Diamond DA 40 N419JS incident near San Carlos, California

Probable cause

The improper assembly of the governor during manufacture.

NTSB analysis

The pilot departed on a local area flight when the engine had higher than normal RPMs. The pilot elected to return to the departure airport and his mechanic inspected the airplane. The mechanic discovered a blister in the engine casing and fragments of metal in the oil. The engine was then disassembled and a ball bearing from the propeller governor was located in the engine which had the potential of resulting in a catastrophic engine failure. Examination of the engine showed that the ball bearings from the governor were able to pass through the oil drain hole of the governor. Follow up examinations of the propeller governor showed that the governor bearing race and plunger were assembled with the bearing race set screw and plunger hole misaligned. When the bearing race set screw was torqued down, the set screw tip flattened against the harder plunger surface. During operation, the set screw/plunger race separated.

Text from the NTSB. Open the NTSB docket for SEA08IA217 for the full record.