Introduction
On Monday, 19 July 1965, at 12:42 MEZ, a tow plane consisting of a Piper J3C Cub HB-OBV and a DFS Meise HB-389 glider departed from Sitten airfield. The first minutes of flight were normal. At 12:50, the tow plane, flying east along the slope at relatively low altitude near Ayent, initiated a turn away from the slope but continued with a slight descent, struck a house, and caught fire. The glider pilot had released before impact and executed a forced landing in the vicinity.
Pilot and Aircraft
The pilot of HB-OBV was born in 1901 and held a private pilot license issued on 20 December 1948, with a towing endorsement valid until 1 May 1966. His total flight experience was approximately 300 hours and 1,100 flights, of which 245 hours and 980 flights were in powered aircraft, mostly on the accident type. His last medical check-up was on 2 May 1964.
The Piper J3C Cub (serial number 12.232, built 1944) was a single-engine high-wing monoplane with a Continental C-90-8F engine rated at 90 hp. Its maximum takeoff weight was 550 kg; weight at the time of the accident was 475 kg. The aircraft was equipped with a standard towing device and a 35-meter nylon tow rope. The tachometer had been inoperative before the flight; no other technical defects were found.
The glider HB-389 was a DFS Meise, single-seat shoulder-wing wooden aircraft built in 1945.
Terrain and Weather
The accident occurred about 5 km northwest of Sitten on the slope of Ayent, near the Blignou district, at coordinates 597.050/124.600, elevation 992 m. Weather was fair with 3/8 altocumulus clouds base 2,300 m, wind 10–20 kt from the southwest, temperature +25 °C at 1,000 m.
Flight and Accident Sequence
At around midday on 19 July 1965, the glider pilot intended a performance flight with a tow start. The tow pilot, who had just returned from a flight, volunteered after breakfast. The tow plane was handed over with about 30 liters of fuel and the note that the tachometer was inoperative.
Takeoff from grass runway 26 at Sitten was at 12:42. The planned release area was the Creta Besse ridge north of the airfield. After a normal initial climb, the tow pilot initiated a right turn slightly earlier than planned and flew east at relatively low altitude over the northern valley side, climbing slowly with good speed. Over Ayent, he began a turn away from the slope to the south. The glider pilot expected a turning climb down the valley, but the tow plane continued turning and began to dive toward the slope. The glider pilot released and flew away from the slope.
The tow plane continued its descending turn, struck a power line, then a chalet, and came to rest burning in the garden at about 12:51.
Damages and Injuries
The tow pilot was found dead at the scene; the glider pilot was uninjured. The tow plane HB-OBV was destroyed; the glider HB-389 was undamaged. Property damage to the chalet was about 5,500 CHF, and to the power line about 300 CHF.
Investigation Findings
Examination of the wreckage showed no damage that could not be explained by impact or fire. The controls functioned properly; the propeller was rotating at impact (exact rpm undetermined); the throttle was at full (deformed and locked); the fuel cock was open; the tow release mechanism worked correctly and the tow rope lay released beside the aircraft.
Autopsy of the pilot revealed extensive arteriosclerosis of the coronary arteries (with severe narrowing in peripheral areas), carotid arteries, and aorta; a fresh hemorrhage was found in an atherosclerotic plaque of a coronary artery. The heart muscle showed increased connective tissue. Medical records since 1949 noted shortness of breath after exercise and, since 1951, signs of diabetes mellitus treated with tablets, though metabolic control was suboptimal due to patient non-compliance.
Cause
The accident was attributed to the occurrence of an acute heart crisis of the pilot.