Accident Overview
On Whit Sunday, 17 May 1964, at approximately 1220 Central European Time, a glider pilot departed from Grenchen airfield in a K-8 glider registered as HB-620 for a cross-country flight along the Jura slopes. Around 1410, while flying near the south slope of Bettlachstock at an altitude of 1050 meters above sea level, the glider's left wing struck a fir tree protruding from the mixed forest. The aircraft then slid over several treetops before crashing forward into a forest clearing. The pilot emerged uninjured, but the glider was 70% destroyed.
Pilot Information
The pilot, born in 1944, held a glider pilot license issued on 17 May 1963, valid until 19 November 1965. He began glider training in April 1962 in Grenchen. His total flight experience amounted to approximately 18 hours and 110 flights, of which 3 hours and 39 minutes (12 flights) were on the accident type. He had completed two flights exceeding one hour in duration. The investigation found no evidence of any health issues at the time of the accident.
Aircraft Details
The glider was a K-8, serial number 506, built in 1959 by Alexander Schleicher in Poppenhausen, West Germany. It was a single-seat high-wing aircraft of mixed construction. The owner and operator was the Segelfluggruppe Grenchen. The airworthiness certificate was issued on 14 March 1959 and valid until 5 March 1965. The maximum permissible takeoff weight was 310 kg; the weight at the time of the accident was about 280 kg. The center of gravity remained within prescribed limits throughout the flight. The airspeed indicator markings were: red at 55 km/h (minimum speed), yellow at 60 km/h (minimum sink rate), white at 75 km/h (best glide angle), and orange at 190 km/h (maximum speed). The aircraft was known for its docile handling characteristics.
Weather Conditions
On the day of the accident, fine weather prevailed in the area, with 3/8 coverage by cirrus clouds at about 8000 meters and visibility of 30 km. In the free airspace between 500 and 2000 meters, winds were from the west-southwest at 5 to 10 knots. At the Bettlachstock slope, a gusty upslope wind from the south-southwest at 5 knots was present.
Flight Sequence and Accident
After takeoff at 1220, the pilot conducted a soaring flight. About 95 minutes later, he was flying eastward in the Grenchen-Solothurn area with an altitude margin of 700 to 800 meters above the terrain. Having experienced several strong gusts and noted fluctuations on the airspeed indicator, he maintained a speed slightly above the white mark, i.e., above 75 km/h. The rate of climb was about 0.5 m/s. He then intended to initiate a right turn to return to Grenchen airfield. Before he could execute this, he suddenly felt a sharp decrease in control pressure. He immediately pushed the nose down and attempted to turn right away from the slope, but the glider yawed left. He was unable to recover before the left wing struck a fir tree tip. The aircraft then slid over several treetops and crashed forward into a clearing.
Damage
The pilot sustained no injuries. The glider was 70% destroyed.
Probable Cause
The accident was probably caused by the relatively inexperienced pilot losing control during slope soaring due to gust influences, as he maintained a flight speed too low relative to the gustiness and his proximity to the slope.