Aircraft and Pilot
The aircraft was a Jodel D-11 single-engine low-wing monoplane (manufacturer's serial number 525-6, built in 1956) owned by the Flugsportgruppe Zürcher Oberland. It was powered by a Continental C85-8 engine of 85 hp and a McCauley propeller. At the time of the accident, the aircraft had accumulated about 1500 flight hours and 5800 landings. The weight and balance were within the prescribed limits.
The pilot, born in 1914, held a private pilot license issued on 26 October 1959. He had approximately 124 hours of total flight experience, of which 6 hours were on the Jodel D-11. His last flight on this type before the accident was on 25 February 1967, when he completed six circuits after a hiatus of over three years. His last medical examination on 15 September 1965 revealed no abnormalities. An autopsy showed severe coronary artery disease and arteriosclerosis. Toxicological tests for alcohol and carbon monoxide were negative. The cause of death could not be determined with certainty due to both severe traumatic injuries and the possibility of acute heart failure.
The passenger, born in 1929, had no flying experience.
Accident Sequence
On 4 March 1967 at 0956, the pilot and passenger departed Speck-Fehraltorf for a local sightseeing flight. They returned around 1100 and entered the traffic pattern for a landing on runway 12. A student pilot in a Cessna 150 (HB-CBL) was also in the pattern. Witnesses reported that after turning onto the base leg, the Jodel appeared to reduce speed. Without any abrupt maneuver, the aircraft banked left and entered a spin from about 150 meters above ground. It completed two to three turns before impacting the ground near the road at Rüti, Fehraltorf, at approximately 1102. The engine was heard idling before the stall and then accelerating. No evasive action was observed.
The student pilot, approximately 200 meters away, saw the Jodel flying slowly. Another pilot observed the Jodel reduce speed, possibly to give way to the Cessna, and then suddenly bank left.
Investigation Findings
Examination of the wreckage revealed no pre-existing technical defects. The fuel system was intact: the rear tank was empty, and the front tank contained fuel residues. The throttle was found in the full-power position, and the ignition was set to magneto 1. The propeller showed signs of engine power at impact. The engine teardown found no anomalies.
The aircraft's minimum speeds were 55 km/h in straight flight, 60 km/h at 30° bank, and 66 km/h at 45° bank. Stall characteristics were normal, but the aircraft showed an increased tendency to stall in poorly flown turns.
The accident was highly probably caused by the aircraft falling below minimum speed during the landing approach.