Cessna U206 N4615X accident near Harpswell, Maine

Probable cause

A total loss of engine power due to maintenance personnel's failure to apply proper torque to the cylinder flange nuts and through bolts, which resulted in shifting of the Nos. 1, 2, and 3 main bearings, loss of lubrication, and failure of the crankshaft.

NTSB analysis

The pilot reported that, while en route to his destination, he heard a “loud bang,” followed by a complete loss of engine power. After determining that the airplane was not within gliding distance of the nearest airport, the pilot performed an off-airport landing, during which the airplane impacted a small drainage ditch, nosed over, and came to rest inverted, resulting in substantial damage.

Disassembly of the engine revealed mechanical damage in multiple locations inside the crankcase. The No. 1 main bearings had shifted slightly out of the support and displayed deformation. The No. 2 main bearings displayed bearing shift signatures. One was missing, and the other had extruded out of the bearing support. Multiple pieces of the missing bearing were found in the oil sump. The No. 3 main bearings were intact and had moved in a clockwise direction when viewed from the back of the engine, had shifted slightly out of the support, and the oil passage holes were partially blocked. The crankshaft was fractured at the crankshaft cheek between the No. 2 main journal and the No. 2 connecting rod journal. The No. 2 connecting rod journal and connecting rod bearings displayed incipient signatures of lubrication distress.

Review of the engine maintenance records revealed that, about 500 flight hours before the accident, all the cylinders had been replaced with overhauled cylinders and new pistons had been installed along with reconditioned lifters and seals.

If a bearing shifts for any reason after removal of one or more cylinders, or during replacement of one or more cylinders, the oil holes in the bearing can become misaligned with the oil passages in the crankcase, partially cutting off the oil supply to the bearing. If the engine is then put back into service, the reduced oil supply to the bearing can cause increased heat and friction that can result in the bearing shifting more, further reducing its oil supply. Eventually, the bearing can shift enough to cause the oil supply to be cut off completely, resulting in a catastrophic failure. This is usually referred to as a "spun bearing."

To prevent a spun bearing, the manufacturer specified a multiple-step torquing process for proper cylinder installation. The process required that the through bolt nuts be torqued on both sides of the engine, even if only one cylinder was being installed. The manufacturer warned that "failure to torque through bolt nuts on both sides of the engine can result in a loss of main bearing crush with main bearing shift and subsequent engine failure." Given the observed damage, it is likely that maintenance personnel did not apply sufficient torque to the cylinder flange nuts and through bolts during installation of the cylinders, which resulted in shifting of the Nos. 1, 2, and 3 main bearings, a loss of lubrication, and failure of the crankshaft.

Text from the NTSB. Open the NTSB docket for ERA17LA307 for the full record.