What happened
While back-taxiing on a runway to prepare for takeoff, a commercial certificated pilot began a 180-degree turn. During this maneuver, the pilot reported hearing a scraping sound accompanied by a wobble and shimmy in the aircraft. Simultaneously, the right brake ceased to function.
The right main landing gear strut collapsed near the axle attach point, causing the right main wheel and tire to roll away from the airplane. As a result of the collapse, the right wingtip and the propeller struck the ground, and the right gear strut was torn loose from its upper attach point. The pilot was not injured.
The investigation
A post-examination of the right wheel and strut conducted by the owner showed that the axle assembly had separated from the lower end of the landing gear strut. The assembly is secured to the gear leg by four hex head bolts, each featuring a nut threaded and torqued onto the bolt shaft.
Investigators found that all four bolts were bent. Two of the nuts had stripped off their respective bolt shafts, while the remaining two bolts had broken approximately mid-shaft. The owner noted that new brake assemblies had been installed on the aircraft roughly five days prior to the accident, and the original bolts were re-used for that installation.
A materials examination by an NTSB engineer of one broken bolt revealed it was bent approximately 35 degrees. The bolt exhibited necking deformation near the fracture site, with features consistent with an overstress fracture in bending.