What happened
While in cruise flight, the pilot noticed the aircraft began to vibrate slightly. In response to this vibration, the pilot decided to return to the airport. During the flight, the engine vibration escalated into violent shaking, prompting the pilot to perform a forced landing. The aircraft landed short of its intended destination, nosed over, and came to rest in an inverted position. There were no injuries reported.
The investigation
An examination of the wreckage determined that the left rear cylinder on the engine had separated from its mounting flange. At the time of the accident, the engine had accumulated approximately 13 hours of operation since an overhaul. During this overhaul, all four cylinders had been replaced with chromed cylinders obtained from an outside source before being processed by the engine overhaul facility.
A metallurgical examination of the failed cylinder identified a fatigue origin located near the bottom of the fin radius, specifically between the tenth and eleventh fins from the inboard end of the cylinder barrel. Investigators also observed severe corrosion attack on the fin immediately adjacent to the fatigue origin. Additionally, corrosion damage was noted in the radius, including a corrosion pit that appeared to be the point from which crack arrest features emanated. This corrosion pit was found to be filled with a non-conductive substance.