Amateur-built S-21 N468MM accident near Creswell, Oregon

Probable cause

A total loss of engine power caused by fatigue cracking and fracture of the crankshaft due to severe wear of the bearings.

NTSB analysis

The pilot had recently finished building the airplane and it had amassed about 55 hours of flight time on the airframe and engine. While the pilot was maneuvering at about 700-900 feet above ground level (agl), the engine made “rough” sounds and vibrated. About 20 seconds later the propeller seized and the pilot made a forced landing in the field below.

A postaccident examination of the engine revealed that the crankshaft was fractured at a crank cheek, just aft of the journal of the No. 2 connecting rod. A materials analysis revealed that the crankshaft fracture resulted from fatigue cracks originating at the surface of the aft radius on the No. 2 rod journal, likely due to high operating stresses on the fillet radii. The metallic particles in oil filters suggested continuous wear on the bearing shells. The worn bearing shells most likely intensified the high operating stress in the crankshaft. The source of the high operating stress acting on the crankshaft and severe wear of the bearings was not determined, but some out of range operating parameters recorded on the Engine Control Unit (ECU) could have contributed or been the source of these stresses, which led to fatigue cracks and severe wear of the bearing shells. Torque for the propeller flange was not measured, but the bolt, though not difficult to remove, likely lost preload torque due to the propeller striking the ground. There was no evidence that the crankshaft failure was a result of a material flaw or defect, and there is no discernible pattern, or recurring failures observed in the six-cylinder engine crankshafts.

Text from the NTSB. Open the NTSB docket for WPR23LA184 for the full record.