Accident Overview
On September 11, 2022, at 1900 central daylight time, a Cessna A188A, registration N2122U, sustained substantial damage during a landing accident near Canby, Minnesota. The aircraft was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot was not injured.
Pilot Account
The pilot reported that during the landing roll, immediately after the tailwheel touched down, the airplane made an abrupt turn. The left main landing gear collapsed, and the left wing struck the ground. The airplane sustained substantial damage to the left wing. The pilot stated that the tailwheel spring cross tube broke at the moment of tailwheel contact. He noted that the failed tube had a wall thickness of 0.055 inch, whereas the superseded part had a wall thickness of 0.125 inch.
Post-Accident Examination
The National Transportation Safety Board Materials Laboratory examined the fractured cross tube. The tube wall at the location of the right joint was flattened and substantially deformed, consistent with ductile overstress deformation. The deformation corresponded to an upward displacement of the attach fitting's right end relative to the adjacent fuselage lug. The portion of the tube that remained within the lug was ovalized, also consistent with overstress deformation from the relative displacement.
Component History
The tube material was identified as alloy 4130 steel, matching the specified material for the part. The measured outside diameter was 0.873 inch with a wall thickness of 0.0481 inch. According to the engineering drawing for the tube at the time of the airplane's manufacture in 1970, the specified nominal outside diameter was 0.875 inch and the wall thickness was 0.049 inch. Thus, the tube was approximately 0.002 inch smaller in diameter and 0.0009 inch thinner than the original design specifications. In 1985, the wall thickness of replacement parts was increased to 0.120 inch, as the aircraft model was no longer in production. The measured hardness of the tube was consistent with values expected for the specified material.