CEN19LA251

The pilot reported that as the airplane touched down on the dry concrete runway, he heard a "snap" noise but that the landing was "smooth and normal." He taxied the entire length of the runway for the rollout but then had difficulty taxiing the airplane straight. The pilot thought that he had a flat tire until he exited the airplane and saw that the left main landing gear had failed. A postaccident examination revealed the left main landing gear upper side brace assembly fractured due to reverse bending fatigue at the circumferential weld between the middle tube and the fork at the inboard end of the assembly. The fatigue initiated from the tip of the weld in areas of incomplete weld penetration. The weld was performed during initial production of the component. The fatigue cracks extended radially outward from the interior surface of the tube, but the fatigue region did not intersect the outer surface. Thus, the cracks likely would not have been detected by visual inspection. The failure of the weld resulted in the displacement of the left main landing gear strut outboard toward the wingtip, which caused substantial damage to the underside of the left wing.

On July 27, 2019, about 1617 central daylight time, a Ryan Navion B airplane, N27AW, sustained substantial damage when it was involved in an accident at the Chicago Midway International Airport (MDW), Chicago, Illinois. The private pilot and three passengers sustained no injury. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that as the airplane touched down on the dry concrete of runway 31R, he heard a "snap" noise emit from the airplane. During the landing, he reported a crosswind from the southwest was present, but the landing was "smooth and normal." He taxied the entire length of the runway for the rollout until he had difficulty taxiing the airplane straight. The pilot thought he had a flat tire, until he exited the airplane and observed the failure of the left side main landing gear (MLG). The left side MLG has the upper side brace assembly part number 143-33165-20. This component consists of a middle tube that is welded to a clevis-shaped fork at the inboard end and a scoop-shaped fitting at the outboard end. The middle tube fits over tube-shaped ends of the fork and the fitting, and the pieces are welded together with circumferential welds at the ends of the middle tube. The fracture in the accident side brace assembly occurred in the middle of the circumferential weld between the middle tube and fork. The failure of the circumferential weld resulted in the displacement of the left MLG strut outboard toward the wingtip. This displacement caused substantial damage to the underside of the left wing. A postaccident examination of the upper side brace assembly revealed portions of the fracture surface had a flat fracture in a plane perpendicular to the tube axis with a smoothly curving boundary, features consistent with fatigue. The fatigue regions were located at the upper and lower sides of the side brace consistent with reverse bending fatigue. Crack arrest lines were observed in the fatigue regions, and the shape of the fatigue regions and arrest lines were consistent with fatigue cracks propagating radially outward from the interior surface of the tube. However, the fatigue regions did not intersect the outer surface. An engineering drawing for the fork used in the brace assembly showed the fork had a fillet radius at the location of the welded joint. The interior surface of the middle tube adjacent to the fracture was machined at an angle consistent with a chamfer, and the location of the chamfered end corresponded to the location of the fillet radius in the fork when fit up together at the circumferential weld location. The presence of a chamfer on the interior surface of the circumferential weld was consistent with incomplete penetration of the weld, which was observed around approximately 90% of the circumference. The weld was performed during initial production of the component. The chamfer was consumed by the weld bead on the aft side of the tube where the weld beadpattern was consistent with the start and stop points for the circumferential weld. A visual inspection is conducted during the preflight process and during maintenance inspections to look for weld defects and cracks in the MLG, including the upper side brace assembly; however, there is no non-destructive inspection method requirement from the manufacturer for this component.