What happened
Approximately 25 minutes into a positioning flight, the pilot experienced a slight vibration followed by a small streak of smoke trailing from the right engine cowling. Upon noticing a flame emanating from the right side of the engine, the pilot shut down the right engine as a precaution and pitched the nose down to increase airspeed, which extinguished the fire almost immediately. The aircraft subsequently landed uneventfully.
The investigation
An examination of the right engine revealed that the number 3 cylinder had separated from the engine case, resulting in substantial damage to the right wing. Investigation of the attachment studs for the number 3 cylinder showed they had failed due to fatigue, likely caused by insufficient preload of the fastening nuts.
Inspectors noted that paint in the nut contact area around one of the cylinder hold down plates remained undisturbed; typically, the friction from proper torquing would remove this paint layer. Additionally, the paint found between the cylinder flange and the hold down plate was thicker than specified in the manufacturer's overhaul manual and was not the correct primer. While logbook records indicated no cylinders had been replaced since the last engine overhaul, evidence suggested the number 3 cylinder had been replaced after the overhaul occurred. Although the overhauler stated new cylinders were coated with a thin layer of zinc chromate, the failed cylinder lacked serial numbers and contained the non-standard, thick paint layer.
Findings
- The failure of the attachment studs was due to fatigue caused by insufficient preload.
- The presence of excessive, non-specification paint between the flange and hold down plate likely contributed to the loss of preload during service.