Fracture of a Landing-Gear Flat Spring

A flat spring for the main landing gear of a light aircraft failed after safe execution of a hard landing. The spring material was identified by chemical analysis to be 6150 steel. The fracture was found to have occurred near the end of the spring that was inserted through a support member about 25 mm thick and attached to the fuselage by a single bolt. Brinelling (plastic flow and indentation due to excessive localized contact pressure) was observed on the upper surface of the spring where the forward and rear edges of the spring contacted the support member. It was indicated by chevron marks that brittle fracture had started beneath the brinelled area at the forward edge of the upper surface of the spring. The origin of the brittle fracture was found to be a small fatigue crack that had been present for a considerable period of time before final fracture occurred. Fracture of the landing-gear spring was concluded to have been caused by a fatigue crack that resulted from excessive brinelling at the support point. Regular visual examinations to detect evidence of brinelling and wear at the support in aircraft with this configuration of landing-gear spring were recommended.