Accident Overview
On May 17, 2024, about 1500 Pacific daylight time, a Cessna 190 airplane, registration N1500D, was substantially damaged during a ground loop while landing at San Luis Obispo County Regional Airport (SBP) in San Luis Obispo, California. The pilot, who was the sole occupant, was uninjured. The flight was operated under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Sequence of Events
The pilot reported that during the landing roll on runway 29, the airplane's right main landing gear began to track under the fuselage. As the airplane slowed, directional control was lost, resulting in a ground loop. The right wing sustained substantial damage during the event.
Post-Accident Examination
After the accident, the pilot inspected the right main landing gear and discovered that the bolts attaching the axle to the main gear spring had fractured. The fractured bolts allowed the right main wheel assembly to separate from the main gear spring. The axle, tapered shim, and attachment bolt were collected and sent to the National Transportation Safety Board's Materials Laboratory in Washington, DC, for examination.
Laboratory Findings
The two fractured lower bolt shafts were retained within the axle. The tapered shim was bent and buckled outboard, downward, and slightly aft. Two curved witness marks were found near the lower edge of the tapered shim, aligned with hole features on the mating axle collar surface. The fractured bolt shafts exhibited a 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. From that point, the fracture continued downward along an outboard and slightly aft slanted plane through the remainder of the bolt. The 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.