What happened
The pilot was conducting cruise flight at 2,800 feet mean sea level when he heard a loud bang, followed immediately by a complete loss of engine power. Although the aircraft had been operating normally with no abnormal engine indications prior to the event, the pilot was unable to restart the engine after the failure. After declaring an emergency, the pilot executed a forced landing into a soybean field. The airplane sustained substantial damage during the impact.
The investigation
An on-site inspection of the engine revealed that the number 3 cylinder and its connecting rod had separated from the engine assembly. Investigators found that all eight hold down studs on the number 3 cylinder deck were broken, showing evidence of fatigue fractures.
A metallurgical examination of the upper front and lower front 1/2 inch thru-studs showed they had fractured through approximately 95% and 90% of their cross-sections, respectively. These fractures were characterized by beach marks and smooth transgranular topography, indicating fatigue failure.
Further testing determined that the hardness of these studs was below the requirements specified in the engineering drawings, indicating they had not been properly heat-treated. The 3/8 inch cylinder stud fractures were found to be secondary to the fatigue fractures of the anchored thru-studs. Additionally, the hardness of other thru-studs within the crankcase was also below the required engineering specifications.
Maintenance records indicated the engine had undergone an overhaul on February 11, 1999, and had flown approximately 532 hours since that time. The investigation noted that the overhaul facility was not required by FAA regulations or the manufacturer's manual to test the hardness of engine thru-studs during the overhaul process.