What happened
On July 6, 1976, a Standard Libelle glider, registration HB-999, departed Zweisimmen for a training flight. The pilot initially intended to fly toward the Valais region but abandoned this plan as cloud cover increased. After encountering thunderstorms over the Alpine chain, the pilot transitioned through the Simmental valley, seeking thermal lift.
While the pilot managed to gain some altitude near Boltigen, the onset of rain and unfavorable air currents caused a continuous loss of altitude. Despite being advised via radio to land in Zweisimmen, the pilot attempted to reach a specific landing field south of Boltigen. During the final approach, the aircraft's altitude became insufficient to clear a 30-meter construction crane. The pilot maneuvered around the crane, passing over high-voltage power lines, but was unable to maintain enough height to reach the intended field. The glider touched down on wet grass approximately 200 meters short of the target, sliding for 8 meters before colliding with a barbed-wire fence. The pilot suffered one fatal injury (serious facial injuries) and the aircraft sustained significant damage to the cowling, wing leading edge, and landing gear.
The investigation
The investigation confirmed that the Standard Libelle was airworthy and that the aircraft's weight and center of gravity were within legal limits. There were no evidence of mechanical failures contributing to the accident. Meteorological data indicated thunderstorm activity and rain showers in the area at the time of the event. The investigation also reviewed the pilot's flight experience and the regulatory requirements for glider training, which mandate that pilots fly around a landing site at a minimum altitude of 300 meters to inspect for obstacles such as fences, ditches, or terrain irregularities.
Findings
- The pilot attempted to continue the flight in search of thermals despite deteriorating weather conditions and increasing cloud cover.
- The decision to attempt a landing at the specific field was made too late, leaving insufficient altitude for a safe approach.
- The pilot failed to fly around the landing area to check for obstacles, a direct result of the lack of altitude reserve.
- The aircraft's descent was exacerbated by downdrafts and rain.