CEN23LA149 · Cessna 182Q

The pilot stated that, about 30 minutes after takeoff, he observed a subtle but abnormal engine vibration, and he began to return to the departure airport. The pilot reported that all engine indications were normal with the exception of one cylinder that displayed a high cylinder head temperature, but he could not recall which one. The engine subsequently lost total power, and the pilot performed a forced landing to a field, during which the airplane nosed over and incurred substantial damage. Examination of the engine revealed that the No. 6 connecting rod was separated from the crankshaft. Removal of the cylinders and pistons revealed damage due to detonation. Borescope examination revealed that the No. 5 piston was melted on the intake side and displayed evidence of severe detonation. Damage to the skirt of cylinder No. 6 prevented removal of the piston. All of the intake and exhaust valves and associated pushrods were intact with no damage noted, and there were no other anomalies detected during the engine examination. An interview of the mechanic who had performed recent maintenance revealed that all six cylinders were replaced with new or repaired units 90.7 flight hours beforethe accident. The mechanic stated that he had not performed a cold valve clearance check when the cylinders were installed. Since the engine could not be rotated, it was not possible to determine the valve clearance after the accident; however, it is unlikely that any valve clearance anomaly, if it had existed before the accident, would have resulted in the partial and then total loss of engine power. The vibration and high cylinder head temperature reported by the pilot are consistent with detonation, which likely led to the failure of the No. 6 connecting rod and a total loss of engine power; however, the reason for the detonation could not be determined based on the available information.

Verbatim NTSB analysis and probable cause from the NTSB dataset