safe-life principle is based on a safety factor applied to data obtained from flight tests and information about properties of the spar material. The life of 11,400 flights for the Viscount inner lower boom was based on safety factors of 3.5 for the ground-air-ground cycle and 5.0 for fatigue damage due to atmospheric gusts. These safety factors were typical for this class of aeroplane. A 50% reduction of mean time to failure does not adequately explain why the inner lower boom in VH-RMQ should have failed before reaching its retirement life. In anticipation that the atmospheric gust spectrum in Australia may be more severe on the Viscount than the spectrum in some other climatic zones the gust spectrum was measured during 14,000 Viscount flights in Australia before 1961. The Department of Civil Aviation accepted the Viscount retirement lives as compatible with the atmospheric gust spectrum these aircraft would encounter during operations in Australia. The airworthiness design requirements applicable to the Vickers Viscount and other safe-life transport category aeroplanes did not require retirement lives to be determined taking account of an unpredictable gross defect of the kind inflicted on the wing spar of VH-RMQ by insertion of the flared bush. Similarly, the airworthiness maintenance requirements did not require periodic inspection for fatigue cracking of the wing spars. VH-RMQ was inspected by Ansett-ANA in May 1968, 922 flights prior to the accident, but it was not a requirement of this inspection that the wing structure be disassembled to allow access to the lower booms. Even if the wing had been disassembled it is unlikely the cracks radiating from the damaged bolt hole could have been detected. Early in the life of the Viscount aircraft type, renewal of the inner lower booms included installation of new mount fittings for attachment of the rear of the two inboard engine nacelles to the lower booms. New fittings were supplied without pre-drilled holes and the holes were drilled during installation to correctly align the engine nacelle with the wing. However, after considerable in-service experience of the boom-renewal process British Aircraft Corporation amended the procedure to allow re-use of the engine nacelle rear mount fittings. Re-use of the old fittings relied on the existing holes aligning closely with the bushes in the new inner lower booms. When new inner lower booms were installed in VH-RMQ in 1958, new engine nacelle rear mount fittings were also installed but