Amateur-built VANS RV-7A N507RV accident near Byron, California

Probable cause

Improper overhaul of the right magneto, which eliminated system redundancy, and resulted in a loss of engine power when the left magneto failed during aerobatic flight maneuvers.

NTSB analysis

Shortly after takeoff, the private pilot began a series of aerobatic maneuvers in the experimental, amateur-built airplane. After executing two rolls, the engine began to run rough, and an instrument panel annunciator indicated that the ignition system had reverted to backup mode. Unable to maintain altitude, the pilot performed a forced landing to a field, where the airplane nosed over and sustained substantial damage.

The airplane's engine was equipped with a composite propeller and an FAA-certified ignition system, which used traditional magnetos combined with an electronic control module. The design was such that, should the electronic system fail or malfunction, the ignition system would revert to "backup" mode, operating on the traditional magnetos. Postaccident examination revealed that the steel magneto drive gear for the left magneto had broken off and fallen into the engine accessory case. Based on the damage signatures to the gear teeth and its shaft, it is most likely that a foreign object within the engine crankcase or accessory case moved into the path of the gear drivetrain during inverted flight, imparting a heavy side load on the gear and separating it from the magneto.

The system was designed to tolerate such a failure and revert to backup mode on the remaining magneto with only a minimal loss of engine power. However, postaccident examination revealed that the right magneto's internal contact point timing ("E" gap) was offset by about 16 degrees. This was likely the result of an improper adjustment during overhaul of the magneto, which was completed at a repair facility about 20 flight hours prior to the accident. This internal error would not have been visible to the pilot (builder), who followed the correct procedures to reinstall the magneto and synchronize it to the engine. Under this condition, the engine-to-magneto timing would have been correct, but the magneto's internal timing was wrong, resulting in a reduction of energy available for the spark plugs. This error would not have been a factor under normal operation, because the electronic ignition system did not use the magneto timing points.

The error should have been noticeable during the pre-takeoff engine magneto check, because the system reverted to backup mode as the ignition was switched from BOTH to either the LEFT or RIGHT magneto position. However, it is possible that engine and environmental variables, or operating conditions at the departure airport's elevation, were sufficient to mask the lower spark energy during ground run-up, but not in the more demanding environment of powered flight.

Text from the NTSB. Open the NTSB docket for WPR15LA262 for the full record.