Flight
On Saturday, 1 September 1973, at 1557 hours MEZ, a pilot flying a Bücker Jungmann (registration HB-UUH) departed for an aerobatic performance at the Dittingen airfield as part of the Dittingen airshow. Starting at approximately 500 meters above the airfield, he performed the following sequence: loop, left spin of two turns, loop, right spin of two turns, left renversement, left roll, two left Immelmann turns, and a final left renversement. Immediately after passing the apex of this last figure, the aircraft entered a left spin. After about two to two and a quarter turns, the pilot managed to stop the rotation, but due to insufficient remaining altitude, a complete recovery was impossible. At 1608 hours, the aircraft struck the ground at a shallow angle on sloping terrain north of the runway, collided with a telephone pole, rotated 180 degrees about its vertical axis, and came to rest after approximately 10 meters.
Pilot
The pilot, born in 1929, held a valid private pilot license with endorsements for aerobatics and radiotelephony, as well as a limited commercial pilot license. He also possessed a special permit A authorizing aerobatics below the legal minimum altitude, allowing maneuvers down to 200 meters above ground. His total flight experience was 528 hours and 39 minutes, with 35 hours on the accident type. In the preceding 90 days, he had flown 42 hours and 46 minutes, of which 10 hours and 49 minutes were on the accident type. No evidence of health issues at the time of the accident was found. A forensic examination by the University of Basel's Institute of Forensic Medicine revealed a blood alcohol concentration of 0‰.
Aircraft
The aircraft was airworthy and certified for flight. No pre-existing technical defects were identified. The maximum permissible takeoff weight was 610 kg; the actual weight at the time of the accident was 560 kg. The center of gravity was 0.51 meters behind the reference plane, within the permissible range of 0.47 to 0.72 meters.
Weather
At the time of the accident, wind was light, predominantly from the west-northwest. Visibility was 20 to 30 kilometers. Cloud cover was 1/8 cirrus or altocumulus. QNH was 1025 mb, air temperature 22°C, and dew point 13°C.
Investigation Findings
The pilot reported that during the final renversement, his right foot slipped off the rudder pedal and became jammed. Subsequently, the aircraft entered a left spin. After approximately 2.5 to 3 turns, he freed his foot and recovered the spin at about 25 to 30 meters above ground.
Film footage of the accident showed that the last renversement was initiated at a low speed. After passing the apex, the aircraft fell into a left spin, completing two to two and a quarter turns. During the second turn, control surface deflections were clearly visible: the rudder was slightly deflected to the right, while the ailerons and elevator indicated that the control stick was pushed to the right and pulled fully aft.
Witnesses reported that the last renversement began at a height of 150 to 200 meters above ground, with the apex at 200 to 250 meters. After the apex, the aircraft entered a left spin with the engine at high RPM.
Runway thresholds at Dittingen airfield were at 558 meters AMSL for runway 11 and 532 meters AMSL for runway 29. The pilot had set his altimeter to 558 meters.
The organization, safety measures, and supervision of the airshow complied with regulations.
Assessment
The assessment noted that entering a spin shortly after passing the apex of a renversement is only possible if the elevator is pulled rapidly and firmly. If the elevator is held neutral, the aircraft will not enter a spin even if the rudder pedal is temporarily jammed. It is believed that the pilot, after recognizing the insufficient altitude at the renversement apex, reflexively pulled the elevator.