On June 22, 2017, about 1325 Pacific daylight time, a Cessna 421B, N380AL, was substantially damaged when it was involved in an accident in Carlsbad, California. The two pilots were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The pilot receiving instruction reported that the accident flight was the airplane’s first flight after extended maintenance. The pilot said they flew to Ramona, California, and conducted an uneventful landing before flying to Mc Clellan-Palomar Airport (CRQ), Carlsbad, California. He indicated that touchdown on runway 24 and rollout were normal but that, as the airplane decelerated to about 10 to 15 mph, it started to wobble. The pilot maintained directional control without using brakes, but the wobbling intensified; suddenly the right main landing gear collapsed, and the right wing and engine struck the ground, resulting in substantial damage to the wing. The flight instructor reported that the approach and landing were normal and that after touchdown, the airplane decelerated "nicely.” He stated that just before they reached taxiway A5, he noticed what felt like a flat tire or oscillation in the gear and asked the pilot to "keep it straight." The instructor recalled that the pilot responded that he wasn't going to brake. A few seconds later, the right main landing gear collapsed, and the airplane rotated toward taxiway N3. The instructor estimated that the airplane’s speed at the time of the gear oscillation was between 5-10 mph. He added that when he learned the bolt that connected the "torque links or scissors" on the right main landing gear assembly had failed, this would explain "how the assembly rotated and then collapsed backwards when the tire became misaligned with the aircraft's momentum." Postaccident examination of the airplane revealed that the right main landing gear torque link pivot bolt assembly was severed, and the two inboard spacing washers, nut, and cotter pin were not located (see Figure 1). Additionally, the landing gear trunnion was partially separated from the wing structure, and the right main landing gear was separated but remained attached via hydraulic hoses.

Figure 1: Service Kit SK421-118 overview of the torque link assembly and associated part with annotated references to the missing parts. Further examination of the right main landing gear revealed that the forward right landing gear trunnion was separated into two pieces, with the lower portion torn away from the upper portion. Scraping was observed on the outboard side of the landing gear strut and brake caliper. The right main landing gear tire exhibited vertical scuffing along its outboard crown. Additional multidirectional striations were observed on the outboard portion of the lower torque link assembly, brake caliper, and lower portion of the landing gear strut assembly. (The upper and lower landing gear strut are interconnected by the upper and lower torque link assemblies. Alignment of the wheel, which is connected to the lower portion of the landing gear strut, is obtained by washers installed between the upper and lower torque link assemblies.) Portions of the right landing gear trunnion, right landing gear brace assembly, and the upper torque link assembly, along with the torque link pivot bolt, were sent to the National Transportation Safety Board Materials Laboratory for further examination. Metallurgical examination revealed that the torque link pivot bolt was fractured through the threaded region at the edge of the pivot barrel. The fracture surface was partially obscured by red rust deposits but exhibited surface texture, surface topography, and deformation patterns indicative of a bending overstress fracture. No indications of preexisting cracking or corrosion were found. The pieces of the trunnion and the rest of the submitted parts exhibited fractures consistent with overstress, with no indications of preexisting fractures or fatigue cracks.