Background
The accident involved a Grunau Baby II single-seat glider, registration HB-148, owned privately and operated by Segelfluggruppe Thun. The aircraft was built in 1935 and had a maximum tow speed of 120 km/h in calm conditions and a maximum flight speed of 150 km/h. At the time of the accident, its weight was 223 kg, within the maximum permissible 250 kg, and the center of gravity was within limits.
On 14 October 1967, an air office inspector noted two deficiencies: the safety coupling for winch launch needed replacement or removal per a technical bulletin, and the control handle markings were missing. These had not been rectified by the accident date.
The student pilot, born 1950, held a learner permit issued 3 August 1967 and had logged 6.03 hours total, with 31 minutes and 4 flights solo, all on HB-148. His instructor, born 1943, was a visiting instructor from Bern. The tow pilot, born 1922, held private and limited commercial pilot licenses and had previously held a glider license.
Incident
On 28 October 1967 at 1708 MEZ, the student took off in aerotow behind a Stinson HB-TRY. The plan was to release at 300 m AGL, fly left spirals, and perform a spot landing. The tow pilot reached the desired altitude above the airfield and wiggled the wings as a signal to release.
The student had his hand on the release handle and pulled immediately. The glider did not release, and speed increased, exceeding the maximum tow speed by at least 30 km/h. The student felt shaking and pulled back on the control stick. Suddenly, the glider released, pitched steeply, and entered a spin. The student jettisoned the canopy, released his seatbelt, and prepared to bail out. The spin stopped, but he nevertheless jumped and opened his parachute.
The pilotless glider entered a flat spin and descended near the parachute, striking the ground heavily.
Investigation
Post-accident inspection found that the release mechanism functioned normally when tested on the ground. The tow rope was in good condition with proper weak links. The safety coupling, which had been criticized, was not used during this flight; the ring coupling was employed and had not been faulted.
The student stated he was certain he pulled the correct release handle, despite the missing label. The instructor had earlier noted that the student tended to apply excessive rudder before release, a habit corrected during a dual check flight just before the accident.
The investigation considered that the student might have initially pulled the wrong handle or not pulled forcefully enough, but the exact reason for the delayed release could not be determined. The glider's behavior after release was attributed to the student's actions.
Probable Cause
The accident was probably due to the glider student not properly performing the release maneuver and subsequently behaving inappropriately in a spin.