Cessna 180K N2590K accident near Boca Raton, Florida

Probable cause

Maintenance personnel’s failure to apply proper torque to the cylinder flange nuts and through bolts, which resulted in shifting of the Nos. 1 and 2 main bearings, loss of lubrication, failure of the crankshaft, and a subsequent total loss of engine power.

NTSB analysis

The private pilot and passenger were conducting a cross-country flight. During the descent from cruise altitude to the destination airport, the airplane began to shake violently. The pilot heard a "loud noise," and the propeller stopped rotating then began slowly windmilling. The engine was not producing power. During the subsequent descent, the pilot realized that the airplane would not reach the airport and decided to conduct an off-airport landing. The ground was soft and sandy, and during the landing roll, the airplane nosed over and came to rest inverted.

Examination of engine maintenance records revealed that the Nos. 3 and 4 cylinder assemblies had been removed and reinstalled about 150 flight hours before the accident. During a postaccident examination of the engine, the cylinder flange nuts and through bolts were checked for security and breakaway torque. One of the nuts attached to one of the hold-down studs for the No.1 cylinder was loose, with a significant gap between the nut and the cylinder flange. The breakaway torque values for each of the No. 2 cylinder through bolts was about 200 in-lbs less than that of the through bolts on the other cylinders. 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. 1 bearing support mating surfaces also displayed fretting, as well as lock tab elongation. The No. 2 bearing support displayed damage consistent with a bearing shift and a spun bearing event. The area around the No. 2 bearing support through bolt holes displayed a significant amount of fretting between the mating surfaces. The crankshaft was fractured at the crankshaft cheek between the No. 2 main journal and the No. 2 connecting rod journal. The Nos. 2 and 3 connecting rod journals displayed incipient signatures of lubrication distress. Further examination of the crankshaft using a magnaflux booth also revealed a crack along the bend radius of the No. 1 journal.

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, proper cylinder installation requires a multiple-step torqueing process. The process requires that the through bolt nuts be torqued on both sides of the engine, even if only one cylinder is being installed. A manufacturer service bulletin 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 Nos. 3 and 4 cylinders, which resulted in shifting of the Nos. 1 and 2 main bearings, loss of lubrication, and failure of the crankshaft.

Text from the NTSB. Open the NTSB docket for ERA15LA014 for the full record.