The accident occurred when the aircraft, which had a landing gear problem on its first approach to Heathrow Airport, carried out an emergency landing on runway 27L with the left main landing gear only partially extended. The flight crew responded to the in-flight emergency with commendable judgment and conducted a skilful landing, with the Airport Emergency Services in full and effective attendance. The evacuation was completed with minor injuries to 5ÿpassengers and 2 crew members. Examination of the left main landing gear found that the gear had been jammed by the No 6 wheel brake torque rod which had disconnected from its brake pack assembly and had become trapped in the keel beam structure. The associated torque rod pin was subsequently found beyond the end of runway 24L at Los Angeles International Airport, the departure airport. CAUSAL FACTORS: 1) Full deployment of the left main landing gear was prevented by the unrestrained end of the No 6 brake torque rod having become trapped in the keel beam structure within the gear bay, jamming the landing gear in a partially deployed position. 2) The torque pin which had connected No 6 brake torque rod to that wheel brake assembly had disengaged during landing gear retraction after take off from Los Angeles, allowing the unrestrained rod to pivot freely about the retained end. 3) The torque pin and its retaining assembly had been subject to higher axial and torsional loads than predicted during aircraft braking in service. These loads were the result of elastic deformation of the wheel axle, brake and torque rod, and due to assembly without the correct axial clearance as a result of prior undetected displacement of the associated bushes. The precise mode of failure of the retaining assembly bolt, nut and cotter pin could not be ascertained in the absence of these parts. 4) This design of wheel brake assembly had satisfactorily passed the related certification wheel brake structural torque test to the requirements of TSO C26c paragraph 4.2(b). However the latter contained no requirement to use a representative axle or other means to reproduce the axle deflections which occur during aircraft braking in service, and did not require post torque test strip assessment of brake assemblies for resultant evidence of overstressing deformation which did not produce component failure.