What happened
During the cruise phase of flight, the aircraft experienced a total loss of engine power. This failure necessitated that the pilot execute a forced landing.
The investigation
An examination of the engine crankshaft and its attached rod components was conducted following the event. Investigators found that both nuts on the number three connecting rod had backed off from their original installed positions. Additionally, one nut on the number four connecting rod had also backed off, though it did not fail during the flight.
Technical analysis regarding bolt preload noted that while preload is related to installation torque, several variables can influence this, including lubrication, thread variations or deformities, surface roughness, and contamination. Torque tests performed during the investigation showed a variance of 18-25 percent between the tightening and loosening of the nuts. While the breakaway torque values fell within the range expected for properly torqued nuts, the uncertainty of the readings prevented a definitive conclusion regarding whether the original installation torque was adequate. Furthermore, the physical condition of the separated number three rod and the damaged number four nuts made further measurements impossible.