What happened
On December 10, 1967, a D.H.C.-1 Chipmunk, registration HB-TUF, was performing a glider towing operation at Birrfeld Airfield as part of a local landing competition. Shortly after takeoff, while flying at a low altitude over wooded terrain, the aircraft experienced a sudden engine failure following a loud bang. The glider being towed, a Grunau Baby HB-494, successfully released from the tow rope and managed to land safely at the airfield.
The pilot of the HB-TUF attempted an emergency landing on a nearby terrace. During this maneuver, the aircraft flew at a low altitude over high-voltage power lines. The trailing tow rope made contact with the uppermost cable, which likely caused a loss of lift by pulling the tail of the aircraft upward. This led to a steep descent and a subsequent crash. The aircraft overturned upon impact, causing fatal injuries to the pilot, who passed away four days later. The aircraft was a total loss.
The investigation
The investigation focused on the mechanical failure of the engine and the circumstances of the emergency landing. Investigators examined the engine, a Gipsy Major 10 Mk. 2, and identified a broken connecting rod. Metallurgical analysis conducted by the Swiss Federal Institute of Materials Science confirmed that the failure of connecting rod number 3 was caused by metal fatigue. Additionally, a crack was discovered on connecting rod number 1, also attributed to fatigue.
Records indicated that the engine had previously experienced a main bearing failure in 1962. Investigators also reviewed technical data from Rolls-Royce, noting that while similar fatigue cases had occurred in other engines, the manufacturer did not stipulate a maximum service life for these specific connecting rods. The investigation also noted that the aircraft was being used for glider towing, an operation for which the Chipmunk had not received civil certification, often involving prolonged use of full throttle.
Findings
- The primary cause of the engine failure was a fatigue fracture of connecting rod number 3.
- The engine failure occurred during a critical phase of flight, at a low altitude immediately after takeoff.
- The terrain available for the emergency landing was uneven and filled with obstacles.
- The trailing tow rope contacted a high-voltage power line, which contributed to a loss of lift and a steepening of the descent angle.