What happened
On May 17, 1964, at approximately 12:20 PM, a pilot departed from Grenchen airfield in a Ka-8 glider, registration HB-620, for a duration flight along the Jura ridges. After roughly 95 minutes of flight, the aircraft was flying east at an altitude of approximately 700 to 800 meters.
As the pilot attempted to initiate a right turn to return to Grenchen, the aircraft encountered sudden turbulence. The pilot noted a sharp drop in control pressure and attempted to maneuver away from the ridge; however, the aircraft instead turned toward the slope. At 2:10 PM, the left wing of the Ka-8 struck the treetop of a fir tree protruding from the forest on the southern slope of the Bettlachstock. The glider continued to glide over several treetops before crashing nose-first into a forest clearing.
The investigation
The investigation focused on the pilot's flight parameters and the environmental conditions at the time of the accident. The pilot, who had approximately 18 hours of total flight experience, had been maintaining a speed slightly above 75 km/h to account for observed gusts. Investigators examined the aircraft's performance, noting that the Ka-8 is known for having docile handling characteristics. The investigation also reviewed the terrain, which consisted of the steep, forested southern slope of the Bettlachstock, and the weather conditions, which included a west-southwest wind of 5 to 10 knots with gusty south-southwest upcurrents near the ridge.
Findings
- The pilot was maintaining a flight speed that was too low relative to the intensity of the gusts and the proximity to the ridge.
- A sudden loss of control pressure occurred, and the pilot's attempt to use ailerons to turn away from the slope likely caused the airflow to stall on the wing closest to the ridge.
- The pilot was relatively inexperienced, with only about 18 hours of total flight time.
- The aircraft sustained approximately 70% damage, but the pilot remained uninjured.