1972 Grenchenberg glider accident involving K8B HB-884

Accident overview and flight progress On 18 June 1972, a K8B glider with registration HB-884 was involved in an accident near Grenchenberg in the Swiss canton of Solothurn. The pilot took off from Grenchen airfield at 1152 local time in an aerotow. After nine minutes, the glider released at 1,200 m QFE over the southern slope of the Grenchenberg. The aircraft then descended to approximately 750 m QFE, a height the pilot maintained for more than two hours by ridge soaring. Around ten other gliders were operating in the same area. ## Sequence of events At about 1430, the pilot was flying on a heading of 235°, approximately 300 m from a wooded mountain slope. North of Lengnau, he decided to turn left, away from the slope, to establish an opposite heading. At that moment, he noticed another K8 glider approaching head-on at the same altitude, offset 100 m to his left. He continued straight. Shortly afterward, HB-884 suddenly turned right to a bank angle of 30°. Unable to stop the movement, the pilot decided to continue the right turn toward the slope. The glider then lost altitude rapidly, struck trees with its right wingtip, and crashed into the forest at 1435. ## Damage and injuries The pilot was not injured. The glider was heavily damaged. Third-party damage on the ground was described as minor. ## Findings The pilot, born in 1955, held a valid glider pilot certificate. His total flight experience was about 40 hours, including about 21 hours of powered flight, with 8 hours 15 minutes on the accident type. In the preceding 90 days, he had flown 10 hours 39 minutes in gliders, including 6 hours 50 minutes on the accident type. The investigation found no indication of health problems at the time of the accident. The Swiss Federal Air Office records showed no accidents or other incidents. The aircraft was airworthy and approved for operation, with no evidence of pre-existing defects. The pilot reported that the aircraft had responded normally throughout the flight. ## Weather and pilot account According to the MZA, at the time of the accident at 1,000 m MSL there was a southwest wind of 10 kt. Visibility was 10–30 km, with 1/8 cumulus cloud with a base around 1,600 m MSL and 4 to 6/8 cirrus above. The weather development included cirrus building from the WNW and weakening convection. The pilot attributed the uncommanded right turn after passing the other K8 to a gust. He said he was flying at about 75 km/h and attempted to restore normal attitude with a brief left control input, without success. ## Assessment The uncommanded right turn may have been caused by a gust. It was probable that the aircraft speed was insufficient at that point. The pilot’s decision, after the first unsuccessful correction, to continue the uncommanded right turn toward the slope was judged to be incorrect.