What happened
During a banner towing flight along the coast over the ocean, the engine of the aircraft began running rough and experienced a significant loss of power while in cruise flight. The pilot attempted several corrective measures, including releasing the banner and applying and removing carburetor heat, but the power loss continued. Because the engine could no longer produce sufficient power for sustained flight, the pilot performed a ditching in the water short of the beach. Upon impact with the water, the aircraft nosed over and came to rest in an inverted position. There were no fatalities reported.
The investigation
Post-accident examination of the engine revealed that the number 4 piston had fractured around its circumference just above the connecting rod pin, with beach marks indicating cyclic fatigue. Fragments of the piston were discovered in the oil sump. Additionally, the aft piston pin end cap showed signs of wear and burnishing, while the forward piston pin end cap had separated from the piston pin due to an overload fracture. The rod journal bearing was also found to be worn and burnished.
Further inspection of the crankshaft showed that the number 4 connecting rod journal was cracked and the connecting rod cheek was fractured perpendicular to the rod journal. This cheek fracture exhibited beach marks consistent with cyclic fatigue. While the irregular shape of the fracture allowed for continued rotation, it also caused the crankshaft to flex. This flexing induced a rotational misalignment of the connecting rod, which led to the cyclic fatigue failure of the piston. This mechanical condition had likely been developing for some time.
Investigators found large amounts of small metal debris and flakes in the oil pressure filter screen, consistent with bearing material from the rod end cap. These contaminants should have been detectable during regular inspections. Maintenance records indicated that 112 hours had passed since the last 100-hour inspection, and there was no evidence that the operator was following the procedures outlined in Lycoming Mandatory Service Bulletin No. 480E, which recommends oil changes and screen checks every 25 hours.