Examination of a 7075-T6 aluminum alloy pylon strut revealed cracks in two locations on the ears of the strut. Because the part was still intact, the cracks had to be forced open so that the fractures could be examined. Scanning electron microscopy (SEM) of the opened cracks showed that the crack surfaces were covered with a mud crack pattern suggestive of stress-corrosion cracking (SCC). The T6 temper is susceptible to SCC. It was concluded that cracking of the strut could have been aggravated by the hard landing experienced by the aircraft. The strut, however, contained stress-corrosion cracks which were present before the landing. It was recommended that an inspection for SCC be made of all pylon struts with a similar service life.

Examination of several fighter aircraft main landing gear legs revealed unusual cracking in the hard chromium plating that covered the sliding section of the inner strut. The cracking was associated with cracks in the 35 NCD 16 steel beneath the plating. A detailed investigation revealed that the cracking was caused by the combination of incorrect grinding procedure, the presence of hydrogen, and fatigue. The grinding damage generated tensile stresses in the steel, which caused intergranular cracking during the plating cycle. The intergranular cracks were initiation sites for fatigue crack growth during service. It was recommended that the damaged undercarriage struts be withdrawn from service pending further analysis and development of a repair technique.

A crack was detected in one arm of the right-hand horizontal brace of the nose landing gear shock strut from a large military aircraft. The shock strut was manufactured from a 7049 aluminum alloy forging in the shape of a delta. A laboratory investigation was conducted to determine the cause of failure. It was concluded that the arm failed because of the presence of an initial defect that led to the initiation of fatigue cracking. The fatigue cracking grew in service until the part failed by overload. The initial defect was probably caused during manufacture. Fleet-wide inspection of the struts was recommended.