that the wing's main spar had fractured in flight causing immediate separation of the half-wing from the rest of the aircraft. Investigation Detailed investigation of the two fracture surfaces showed that metal fatigue had caused cracks to grow across the lower boom (or lower flange) of the right wing's main spar until they affected approximately 85% of the cross-sectional area. With so much of the lower boom affected the wing could no longer support the weight of the aircraft, the lower boom suddenly broke in two and the outer half of the right wing separated from the inner half. The mandatory retirement life of the lower boom in the inner wing was 11,400 flights. A pair of new inner lower booms had been installed in VH-RMQ in 1964 and had been in service for only 8,090 flights. The investigation concentrated on determining why the inner lower boom had failed at 70% of its retirement life. The fatal fatigue crack in the inner lower boom had been initiated at a bolt hole at Station 143, the rearmost of five bolt holes for attachment of the inner engine nacelle to the lower boom. These holes were ⅞ inch (2.22 cm) diameter and were anodised to resist wear and corrosion. A cadmium-plated steel bush of length 1 ⅝ inch (4.13 cm), chamfered at one end, was pressed into each hole. Each bush was an interference fit in the hole to improve fatigue resistance and substantially increase the retirement life of the inner lower boom. The investigation determined that some years before the accident, the bush at Station 143 had been pushed upwards so the chamfer and 0.055-inch (1.40 mm) of the parallel-sided portion protruded beyond the upper surface of the boom. The exposed end of the bush was then struck with a conical tool applied to the bore. This action slightly flared the exposed end and left the external diameter 0.0038 inch (0.097 mm) oversize. The bush was then pushed upwards out of the hole and re-inserted from the lower surface. As the bush was being re-inserted its flared end broached the anodised material and a small amount of aluminium from the wall of the hole. This broaching action scored the wall of the hole and left its diameter slightly oversize so the bush was not an interference fit anywhere except at its flared end. Scoring of the wall of the hole and