Event Sequence
Approximately 25 minutes into a positioning flight, the pilot detected a slight vibration. Minutes later, a small streak of smoke was observed trailing along the right engine cowling. The pilot shut down the right engine as a precaution. Subsequently, a flame emanated from the right side of the engine. The pilot pitched the nose down, increasing airspeed, which extinguished the fire almost immediately. The aircraft landed without further incident.
Examination Findings
Post-flight examination of the right engine revealed that the number 3 cylinder had separated from the engine case. The right wing sustained substantial damage. Detailed examination of the number 3 cylinder attachment studs showed they had failed in fatigue, most likely due to insufficient preload of the fastening nuts. Paint in the nut contact area around one of the cylinder hold-down plates was present and undisturbed. Adequate friction during attachment and torquing would be expected to disturb and remove the paint layer, as was observed at other locations on the hold-down plate. Additionally, the paint between the cylinder flange and hold-down plate was not the primer specified by the engine manufacturer's overhaul manual and was applied at a thickness greater than that specified. The presence of this paint could have led to an in-service loss of preload, even if sufficient torque was applied to the cylinder nuts at installation.
Maintenance History
Logbook records contained no entries indicating that any right engine cylinder had been removed or replaced since the last engine overhaul. However, evidence showed that the number 3 cylinder had been replaced after the overhaul. According to Federal Aviation Administration records, the cylinders installed during the overhaul were new and had serial and cylinder class numbers stamped on them. The overhauler stated that new cylinders were received with a thin coat of zinc chromate, and the engine was painted after reassembly. The failed number 3 cylinder did not have serial numbers and had paint between the cylinder flange and hold-down plate.