What happened
While operating in cruise flight at an altitude of approximately 8,500 feet, the pilot of the aircraft heard a loud bang, followed by a decrease in engine oil pressure. Shortly after this event, engine oil was observed on the windscreen, and the engine subsequently lost all power. The pilot then initiated a forced landing into a field.
During the landing sequence, the aircraft impacted the terrain, which caused the nose and right main landing gears to collapse. There were no injuries reported following the accident.
The investigation
An examination of the aircraft revealed that the right wing had buckled upon impact. Maintenance records indicated that the most recent annual inspection was completed on November 11, 2005, at a total airframe time of 3,358 hours. At the time of the accident, the engine had accumulated 257 hours since its major overhaul and 49 hours since the annual inspection. Aside from an oil change, no other maintenance had been performed on the engine since the November inspection.
Upon disassembly of the engine, investigators found that the connecting rods had fractured and separated. The pistons were also found to have separated from the connecting rods. All six pistons exhibited cracks and fractures along the piston pin bosses, along with mechanical damage on the bottom side of the pistons. Additionally, the connecting rod and main bearings showed evidence of discoloration and heat signatures, and the crankshaft counterweights had separated from the crankshaft.
Findings
Metallurgical examination of two of the pistons revealed that the fractures along the piston pin bosses were consistent with overstress separation in a casting.