Accident Description
According to the pilot, shortly after departing runway 18, she heard a sound described as a loud backfire, a single bang, followed by engine power loss. The pilot selected a nearby field and performed a forced landing. During the landing, the nose gear collapsed, resulting in substantial damage.
Engine Examination
An examination of the engine revealed that the crankshaft gear had separated from the crankshaft and was found loose inside the accessory case. The upper portions of five of the six screws used to secure the gear to the crankshaft were located in the accessory case. Metallurgical examination of the fracture surfaces showed signatures consistent with fatigue fractures.
Screw Analysis
One screw was sectioned and tested for hardness, yielding results of 29, 30, 30, and 31 Rockwell C Hardness (HRC). According to TCM, the drawing specifies an H5 fastener with a hardness requirement of 32 through 37 HRC. Additionally, the head diameter of the screw measured .40 inches or less, while the TCM drawing specifies a diameter of .44 inches. According to TCM, this discrepancy could have resulted in inadequate torque for this installation.
Other Findings
An examination of the airplane flight controls and engine controls revealed no anomalies.