N255BZ

On June 3, 2017, about 1130 Pacific daylight time, an experimental, amateur-built Sonex, N255BZ, was substantially damaged when it was involved in an accident near Garfield, WA. The private pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot, who was also the builder of the airplane, reported that, during cruise flight at 4,000 ft mean sea level, he experienced an increasing vibration followed by a loud bang. He subsequently saw the propeller separate from the airplane and he performed a forced landing to a paved road. During the landing roll, the airplane impacted a power pole guy wire, resulting in substantial damage to the left wing (see Figure 1).

Figure 1. Accident site.

Examination of the airplane revealed that the propeller assembly and forward section of the crankshaft separated from the engine. The propeller and separated crankshaft section were not located.

An examination of the engine crankcase and crankshaft was conducted at the aircraft manufacturer. The crankshaft separation surface was located at the forward main bearing journal and exhibited beach marks propagating from the woodruff key slot and extending through most of the separated surface area. (see Figure 2.) 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 remained in their holes in the mount. (see Figure 3.) A 1/4-inch area of fretting was also noted around the outer circumference of the mounting flange and flywheel mating area. The trigger shaft, which is attached to the flywheel, displayed scuffing and material deformation at the locations of the flywheel mounting screw heads.

Figure 2. Crankshaft separation surface area.

Figure 3. Flywheel mounting flange.

The pilot reported that he found no torque values in the manufacturer’s assembly manual during assembly of the flywheel. He was uncertain of the torque that he applied to the screws, although he thought that it was sufficient. The pilot further reported that it is his common practice to apply Loctite to all critical fasteners, and he could not specifically remember adding it to the fasteners during the assembly. According to the AeroVee 2.0 Assembly Manual, the trigger shaft is installed on the flywheel with four screws and Loctite 242 applied to the screw threads. No torque value is stated in this step. The flywheel and flywheel mounting flange are preassembled for the engine kit.

Federal Aviation Administration (FAA) Advisory Circular 43-13-1B Table 7-1 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.