Accident Overview
On September 10, 2021, about 1040 mountain daylight time, a Cessna 175 airplane, registration N9343B, was substantially damaged when it was involved in an accident near Hot Springs, South Dakota. The pilot and three passengers were not injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
Landing and Ground Roll
The pilot reported that the airplane touched down “with a gentle skip then easing forward on the yoke pinned my main gear firmly on the turf.” During the landing roll, the airplane began to veer to the right, and the pilot applied rudder inputs to correct. The airplane subsequently spun around and came to rest upright with the left main landing gear leg separated. The pilot stated the airplane was modified to conventional (taildragger) landing gear through a supplemental type certificate (STC) SA383GL kit in 2013.
Postaccident Examination
Postaccident examination of the airplane revealed the left wing was bent, and the main landing gear box structure was wrinkled. An attachment bolt for the left main landing gear strut was found fractured and separated from the strut.
NTSB Materials Laboratory Examination
The National Transportation Safety Board Materials Laboratory examined the fractured bolt and the attachment strut plate. According to the STC SA383GL, the inboard end of each main landing gear strut was attached to the upper side of the strut plate. Each strut inboard attachment bolt was threaded into a self-locking nut plate attached to the lower surface of the strut plate. Examination of the bolt revealed that most of the bolt threads were sheared, and the bolt shank was fractured through the uppermost thread adjacent to the grip. The bolt fracture surface had a dished fracture surface with matte gray features consistent with ductile overstress fracture.
Bolt thread remnants were observed trapped between threads on the inner diameter of the nut plate, including between the threads at the bottom of the nut plate. The 0.49-inch-thick strut plate was bent downward at the forward and aft ends relative to the middle. The upper plate surface had a machined surface finish and did not show any evidence of fretting contact damage.
The bolt head markings were consistent with an Army-Navy (AN) standard bolt. The measured average bolt hardness corresponded to an estimated ultimate tensile strength (UTS) of 142,000 pounds per square inch (psi); the expected minimum tensile strength is 125,000 psi.
The composition of the strut plate was identified as consistent with aluminum alloy 2014. The measured average plate material hardness was above the typical hardness range for an alloy 2014 plate.