Cessna 152 N152GB accident near Hillsboro, Oregon

Probable cause

The partial loss of engine power during a go-around due to the loose copper electrode finger inside the left magneto.

NTSB analysis

While conducting takeoffs and landings during an instructional flight, the student pilot initiated a go-around. Shortly thereafter, the engine began to run rough and lost partial power. The instructor attempted to troubleshoot the engine, but was unable to restore engine power, and initiated a forced landing to a grass area beyond the end of the runway, during which the airplane nosed over. Examination of the engine revealed that the left magneto was producing spark randomly across its four posts. Internal examination of that magneto revealed that the distributor gear was intact; however, the copper electrode finger was found displaced from the plastic gear assembly.

A service bulletin issued by the magneto manufacturer about three years before the accident stated that some units contained distributor gears that exhibited loose electrode fingers, the symptoms of which included "unusual RPM drop during magneto check, difficulty starting, and/or rough running engines." A separate service bulletin issued 38 years before the accident specified that the magnetos should be inspected internally every 500 hours. The operator reported that, at the time of the accident, the magneto had accumulated 402.7 hours since its most recent internal inspection. The operator further reported they conducted internal inspections of the magnetos every 750 hours. It is likely that the loose copper electrode finger resulted in the partial loss of engine power during the go-around.

Text from the NTSB. Open the NTSB docket for WPR18LA148 for the full record.