What happened
Shortly after departing runway 18, the pilot reported hearing a single loud bang, described as sounding like a backfire. Immediately following this sound, the engine lost power. The pilot subsequently selected a nearby field to perform a forced landing. During the landing sequence, the nose gear collapsed, which resulted in substantial damage to the aircraft.
The investigation
An examination of the engine was conducted to determine the cause of the power loss. Investigators found that the crankshaft gear had separated from the crankshaft and was loose within the accessory case. Specifically, the upper portions of five of the six screws used to secure the gear to the crankshaft were located inside the accessory case.
A metallurgical examination of the fracture surface showed signatures consistent with fatigue fractures. One screw was sectioned and tested for hardness, yielding results of 29, 30, 30, and 31 Rockwell C hardness (HRC). According to TCM, the technical drawing specifies an H5 fastener with a required hardness of 32 through 37 HRC. Additionally, the head of the screw measured 0.40 inches or less in diameter, whereas the TCM drawing specifies a diameter of 0.44 inches. TCM indicated that these discrepancies could have resulted in inadequate torque during the installation. An examination of the engine and flight controls revealed no other anomalies.