Piper PA 46 350P N786MD accident near Tahlequah, Oklahoma

Probable cause

The total loss of engine power, which resulted from fatigue failure of two engine through bolts and the No. 2 cylinder attachment studs due to the lack of sufficient clamping force.

NTSB analysis

The private pilot reported that he was in cruise flight when the engine experienced a total loss of power. Oil covered the windshield and smoke filled the cockpit, making it difficult for him to see out of the airplane. He attempted to land at an airport but conducted a forced landing to a field about 300 ft short of the runway. The right wing separated from the fuselage when it hit a hay bale.

The examination of the engine revealed a large hole in the engine case near the No. 2 cylinder. The engine maintenance records indicated that when the engine had a major overhaul about 8 years before the accident, the engine crankcase was repaired at a facility specializing in crankcase repairs. The facility typically used glass bead media to clean engine crankcases instead of "mildly abrasive organic substances" as required by the engine manufacturer. About 1 year before the accident, about 825 hours since the major overhaul, all 6 cylinders were replaced. The engine failure occurred about 198 hours after the cylinders were replaced.

The metallurgical examination of the eight through bolts revealed that two of the through bolts and the No. 2 cylinder studs had fracture surfaces that were consistent with fatigue. The fatigue in the studs and through bolts likely resulted from insufficient clamping force due to insufficient torque applied to the attachment nuts during installation or could have developed in service as asperities on the clamping surfaces worked down over time, leading to a reduction in clamping force.

Text from the NTSB. Open the NTSB docket for CEN17LA292 for the full record.