Aircraft and Occupants
The accident aircraft was a Jodel D-11-2, registration HB-SUO, built in 1956 with serial number 401-4. It was powered by a Continental C90-8F engine and had a fixed tailwheel landing gear. At the time of the accident, the aircraft weighed approximately 513 kg, within the maximum allowed weight of 620 kg. The center of gravity was also within limits.
The aircraft was occupied by two pilots. The pilot in the left seat was born in 1921 and held a private pilot license (issued 4 April 1963, first issued 13 March 1949) and a restricted commercial pilot license (issued 23 August 1962), both valid until 25 November 1971. He had endorsements for towing and limited radiotelephony. His total flight experience was 589 hours, including 82 hours on the Jodel D-11-2. The pilot in the right seat was born in 1941 and held a commercial pilot license (issued 10 December 1970, first issued 11 November 1968) valid until 15 October 1971, with endorsements for night flight, aerobatics, and instrument flight. He also held a provisional flight instructor certificate. His total flight experience was approximately 1535 hours, including 272 hours on light aircraft and 13 hours on gliders. No flights on the Jodel type were recorded in his logbook. Both pilots had valid medical certificates and no accident or violation history.
Winch and Glider Operations
The winch used for glider towing was owned by the Segelfluggruppe Cumulus, Winterthur. It was a double-drum motor winch mounted on a light Opel truck, powered by a GMC Oldsmobile Rocket V8 engine. The winch cables were 1250 m long each, with a diameter of 4.2 mm. The glider towed was a Schleppflugzeug (presumably a glider) with registration HB-835. The winch was positioned about 50 m outside the eastern airfield boundary marker, and the glider's takeoff point was about 1000 m from the winch, approximately 100 m west of runway 10 threshold.
Sequence of Events
On Saturday, 10 July 1971, the pilot of HB-SUO departed from Sitterdorf airfield at 1530 hours local time for a flight to Amlikon. Upon arrival, the aircraft landed at 1545 hours. Shortly after, the pilot took off for the return flight to Sitterdorf. During the initial climb, at an altitude of approximately 40 to 80 meters above the airfield boundary, the aircraft collided with the falling cable of a glider that had been launched by winch towing just before. After the collision, the Jodel entered a near-vertical descent and struck the ground in the bed of the Thur River at 1548 hours. A fire broke out immediately upon impact, destroying most of the aircraft.
The winch operator, born 1950, held a glider pilot license and had been serving as a winch operator since 1967 with no previous incidents except routine cable breaks. The flight director, born 1934, held a glider pilot license and a glider instructor certificate.
Weather Conditions
At the time of the accident, the weather was reported as: cloud cover 1/8, with cumulus clouds base at 2200 m MSL; visibility 25 km; surface wind from the northeast at 10 knots; light turbulence due to bise and thermals; temperature 30°C; relative humidity 35%; QNH 1020 mb; pressure at field elevation 970 mb. Sun elevation was 45°, azimuth 255°.
Regulations
The accident occurred during a private VFR flight from Amlikon to Sitterdorf. The airfield at Amlikon was operated by the Segelfluggruppe Cumulus, which was conducting winch towing operations at the time. The relevant regulations included rules regarding collision avoidance and the obligation of pilots to observe other traffic in the vicinity of an airfield.