What happened
On July 1, 1972, a Pilatus Porter, registration HB-FGC, was conducting a skydiving training flight from Triengen airfield. The aircraft was carrying a skydiving instructor and six students. The flight proceeded as planned for the first round of jumps, but during the downwind leg of the second planned pass, a loud bang was heard, followed by a total loss of engine power.
Recognizing the imminent danger of a forced landing, the pilot ordered the remaining passengers to evacuate the aircraft if possible. Three students successfully exited the plane. The instructor remained on board, as the altitude was deemed too low for a safe parachute deployment. During the approach, the pilot's visibility was severely compromised because oil had sprayed onto the windscreen, making it opaque. The aircraft failed to stop on the runway, overshot the end of the strip into a grain field, and eventually rolled over.
There were no injuries among the pilot or the remaining passenger.
The investigation
The investigation focused on the mechanical failure of the Avco-Lycoming GSO-480-B1A6 engine, which had only 17 flight hours since a recent factory overhaul. Upon disassembly, investigators discovered that the engine failure was caused by a fatigue fracture of connecting rod number 4.
Fragments from the broken rod pierced the engine casing, creating a hole roughly the size of a fist. This allowed engine oil to escape and coat the windscreen. The fracture originated at an internal edge of the I-beam section of the rod where it meets the rod cap. While the fracture surface showed approximately 50% fatigue, the remaining area exhibited a coarse grain structure. Additionally, fine cracks of unknown origin were found near the fracture site, which may have contributed to stress concentrations.
Findings
- The primary cause of the engine failure was a fatigue fracture in the fourth connecting rod.
- The engine casing was breached by debris from the broken rod, leading to significant oil leakage.
- The oil spray onto the windscreen significantly hindered the pilot's ability to maintain visual references during the emergency landing.
- The pilot's actions during the engine failure were deemed appropriate given the emergency circumstances.