What happened
On April 14, 1971, an AS-K 13 glider, registration HB-926, was conducting a passenger flight along the Jura mountains when it encountered severe turbulence. While flying east along the Bettlachstock at approximately 1,200 meters, the aircraft was flying at a speed of roughly 78 km/-h. As the glider crossed a terrain ridge on the eastern flank, it entered a zone of turbulence and experienced a sudden loss of control, slipping toward the slope.
In an attempt to recover the aircraft, the pilot executed a right-hand climbing turn to move away from the terrain. During this maneuver, the left wing and the underside of the fuselage struck a fir tree located near a depression in the landscape. Despite the impact, the pilot managed to navigate the damaged aircraft back to Grenchen airfield, performing an emergency landing in a field approximately 400 meters west of the main runway. There were no injuries to the pilot or the passenger, though the aircraft sustained approximately 40% damage.
The investigation
The investigation, opened at Grenchen airfield, examined the pilot's qualifications, the aircraft's mechanical state, and the environmental conditions at the time of the accident. Investigators reviewed the pilot's flight logs and the specific authorization for passenger flights. They also analyzed the flight dynamics by conducting a control flight with identical weight and center of gravity configurations to determine the aircraft's stability and stall characteristics.
Findings
- The primary cause of the accident was that the aircraft entered a turbulence zone at an insufficient airspeed, which led to the slip and subsequent impact.
- The pilot was flying at a speed suitable for minimum sink in calm air, but failed to increase speed to provide a safety margin for the expected turbulence near the ridge.
- The passenger was permitted to take control of the aircraft at various intervals, which contributed to the lack of necessary precautionary speed adjustments.
- The pilot was not legally authorized to carry passengers at the time of the accident, as he had not yet met the required minimum of six hours of glider flight time and 20 landings within the preceding 12 months.