Amateur-built SONEX N255BZ accident near Garfield, Washington

Probable cause

The pilot’s improper torque of screws that attached the flywheel mounting flange to the crankshaft during engine assembly, which resulted in heavy vibrations and the subsequent in-flight loss of the propeller.

NTSB analysis

The pilot reported that, during cruise flight, the airplane began vibrating, followed by a loud bang. The pilot saw the propeller separate from the airplane and he subsequently performed a forced landing to a road. During the landing roll, the left wing impacted a guy wire, resulting in substantial damage to the left wing.

Postaccident examination revealed that the crankshaft separated at the forward main bearing journal. The area of separation exhibited beach marks propagating from the woodruff key area throughout most of the separated surface area. The flywheel mounting flange attached to the crankshaft and the flywheel mating surface showed material deformation at several screw holes and two fractured screw shafts that remained in their holes in the mount. A 1/4-inch wide area of fretting was noted around the outer circumference of the mounting flange and flywheel mating area. The trigger shaft, which was attached to the flywheel, displayed scuffing and material deformation at the locations of the flywheel mounting flange screw heads.

The pilot/builder reported that he found no torque values in the manufacturer’s assembly manual during assembly of the flywheel and he was uncertain of the torque that he applied to the screws, although he thought that it was sufficient. It is likely that he did not adequately torque the screws during installation of the flywheel, which allowed the screws to loosen over time, resulting in elongation of the screw holes and fretting on the flywheel mounting flange and flywheel mating surface. The loose flywheel assembly would have increased vibration throughout the engine, along with torsional loads that ultimately resulted in the separation of the forward crankshaft and propeller assembly.

A Federal Aviation Administration (FAA) Advisory Circular shows the recommended torque to be used for a specified fastener. Builders can also contact the manufacturer, who can supply them with a general torque table. The maintenance history of the airplane was not obtained during the investigation.

Text from the NTSB. Open the NTSB docket for WPR17LA119 for the full record.