Aircraft and Pilot
The accident aircraft was a D.H.C.-1 Chipmunk, registration HB-TUF, built in 1950 (serial number CL-0139). It was powered by a four-cylinder Gipsy Major 10 Mk. 2 engine (serial number 10982, built 1951) producing 145 hp at sea level. The aircraft was owned and operated by Fliegerschule Birrfeld of the Aargau section of the Aero-Club of Switzerland. The pilot, born in 1946, held a private pilot license issued on 30 March 1967 with a glider towing endorsement from 7 April 1967, and a glider pilot license from 24 August 1964. His total motor flight experience was 34 hours 17 minutes (199 flights), of which 26 hours 32 minutes were on the Chipmunk (179 flights, including 99 glider tows). His glider experience totaled 233 hours 55 minutes in 283 flights. He had no previous incidents.
Accident Sequence
On 10 December 1967, the pilot was towing a glider as part of a contest organized by the Akademische Fluggruppe Zürich at Birrfeld airfield. Before the first flight, he warmed the engine longer than normal due to the low ambient temperature. After a first tow from 1108 to 1113 hours, a second glider (a Rhönlerche II, HB-797) was prepared. The tow aircraft and glider took off on runway 08 at 1118 hours. About 1.5 to 2 minutes later, at an altitude of approximately 60 m above ground level over a forest patch east of the airfield, a loud bang was heard and the engine stopped. The glider pilot noted the towline tension drop and immediately released; he then executed a 180-degree turn and landed normally on the airfield against the prescribed direction due to low altitude. The motor pilot apparently intended to land on a terrace east of the forest. He made a 90-degree right turn and flew low over a high-tension power line. Shortly after, the aircraft lost speed and crashed steeply onto a meadow, overturning and coming to rest inverted. Bystanders freed the trapped pilot, who was severely injured and taken to hospital in Brugg, where he died on 14 December 1967.
Investigation Findings
Examination of the wreckage revealed that connecting rod No. 3 had fractured due to material fatigue. Inspection by the Swiss Federal Materials Testing and Research Institute also found a fatigue crack 6–8 mm deep in connecting rod No. 1 at the same location. Connecting rods No. 2 and 4 showed no anomalies. Rolls-Royce Ltd., the engine manufacturer, conducted fatigue tests on similar connecting rods from Swiss Chipmunk aircraft and concluded that laboratory tests could not determine a safe service life, as the stresses required for fracture were not achievable in the engine and no differences were found between new rods and those with 7,000 operating hours. Rolls-Royce noted that the Chipmunk had not been granted civil certification for glider towing, and that full-throttle usage during towing may be disproportionately long compared to normal flight. The towing coupling functioned normally after the accident, and the towline was found separated with intact weak links, having contacted the top wire of a power line as evidenced by a section of rime ice missing on that wire.
Conclusion
The accident was directly caused by a connecting rod fracture that led to an immediate engine failure shortly after takeoff at low altitude over obstacle-rich terrain, resulting in a crash during the forced landing attempt.