Accident Summary
On June 4, 2020, at 1821 Pacific daylight time, a Piper PA30 airplane, registration N8244Y, was substantially damaged during a landing at Salem, Oregon. The pilot and passenger were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Flight Sequence
The pilot reported that after departure, the landing gear was raised and an unusual "clunk" was heard. However, the gear retraction light illuminated normally, and a mirror on the left engine nacelle indicated the nose gear was retracted. The flight continued. Approaching the destination, the pilot lowered the landing gear, but the green "down and locked" light did not illuminate. The nose landing gear door was open, but the nose gear was not extended. The pilot climbed and attempted troubleshooting, including the emergency gear extension procedure, but the system was totally jammed. The pilot diverted to a nearby airport, performed a flyby, and ground personnel reported the main landing gear was partially extended and the nose gear was not extended. The pilot landed the airplane, and both occupants egressed without injury. The airplane sustained substantial damage to the keel beam and the right-wing spar.
Postaccident Examination
Examination revealed that the right main landing gear trunnion was fractured. The trunnion was sent to the NTSB Materials Laboratory for analysis. The fracture surfaces had been forced together and taped before receipt, which inhibited examination. Most undamaged portions exhibited a rough, fibrous texture with a dull luster. The inner mating fracture surface was less damaged and was sectioned below the fracture surface for further examination.
The sectioned portion was examined with a scanning electron microscope. Most of the surface showed features consistent with overstress fracture. However, two opposite-facing areas exhibited flatter, darker features at a tapered area where the trunnion and gusset were joined. These areas showed evidence of fatigue striations consistent with propagation from the outside surface inward. Due to post-fracture damage, only one crack initiation site could be observed, which exhibited an inward concave geometry inconsistent with a corrosion pit or pore.
The cross-sectional thickness of the tapered area where fatigue was found was 0.4 inches, while the welded gusset was 0.8 inches thick and the boss had 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 fatigue cracks propagated far enough into the boss cross section, the remainder of the part fractured under overstress during the last landing.
Chemical composition of the sectioned trunnion was examined using X-ray fluorescence and energy dispersive X-ray spectroscopy, and found to be consistent with AA 2014 aluminum alloy. Hardness and conductivity met specifications for peak hardened temper for this alloy. The welded gusset had a different composition, consistent with an aluminum casting or welding alloy. Its electrical conductivity was higher, and hardness was softer than the Rockwell B scale. Hardness of the boss material decreased closer to the weld, consistent with local softening due to heat from welding.
Probable Cause
The source does not provide a stated probable cause.