What happened
On the afternoon of July 14, 1963, a Bü-131 Jungmann aircraft, registration HB-UTG, departed from Grenchen airfield for a private recreational flight accompanied by one passenger. During the return leg of the flight, the aircraft began experiencing intense engine vibrations. Fearing a potential crankshaft failure and seeking to prevent further engine damage, the pilot executed an emergency landing in an open field near Selzach, adjacent to the Solothurn-Grenlychen highway. The landing was completed successfully at approximately 15:55 local time without further incident.
The investigation
The investigation focused on the mechanical integrity of the HM 504-A2 engine. While the engine had undergone a major overhaul in October 1960, which included the installation of new original connecting rod bolts, investigators examined the component failure in detail. The inquiry also reviewed the maintenance logs to ensure compliance with recent regulatory updates issued by the Federal Office of Civil Aviation regarding component life limits.
Findings
Technical analysis of the engine revealed a broken connecting rod bolt at piston bearing number four. The investigation found no evidence of external factors or improper maintenance contributing to this specific failure. Although the engine had accumulated 243 hours and 12 minutes of total flight time since its last overhaul, the calculation of operational hours—which applied a fourfold multiplier to the 25 hours and 35 minutes of aerobatic flight—resulted in a total of 319 hours and 57 minutes. This was well within the 400-hour limit mandated by technical directive 721-504 for replacing these specific bolts. The investigation concluded that the pilot acted appropriately, and the emergency landing prevented more significant damage.
Safety action
There were no injuries to the occupants, and the Bü-131 Jungmann sustained no damage during the landing. No injuries or property damage were reported.