CEN19LA322

The airline transport pilot reported that, upon landing, he immediately felt a shift to the left. The left main landing gear fractured and collapsed, and the airplane went off the runway. Postaccident metallurgical examination of the landing gear revealed that the fracture occurred from a thumbnail-shaped fatigue crack in the gear spring strut with multiple origins near the inboard attachment location at the forward edge of its upper surface. Final overstress fracture propagation direction was consistent with a downward-and-aft bending failure mode; thus, the fracture was more likely due to overload rather than the fatigue crack. This was consistent with drag loading of the left main landing gear wheel in combination with side-loading of the outboard side of the left main landing gear wheel, such as that encountered during a crosswind landing, ground loop, and/or hard left turn event. The loads experienced by the landing gear during the accident landing could not be determined, and a structural analysis was not performed to determine if the loads exceeded design load limitations.

On September 6, 2019, at 1230 central daylight time, a Cessna 195A airplane, N9895A, impacted terrain after it veered off runway 30 at Pocahontas Municipal Airport (POH), Pocahontas, Iowa. The airplane sustained substantial damage. The airline transport pilot and a passenger were uninjured. The airplane was registered to and operated by the pilot under Title 14 Code of Federal Regulations Part 91 as a personal flight that was not operating on a flight plan. Day visual meteorological conditions prevailed at the time of the accident. The flight originated from Bismarck Municipal Airport (BIS), Bismarck, North Dakota, and was destined to POH. The pilot stated the wind was from the north and he determined that runway 30 was an appropriate runway to land on. He stated that during the landing touchdown, he immediately felt a shift to the left. The left main landing gear separated and collapsed, and the airplane went off the runway. The airplane came to a stop with only the tail remaining on the runway. The airplane sustained substantial damage that included wrinkling and upward bending of the left wing. The fractured left main landing gear spring strut was fractured through and shipped to the National Transportation Safety Board Materials Laboratory for examination. The spring strut fractured at approximately 7 inches from the inboard edge at the forward side and approximately 6 inches from the inboard edge at the aft side. A light gray paint coating appeared over the entire part surface and a darker gray paint coating was only present on the outboard piece with a clearly delineated inboard boundary. Both gray coatings were chipped and absent from numerous locations over the part surface thereby exposing areas of the underlying light gray coating as well as areas of brownish-red oxidized metal. A visibly distinct band of brighter light-gray coating and exposed oxidized metal was apparent on the upper and lower surfaces of the spring strut on both sides of the fracture as indicated between the white dashed lines. The upper surface of the spring strut was visibly smoother than the lower surface, which exhibited a dimpled texture. The mating outboard fracture surface of the strut exhibited significant post-fracture mechanical damage and oxidation which obscured some of the finer fracture details. The fracture origin was located on the upper surface at the forward edge of the spring strut. Chevron marks radiated outward from this location along the upper surface and toward the lower surface consistent with an aft and downward crack propagation direction. Closer examination of the origin area revealed a small discolored thumbnail shaped crack and ratchet mark features which were consistent with fatigue cracking from multiple origins. The upper surface of the spring strut adjacent to the thumbnail shaped crack at the fracture origin was absent of gray coating and had exposed oxidized metal over an area approximately 1 inch inboard of the fracture surfaceand approximately 0.75 inch aft of the forward edge. This area was pitted and rougher than the surrounding upper surface where gray coating was intact. Reddish-orange staining was also observed near the fracture origin on the surrounding gray coating. The thumbnail shaped pre-existing crack region was relatively flat and smooth compared to the rest of the fracture surface. Crack arrest lines were evident progressing from at least three origins separated by two visible ratchet marks. Smaller, semi-elliptical sections bounded by crack arrest lines within the thumbnail crack were brownish-red in color, and progressively lighter in color away from the origins. The pre-existing crack measured approximately 0.131 inch along the upper surface of the spring strut and extended approximately 0.058 inch into the part. The fracture surface morphology outside of the thumbnail shaped pre-existing crack was dull and rough in appearance, consistent with overstress separation. The loads experienced by the landing gear during the accident landing are unknown, and a structural analysis was not performed to determine if the loads exceeded design load limits.