Accident Overview
On Sunday, 14 July 1963, at 1515 Central European Time, the pilot of a Bü-131 Jungmann, registration HB-UTG, departed from Grenchen airfield for a private scenic flight with one passenger. During the return leg, the aircraft began to experience severe engine vibrations. Suspecting a crankshaft failure and aiming to prevent engine seizure, the pilot decided to perform a forced landing near Selzach, close to the Grenchen-Solothurn highway. The landing was executed without further incident.
Aircraft and Pilot Information
The aircraft was a Bü-131 Jungmann, a single-engine biplane with two tandem seats, of mixed construction. It was built in 1937 by Aktiengesellschaft für Dornier-Flugzeuge in Altenrhein, with serial number 14. The owner and operator was Fliegerschule Grenchen. The aircraft had a valid airworthiness certificate issued on 24 April 1963, valid until 11 September 1963. Weight and balance remained within prescribed limits throughout the flight.
The engine was a Hirth HM 504-A2, an air-cooled inverted four-cylinder inline engine producing 105 hp. It was built before 1939, with serial number 412/2691. The engine underwent a major overhaul in October 1960, during which all connecting rod bolts were replaced with new original bolts. Since the overhaul, the engine had accumulated 243 hours and 12 minutes of flight time (217:37 normal flight, 25:35 aerobatics).
The pilot was born in 1919 and held a restricted commercial pilot license issued on 15 August 1958, valid until 15 July 1963. His total flight experience was approximately 500 hours.
Flight and Forced Landing
After taking off from Grenchen, the pilot completed the scenic flight and was returning when the aircraft began to vibrate heavily. Believing the crankshaft might have broken and wanting to avoid an engine separation, the pilot opted for an immediate forced landing. He chose an open area near the highway at coordinates 602.550/228.370, elevation 430 meters, in the municipality of Selzach. The landing was completed successfully at 1555, with no damage to the aircraft or significant ground damage.
Investigation and Findings
Post-accident examination of the engine revealed a broken connecting rod bolt on the No. 4 connecting rod. No other anomalies were found that could explain the failure. The investigation reviewed maintenance and operational records and found no discrepancies that would have caused the bolt to break. Compliance with the Swiss Federal Air Office's Technical Bulletin No. 721-504, issued on 31 May 1963, was checked. This bulletin required replacement of connecting rod bolts for the HM-504 engine after 400 flight hours, counting aerobatic hours fourfold. The computed engine time since overhaul was 319 hours and 57 minutes (217:37 normal + 102:20 aerobatic, the latter being 25:35 multiplied by four), which was below the 400-hour limit.
Conclusion
The investigation commission unanimously concluded that the forced landing was necessitated by an engine failure resulting from a broken connecting rod bolt.