Accident Overview
On 20 July 1967, at 1539 MEZ, a Bücker Jungmann (Bü 131) aircraft, registered HB-URP, was involved in an accident near Niederbipp, Canton Bern, Switzerland. The aircraft was operated by the Swiss Air Transport School as a dual-control flight. The flight instructor and a student pilot were on board. During the flight, a total engine failure occurred due to a crankshaft fracture. The instructor took control and executed a forced landing in a field. Upon landing rollout, the aircraft's landing gear struck a wire fence, causing the aircraft to overturn. Both occupants were uninjured, but the aircraft sustained substantial damage.
Aircraft and Crew Information
The aircraft was a Bücker Jungmann (Bü 131), a single-engine biplane with fixed tailwheel landing gear. It was built in 1939 and owned by the Motorfluggruppe Biel of the Swiss Aero-Club. The engine was a Hirth HM 504 2A producing 105 hp. The aircraft's total time was 1682 hours and 25 minutes at the time of the accident. The engine had accumulated 235 hours since its last major overhaul.
The flight instructor, born in 1937, held a private pilot license issued in 1959 and a commercial pilot license issued in 1966, with an extension for training private pilots. His total flight experience was 1617 hours plus 404 hours in airline operations. The student pilot, born in 1944, held a private pilot license issued in May 1967 and had approximately 50 hours of flight experience.
Circumstances of the Accident
The dual flight was a geography training flight originating from Grenchen. While cruising over the Jura region near Hohe Winde at an altitude of approximately 1500 meters above mean sea level, engine vibrations occurred and engine speed dropped from 2300 to 2000 RPM. The instructor took over control and headed toward Lebern intending to return to Grenchen. Over Lebern, vibrations intensified, prompting the instructor to reduce throttle and switch off the ignition. Gliding to Grenchen was not possible, so he selected a field north of the Oensingen-Oberbipp road for a forced landing. The touchdown was successful, but during rollout, the wheel brakes were unavailable from the front seat, causing the aircraft to roll into a wire fence and overturn.
Technical Findings
Post-accident technical examination identified the cause of the engine failure as a crankshaft fracture. The fracture occurred approximately 250 operating hours after the engine's last major overhaul. The report notes that crankshaft fractures were not uncommon for the Hirth HM 504 engine, which was about 30 years old at the time. The engine's time-between-overhauls had been limited to 400 hours at the time of the accident, later reduced to 250 hours.
Probable Cause
The accident resulted from the aircraft colliding with a wire fence and overturning during the rollout of an otherwise successful forced landing. The forced landing was necessitated by a total engine failure caused by a crankshaft fracture.