WPR20LA252

The pilot reported that, after departure, he raised the landing gear and heard an unusual "clunk"; however, he did not observe any anomalies at the time and continued the flight. As the airplane neared the destination airport, the pilot extended the landing gear, but the landing gear did not fully extend. After troubleshooting, including attempting the emergency extension procedure, the pilot landed the airplane with the gear partially extended, during which the airplane sustained substantial damage to the keel beam and the right-wing spar. Postaccident examination revealed that the right main landing gear trunnion was fractured as the result of two fatigue cracks that had developed at a welded gusset joined to a protruding boss. Once these fatigue cracks had propagated far enough into the boss cross-section, the remainder of the part fractured by overstress, likely during the last landing. The presence of two opposite-facing fatigue cracks was consistent with reverse bending fatigue. The fatigue cracks initiated at a tapered neck-shaped area, where the gusset and trunnion were joined. The cross-sectional thickness at this location was half the thickness of the gusset and a quarter of the diameter of the boss. This configuration would introduce a stress concentration at the locations of the fatigue cracking. The trunnion boss was consistent with a heat-treated aluminum alloy. The hardness approaching the welded joint decreased, consistent with softening from the heat input. The reduced hardness would increase susceptibility to fatigue crack initiation at the areas of stress concentration. It is likely that, over time, the fatigue cracks propagated as the joint flexed or bent back and forth, and the stress concentrations at the neck-shaped region were exacerbated by the reduced mechanical properties of that region. The fracture surfaces were damaged after the accident, and, due to the amount of post-fracture damage, no reliable fatigue analysis could be performed.

On June 4, 2020, at 1821 Pacific daylight time, a Piper PA30 airplane, N8244Y, was substantially damaged when it was involved in an accident at Salem, Oregon. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. The pilot reported that after departure, he raised the landing gear and heard an unusual "clunk," but the landing gear retraction light illuminated normally and the mirror on the left engine nacelle indicated that the nose gear was retracted. The pilot continued the flight, and as the airplane neared the destination airport, he lowered the landing gear. The green “down and locked” light did not illuminate, and the nose landing gear door was open, but the nose landing gear was not extended. The pilot climbed the airplane and began to troubleshoot the issue, including performing the emergency gear extension procedure; however, “the system was totally jammed.” The pilot diverted to a nearby airport, where he performed a flyby, and ground personnel indicated that the main landing gear was partially extended, and the nose landing gear was not extended. The pilot landed the airplane and he and the passenger egressed without injury. The airplane sustained substantial damage to the keel beam and the right-wing spar. Postaccident examination revealed that the right main landing gear trunnion was fractured. The trunnion was removed from the airplane and sent to the NTSB Materials Laboratory for analysis. The fracture surfaces had been forced together and taped before receipt by the laboratory, which inhibited examination. Most of the undamaged portions of the fracture surfaces exhibited a rough, fibrous texture with a dull luster. The inner mating fracture surface was less damaged and was sectioned below the fracture surface to facilitate further examination. The sectioned portion was examined with a scanning electron microscope. Most of the surface exhibited features consistent with overstress fracture; however, there were two opposite facing areas exhibiting flatter, darker features. These areas were located at a tapered area where the trunnion and gusset were joined and showed evidence of fatigue striations that were consistent with having propagated from the outside surface inward. Due to post-fracture damage, only one crack initiation site was able to be observed, and this site exhibited an inward concave geometry inconsistent with a corrosion pit or pore. The cross-sectional thickness of the tapered area in which the evidence of fatigue was found was measured as 0.4 inches. In contrast, the welded gusset was 0.8 inches thick, and the boss exhibited a diameter of 1.75 inches. This configuration would introduce a stress concentration at the locations of the fatigue cracking. The presence of two opposite-facing fatigue cracks was consistent with reverse bending fatigue, suggestive of the gusset bending back and forth at the interface. Once these fatiguecracks had propagated far enough into the boss cross section, the remainder of the part fractured under overstress during the last landing. The chemical composition of the sectioned trunnion was examined using x-ray fluorescence (XRF) and energy dispersive x-ray spectroscopy (EDS). From the data obtained using these techniques, the trunnion was found to be consistent with an AA 2014 aluminum alloy. The hardness and conductivity met specifications consistent with peak hardened temper for this alloy. The welded gusset however, exhibited a different chemical composition. The composition was consistent with an aluminum casting or welding alloy. The electrical conductivity was higher, and the hardness was softer than the Rockwell B scale and was, therefore, not measured conclusively. The hardness of the boss material was found to decrease as it was probed closer to the weld. These data were consistent with the boss being locally softened due to the heat inputs from the welding process.