Accident Sequence
On 11 September 1960, at approximately 1049 Central European Time, the pilot departed from Sion in the Bücker 131 Jungmann, registration HB-UTV, for a planned one-hour flight to the Upper Valais region. The flight plan indicated a route via Brig, Ernergalen, Blindenhorn, Tumetschhorn, and Saflischhorn. Shortly after 1100, witnesses observed a biplane performing two loops over the Vispertal before continuing up the valley. At about 1120, the aircraft entered the Binntal from the Rhone valley. After flying over Ausserbinn, it appeared over the Eggenalp at an altitude of approximately 1700 m above mean sea level. Over the alp, the pilot executed several steep turns at very low height above ground, repeatedly changing engine rpm and rocking the wings. At approximately 1127, while in a right turn at an altitude of not more than 50 m above ground, the aircraft descended steeply at an angle of at least 50 degrees with considerable engine power. A witness described the final maneuver as an abrupt stall. The onboard clocks stopped at 1127, and both airspeed indicators were stuck at 160 km/h.
Investigation
The investigation was conducted by the Swiss Federal Commission for Aircraft Accident Investigation. The pilot held a private pilot license issued on 4 March 1951, valid until 5 June 1961. He had a total flight experience of 205 hours and 10 minutes across 850 flights. In 1960, he had flown only 3 hours and 33 minutes in 15 flights, with the last flight on 29 June. On the accident aircraft type, he had accumulated 23 hours and 3 minutes in 87 flights over six years. His flight training record noted some difficulties transitioning to the Cessna 170, but otherwise his qualifications were unremarkable. The pilot had previously engaged in unauthorized low flying in 1957 and acrobatics in 1959. No health issues or suicidal intent were found.
The aircraft was a Bücker 131 Jungmann, serial number 88, manufactured by Dornier in 1939. It was owned by the Valais section of the Aero Club of Switzerland. The aircraft was equipped with a 105-horsepower Hirth HM504-A2 engine and a wooden K+W propeller. At the time of the accident, its weight was approximately 540 kg, within the maximum allowable 670 kg, and the center of gravity was within limits. The total operating time since new was 1354 hours and 56 minutes, with 13 hours and 19 minutes since the last major inspection. No technical defects were found that could have contributed to the accident.
The accident site was on the Eggenalp near the hamlet of Zenhäuser, at coordinates 654.720/136.720, elevation 1660 m. The terrain consisted of a spur formed by the confluence of the Rhone and Binna rivers, with forested slopes on both sides. Weather on the day was fine with high pressure over the Alps, good visibility, light northeast wind above 2000 m, and local thermal turbulence below.
Probable Cause
The accident occurred because the pilot lost control of the aircraft while executing steep turns at low altitude in terrain with restricted visibility. No specific reason for the pilot's behavior could be determined.