Amateur-built CH701 N8787G accident near Bothell, Washington

Probable cause

A loss of engine power due to the fatigue failure of the engine’s flywheel screws that were likely overloaded when the engine was involved in a previous accident.

NTSB analysis

The pilot reported the takeoff and en route portions of the flight were uneventful. While descending to the traffic pattern altitude at the destination airport, the engine lost power and the propeller stopped. The pilot made an emergency landing on a vacant road, colliding with powerlines.

A postaccident examination of the engine revealed that the flywheel screws had separated due to fatigue, resulting in an engine failure. Examination of the screws showed multiple fatigue cracks and wear consistent with low-stress bending fatigue.

Maintenance records revealed that the engine had been installed in another airplane that was involved in an accident where the propeller was damaged. According to the engine manufacturer, abrupt propeller stoppage can produce a spike of torque along the crankshaft, overloading the flywheel screws and causing failure. There was no evidence that the screws were replaced after that accident.

Text from the NTSB. Open the NTSB docket for WPR22LA263 for the full record.