Incident Overview
During cruise flight at 2,800 feet mean sea level, the pilot reported that the airplane was operating normally with no abnormal engine indications. Suddenly, a "loud bang" was heard, followed by a complete loss of engine power. The pilot attempted to restart the engine but was unsuccessful. An emergency was declared, and the pilot executed a forced landing to a soybean field. The airplane sustained substantial damage during the landing.
Engine Examination
On-site inspection revealed that the number 3 cylinder and its connecting rod had separated from the engine. Further examination of the engine showed that all eight hold-down studs on the number 3 cylinder deck were broken and exhibited fatigue fractures. Metallurgical examination of the upper front and lower front 1/2 inch thru-studs indicated they had fractured through approximately 95% and 90% of their cross-sections, respectively, by fatigue, characterized by smooth transgranular topography and beach marks.
The hardness of these studs was found to be below the engineering drawing requirements. The examination indicated that the studs were not properly heat-treated. The fractures of the 3/8 inch cylinder studs were secondary to the anchored thru-stud fatigue fractures. Additionally, the hardness of the other thru-studs in the crankcase was also below the requirements outlined in engineering drawings.
Maintenance Background
According to maintenance records, the engine had been overhauled at an overhaul facility on February 11, 1999. Since that overhaul, the engine had accumulated about 532 hours of flight time. The overhaul facility did not test the engine thru-studs for hardness during the overhaul, nor was it required to do so by Federal Aviation Administration regulations or the engine manufacturer's overhaul manual.