What happened
The aircraft was performing cruise flight at 9,000 feet over the ocean when a loud bang originated from the engine compartment. Following this noise, the windshield became covered with oil. The pilot reported that the engine continued to produce partial power for a period before both the engine and the propeller ceased operation. Due to the loss of power, the pilot performed a ditching in the ocean.
The investigation
Examination of the wreckage identified an 8-inch by 5-inch hole in the top of the engine case. Investigators found that the camshaft was exposed and had completely fractured aft of the #3 intake lobe.
Further inspection of the internal components revealed that the #4 connecting rod was fractured approximately at its midpoint. The upper portion of this connecting rod remained attached to the piston pin, while the piston itself was stuck in the cylinder. The lower half of the #4 connecting rod and its associated bearing cap were not attached to the crankshaft and were not recovered from the site.
While the #2 and #6 connecting rods showed evidence of heat damage, the #4 rod did not. Metallurgical analysis of the recovered sections of the #4 connecting rod indicated failure modes consistent with overstress, though no primary failure mode could be identified on the available components. Additionally, engine maintenance records indicated that the #2 cylinder had been removed, repaired, and reinstalled 1,020 flight hours before the accident.