (L-4H) with a Continental C-90-12-F engine. It was privately owned and operated. The ski equipment consisted of wooden skis, 150 cm long and 27 cm wide, mounted on the wheel axle. The skis were restrained by cables: a rear steel cable and a front rubber cord with a parallel steel cable, attached to the fuselage via hooks. The design allowed approximately 15° of movement. The ski equipment had been built in 1954 by the owner based on an American design and had been submitted for inspection by the Federal Air Office at that time, with no objections. However, no further official inspection or extension of the aircraft's approval for ski use was conducted. A crack on the right ski had been repaired in winter 1961/62.
Pilot Information
The pilot, born 1931, held a private pilot license since November 1961. Total flight experience was about 100 hours and 365 flights, including 13 hours on the accident type and 6:49 hours on aircraft with skis.
Terrain and Weather
The accident occurred at Wangen-Lachen airfield, which has a 440-meter grass runway 09/27. A brook covered with railway sleepers crossed the runway about 120 meters from the western edge. At the time of the accident, there was about 50 cm of snow on the field, with the snow above the sleepers melted to a depth of about 20 cm over a width of about 2 meters. The accident site was near the eastern edge of the field. Weather conditions: wind from the west at 10 knots, gusting 15-20 knots, visibility 50 km, scattered clouds at 1800 m, temperature +8°C.
Accident Sequence
At 1450, the owner and a friend conducted four flights together using the ski-equipped aircraft. At 1535, the friend began a solo flight. He took off on runway 09, flew a left circuit, and landed normally at 1539, touching down about 60-80 meters past the threshold. After rolling about 40-50 meters, he initiated a go-around. During this maneuver, the aircraft crossed the depression in the snow above the brook. The takeoff was normal. At about 15-20 meters altitude, severe oscillations of the right ski were observed from the ground and noticed by the pilot. The ski tip rose partially to cabin height. The pilot continued climbing to about 70-100 meters, flew a wide left circuit, and lined up for landing on runway 27. To minimize ski movement, he approached with a nose-high attitude and a slight left bank. About 50 meters before the runway threshold, at 1542, the right ski contacted the snow, dug in, and was largely torn off along with the landing gear. The aircraft slid on the left ski, turning right, and came to rest on its nose and left wing approximately 50 meters later, rotated about 45° to the right.
Damage and Findings
The aircraft sustained severe damage, particularly to the landing gear and left wing (approximately 30% value loss). Examination of the right ski attachment revealed: the rear cable had fractured due to overstress; the front cable was broken near the upper connection, the rubber cord was intact, and the carabiner hook was broken at the loop and extension. The sequence of these failures could not be determined. The strength of the cables and connections was satisfactory, but the carabiner hook had only 30% of the breaking strength of the cables. No ground damage occurred.
Probable Cause
According to the investigation commission, the accident was caused by the failure of a ski attachment element due to terrain-induced overstress during takeoff, followed by the failure of additional attachment elements under aerodynamic forces after lift-off.