Cessna U206G N156SA accident near Louisa, Virginia

Probable cause

The pilot's decision to continue operation of the airplane with known mechanical issues (fluctuating oil pressure and metal in the oil filter), which was the result of undertorqued cylinder through bolts. Contributing to the accident was the mechanic's failure to properly torque the engine through bolts, which led to the eventual failure of the crankshaft.

NTSB analysis

The pilot reported that, during the flight before the accident flight, he noted that the oil pressure was slowly fluctuating, so he chose to change the engine oil and filter. Airport personnel provided the supplies and observed the pilot perform the maintenance. The pilot removed and then opened the oil filter and "an excessive amount of metal" was observed. The pilot's mechanic had replaced four cylinders about 4 months earlier, and the pilot assumed that the metal was chrome from the overhauled cylinders and was the result of "break-in." Although airport personnel expressed concerns about the metallic debris, the pilot chose to finish the oil change and continue with his next flight. About 30 minutes after departure, the engine made "a strange sound," and it lost power about 20 seconds later. The pilot conducted a forced landing, and the airplane landed hard in an open field, which resulted in structural damage to the airframe.

Disassembly and examination of the engine revealed that the engine cylinder through bolts on the Nos. 1 through 5 cylinders were significantly undertorqued. The crankshaft was fractured near the No. 2 main bearing. The areas adjacent to the No. 2 bearing on the inside of the engine case exhibited rotational scoring, indicating that bearing movement had occurred before the crankshaft failure. The Nos. 1 and 3 main bearings also exhibited evidence that bearing movement had occurred before the engine failure. It is likely that the mechanic did not properly torque the through bolts when he replaced the four cylinders, which allowed the bearings to move and led to the eventual failure of the crankshaft.

Text from the NTSB. Open the NTSB docket for ERA14LA193 for the full record.