Amateur-built Q N3QP accident near Spring Hill, Florida

Probable cause

The pilot’s improper modification of the pull starter motor, which resulted in its failure and the subsequent loss of engine power.

NTSB analysis

The pilot was also the owner and builder of the experimental amateur-built airplane. While returning to the airport after a local flight, the engine lost oil pressure, so the pilot diverted toward the nearest airport. The engine then began to run roughly and lost partial power. The airplane was not able to glide to the runway and collided with a ditch in a field during the forced landing. The pilot described that his postaccident examination of the engine revealed that the pinion gear on the pull starter motor fractured into several pieces that entered the accessory gear case, likely seizing the oil pump.

About 4 years and 100 flight hours before the accident, the pilot had modified the pull starter motor from its original design so that the motor would fit in the airplane. Postaccident metallurgical examination found that the pull starter motor’s pivot lever was shortened. The pivot lever arm was designed to move the starter clutch and engage the pinion (small) gear to the crankshaft gear. The pinion gear teeth displayed indentations in their surfaces that indicated that they were only partially engaged to the crankshaft gear when the starter was being operated, a condition which was likely the result of the shortened pivot lever. This ultimately resulted in increased stress on the gear teeth during engine start and the subsequent failure of the gear in overstress. The liberated gear teeth likely damaged and seized the engine oil pump, and resulted in the partial loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA22LA159 for the full record.