Incident Overview
During preflight examination, the first officer reported no abnormalities with the aircraft, and it was deemed suitable for flight. However, during taxi, the airplane unexpectedly dipped to the left and skidded to a halt. The flight crew suspected a tire blowout and, after contacting ground control, was informed by the aircraft behind that the left outboard main tire was smoking. Airport emergency authorities responded and observed that the left main landing gear had failed, with no smoke or fire present.
Maintenance Findings
Maintenance personnel confirmed that the bogie on the left main gear had failed. Specifically, the left main landing gear truck beam had fractured into two pieces between the aft axle and the truck pivot pin. The fracture occurred approximately 14 inches forward of the aft axle centerline. Notably, the truck beam shield was missing from the wreckage, although the attach straps were present. The Boeing 757 Configuration Deviation List (CDL), current at the time of the incident, allowed the main landing gear truck beam shields to be missing for flight, and there was no requirement to inspect the landing gear for damage prior to dispatch with the shield missing. Maintenance personnel had not identified the missing truck beam shield on the left main landing gear, and it was not listed as a missing item under the CDL.
Metallurgical Analysis
Metallurgical analysis revealed that the truck beam fracture resulted from stress corrosion cracking (SCC). The SCC was initiated by damage to the protective finishes on the lower surface of the beam. The material used for the beam is highly susceptible to SCC at the specified high strength levels. The operator reported that the landing gear was delivered new with the airplane on July 29, 1996, and had accumulated 7,517 cycles since new.