Primary finding
Probable cause
The pilot’s failure to maintain directional control during the landing roll, which resulted in a failure of the right main landing gear and subsequent ground loop.
Investigator assessment
Analysis narrative
The pilot reported that, during the landing roll, the airplane’s right main gear began to track under the fuselage. As the airplane slowed, he was unable to keep the aircraft under directional control. The airplane then ground-looped, substantially damaging the right wing. Metallurgical examination revealed the attachment bolts for the right main landing gear failed in downward and slightly aft bending. The two lower bolts fractured in overstress. The upper bolt was bent, and the nut was stripped. A side load on the right main landing gear wheel would have placed the axle in downward bending and drag forces during landing and roll out would have resulted in an aft loading component. The damage signatures observed were consistent with separation of the axle from the landing gear spring as a result of a ground loop.
Source record
Factual narrative
On May 17, 2024, about 1500 Pacific daylight time, a Cessna 190 airplane, N1500D, was substantially damaged when it was involved in an accident near San Luis Obispo, California. The pilot was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, during the landing roll on runway 29 at San Luis Obispo County Regional Airport (SBP), San Luis Obispo, the airplane’s right main gear began to track under the fuselage. As the airplane slowed, he was unable to maintain directional control, which resulted in a ground loop, substantially damaging the right wing. Following the accident, the pilot examined the airplane’s right main landing gear and found the bolts attaching the axle to the main gear spring had fractured. The fractured bolt allowed the right main wheel assembly to separate from the right main landing gear spring. The right main landing gear axle, tapered shim, and attachment bolt were collected and submitted to the National Transportation Safety Board’s Materials Laboratory in Washington, DC, for examination. The two fractured lower bolt shafts were retained in the axle. The tapered shim was bent and buckled outboard, downward, and slightly aft. Two curved witness marks were observed near the lower edge of the tapered shim. These witness marks were aligned with the hole features on the mating axle collar surface. The fractured bolt shafts retained in the axle exhibited a relatively rough, matte gray fracture surface morphology. Radial features were observed emanating from a thread root at the upper edge of both bolts over a relatively flat fracture surface through approximately 40% of the bolt cross section, at which point the fracture continued downward along an outboard and slightly aft slanted plane through the remainder of the bolt. The relatively flat fracture surface at the top of the bolt was consistent with overstress fracture from a downward bending load before the final fracture in downward and slightly aft bending.?