What happened
On July 20, 1967, a Bücker 131 Jungmann, registration HB-URP, was conducting a dual-control geography flight for the Swiss Flight School. The flight originated from Grenchen, traveling over the Jura mountains. During the flight, the aircraft experienced significant engine vibrations and a drop in RPM from approximately 2300 to 2000 near the Hohen Winde area.
As the aircraft progressed toward Lebern, the vibrations intensified, forcing the flight instructor to reduce throttle and shut off the ignition. Unable to reach Grenchen via gliding, the instructor performed an emergency landing on a field adjacent to the Oensingen-Niederbipp road. While the touchdown was successful, the aircraft's nose wheel brakes were unavailable to the instructor in the front seat during the rollout. Consequently, the aircraft struck a wire fence and subsequently capsized. Both occupants, the flight instructor and a student pilot, escaped the incident uninjured, though the aircraft sustained substantial damage.
The investigation
Investigators examined the mechanical state of the engine and the circumstances of the landing site. The technical examination revealed that the engine failure was caused by a crankshaft fracture. This failure occurred approximately 250 flight hours after the engine's last major overhaul. The investigation also noted that the landing area, while suitable for an emergency, was relatively short, which contributed to the difficulty of the rollout.
Findings
- The primary cause of the engine failure was a crankshaft fracture in the Hirth HM 504 engine.
- The aircraft's capsize was caused by the collision with a wire fence during the landing rollout.
- The lack of available braking control for the instructor in the front seat during the rollout contributed to the inability to avoid the obstacle.
- The engine type (HM 504) was known to have a history of crankshaft failures, leading to reduced operational intervals between overhauls.