Cessna Aircraft Co LC41-550FG N932AC incident near Seal Beach, California

Probable cause

Improper maintenance performed on the left magneto which led to a loss of engine power. Contributing to the incident was the pilot's failure to check the magnetos in flight in compliance with manufacturers guidance regarding rough running engine. The pilot's decision to continue the flight after experiencing the rough running engine contributed to the necessity for a forced landing.

NTSB analysis

The commercial pilot reported that the engine started "missing" [running rough] just after departure but he corrected the condition by leaning the mixture. While in cruise the engine started "missing" again but smoothed out after he turned towards a nearby airport, at which point the pilot decided to continue to his destination. When approximately 4 miles away from his destination the engine started to run rough, then quit. The pilot made a forced landing in to an open area. The pilot stated that he did not perform a magneto check at any time after the engine began running rough.

Examination of the engine magnetos revealed that the left magneto gear washer and nylon washer had been installed in reverse order resulting in rapid bushing wear and excessive radial play of the distributor drive gear. This wear led to a migration of the internal timing in the left magneto that could have led to misfires or kickbacks during start that caused shock to the gear train and damaged one or more distributor gear teeth in the right magneto. Damage to one or more gear teeth led to a cascade of gear tooth fractures as the right magneto continued to operate. Eventually the right magneto drive gear decoupled from the engine resulting in cylinder 3 or 6 receiving a spark each time the points were opened during cam rotation, causing random ignition of the fuel-air mixture in those cylinders.

The pilot's failure to follow the Pilot's Operating Handbook guidance to check the operation of the individual left and right magnetos once the engine began running rough likely missed the opportunity to regain engine power utilizing the functional left magneto.

Text from the NTSB. Open the NTSB docket for WPR16IA044 for the full record.