Incident
On 14 August 1971, at 1218 hours MEZ, a glider pilot departed Zweisimmen airfield on a local flight under tow. After releasing at 1800 m/M west of the Niderhorn (2078 m/M), the pilot climbed circling to approximately 2100 m/M. Strong northwest winds carried the glider over the Niderhorn ridge into the rear Diemtigtal valley, where it encountered strong downdrafts. The pilot attempted to fly eastward to gain altitude but was unsuccessful. As rain squalls entered the valley, he decided to make an outlanding on the south slope northeast of Enetchirel (municipality of Diemtigen/BE).
The landing approach was made on a heading of 075° in strong gusty conditions. About 150 meters after crossing a telephone line, the glider touched down on sloping terrain with a gradient of approximately 1:10 in the landing direction. After a ground roll of some 150 meters, the left wing contacted the ground, causing the glider to turn left. At 1250 hours, the tail struck a barbed wire fence. The pilot remained uninjured, but the glider sustained heavy damage. There was no third-party damage.
Aircraft and Pilot
The glider was a K 8 B, registration HB-764, with a valid airworthiness certificate. The pilot, born in 1913, held a valid glider pilot license with passenger endorsement and had accumulated 280 total glider flight hours, including 78 hours and 30 minutes on the accident type. No evidence suggested any pre-existing health impairment. The pilot had two previous landing approach accidents in 1955 and 1968 (Report No. 445 of the Swiss Accident Investigation Commission).
Weather
According to the Swiss Meteorological Institute (MZA), the accident area had total cloud cover of 6-7/8 eighths, with 3-5/8 eighths of cumulus and cumulonimbus, base 2000-2500 m/M. Precipitation in the form of showers or thunderstorms began around 1215 hours and moved northeast after approximately 1245. Visibility outside precipitation exceeded 20 km; within showers it was about 5 km or less. Wind direction was dependent on the current shower or thunderstorm position. Gusts of 15-20 knots were likely with thunderstorms, possibly up to 30 knots. Turbulence was light thermal, becoming strong near cumulonimbus. At the accident site, an east wind of 6-9 knots was reported.
Investigation Findings
The investigation found no evidence of any pre-existing technical defects in the aircraft. The pilot acted imprudently by allowing north winds to drift him into the Diemtigtal valley. His decision to land downhill despite the considerable slope was objectively wrong. Under the given circumstances, the pilot appeared to be overtasked in flying and flight tactics.
Conclusion
The commission unanimously concluded that the accident resulted from the pilot continuing the flight into an area with unfavorable flight conditions, being forced to make an outlanding in difficult terrain, and colliding with a fence due to improper landing tactics.