1970 K8B Glider Accident Near Weissenstein, Switzerland (HB-958)

Accident Overview and Investigation Context On 23 July 1970, a single-seat glider, K8B HB-958, crashed into a wooded slope west of the Geissflue ridge near Weissenstein in the Swiss canton of Solothurn. The accident occurred during a glider flight and resulted in serious injuries to the pilot and destruction of the aircraft. The Swiss Federal Accident Investigation Commission issued its final report following a commission meeting on 26 March 1971. ## Aircraft and Pilot Details The aircraft was a K8B glider, serial number 8780, built in 1969 by Alexander Schleicher, Segelflugzeugbau Poppenhausen a.d. Wasserkuppe, West Germany. It was owned and operated by the Fluggruppe der Gewerbeschüler Zürich (FGSZ). The glider had a total operating time of 97 hours 53 minutes and 112 landings. Its airworthiness certificate was valid from 3 June 1969 to 31 March 1971. The pilot was a female glider pilot born in 1948. She held a glider pilot licence issued on 30 May 1960, valid until 6 June 1972, with aircraft types Rhönlerche, ASK-13, K-8, and K-7 recorded. She also held a special permission for passenger flights issued on 26 September 1969, valid until 6 June 1972. Her total flying experience since beginning training on 22 May 1966 at Birrfeld was 51 hours 30 minutes with 161 landings. In the three months before the accident, she had flown 11 hours 06 minutes in 12 flights, including 9 hours 08 minutes in 8 flights on the accident type. Her last medical examination was on 3 June 1970, and she was found fit. ## Flight Sequence and Accident The pilot took off from Grenchen airfield at 1130 hours by aerotow for a cross-country attempt, described as a 50 km Silver-C. She released approximately 950 m above Hotel Weissenstein QFE and continued westward to the height of Bettlacherstock. Another glider, HB-970, was also in the area, initially 100–200 m lower than HB-958. The two aircraft flew in the same area at the same altitude for about 15 minutes. The pilot of HB-970 observed HB-958 making turns toward the slope at relatively low speed and with large rudder deflection. He reported seeing HB-958 lose altitude rapidly in a left spiral, possibly a left spin. The pilot of HB-958 stated that she observed HB-970 and had the impression that it was closing in closely and flying faster than she was. To avoid a possible collision, she turned left away from the slope at about 70 km/h, which she described as relatively abrupt. During this steep turn, she lost altitude, which she attributed to a slight stall over the left wing. She recovered, flew back toward Geissflue, and regained altitude. Finding the weather conditions unsuitable for cross-country flight, she abandoned that intention. Over the ridge, she initiated an abrupt left turn and the aircraft entered a left spin. Because of insufficient altitude, she was unable to recover, and the glider crashed into the forest. ## Terrain, Weather, and Barograph Findings The accident site was at 1,290 m above sea level, on the steep north slope of the wooded ridge between Geissflue and Hasenmatt, on a hiking path about 8 m below the crest. Tree heights on the crest and slope were about 10 m. Meteorological data for 1415 hours in the accident area indicated total cloud cover of 1/8, wind from NW-N at 3–6 knots with local thermals, visibility 20 km, cloud layer 1/8 Cu with base at 2,000 m, temperature +28°C and 40% humidity at 500 m, +18°C and 70% at 1,500 m, QNH 1018 mb, and local moderate thermal turbulence near the ridge with eddies near the crest. The sun’s elevation was 54 degrees and azimuth 230 degrees. The barograph, a Peravia Bl-250 last calibrated on 13 May 1970, was undamaged and still running when the investigation leader arrived. Its record showed small altitude changes during the flight, indicating no strong updrafts or downdrafts. At 1237 hours, when the aircraft was at 1,230 m, a sudden altitude loss of about 55 m was recorded. After a flight time of 2 hours 28 minutes, at 1358 hours, following a weak climb, the crash occurred. The starting altitude was 1,355 m, with an altitude loss of about 65 m. ## Damage and Technical Findings The pilot was seriously injured, and the aircraft was destroyed. No significant third-party damage occurred on the ground. The wreckage showed the aircraft had crashed downslope, in a south-to-north direction, in a left turn with about 25 degrees longitudinal inclination, nose low, into the forest. At impact, the aircraft had a slight left bank. The cockpit was slightly bent to the left in the direction of flight, and the right side wall was strongly pushed outward. The wings and the right half of the horizontal stabilizer were destroyed. The brake flaps were retracted at impact. The ailerons, elevator, rudder, and trim tab were correctly connected and secured. Pushrods, control cables, and their guides showed no pre-existing defects. The landing gear, rear fuselage, and rudder were only partially damaged. The wreckage investigation found no indications of pre-existing technical defects. ## Probable Cause and Contributing Factors The accident was caused by the aircraft entering a spin due to abrupt rudder control at low airspeed and low altitude above the ground, after which it crashed into a forest. The aircraft’s centre of gravity was in the rear part of the allowable range, which, according to the flight and operating manual, can influence spin characteristics. The manual noted that a rearward centre of gravity makes it easier for the aircraft to enter a spin. The pilot was not able to recover from the spin within the available altitude.