On 31 December 1968 a Vickers Viscount aircraft departed from Perth, Western Australia for a flight of 724 nautical miles (1341 km) to Port Hedland. The aircraft crashed 28 nautical miles (52 km) short of its destination with the loss of all twenty-six people on board. More than half of the right wing, from outboard of the inner engine to the wingtip, including the outer engine and its propeller, broke away from the rest of the aircraft in flight and struck the ground a significant distance from the main wreckage. Investigation by the Australian Department of Civil Aviation and British Aircraft Corporation concluded that a mysterious action during maintenance led to extensive fatigue cracking in the right wing spar. This accident remains the third worst in Australia's civil aviation history.
MacRobertson Miller Airlines Flight 1750, a Vickers Viscount registered VH-RMQ, took off from Perth Airport at 8:36 am local time. On board were two pilots, two air hostesses and twenty-two passengers. The aircraft climbed to an altitude of 19,000 feet (5,800 m) for the 189-minute flight.
At 11:34 am the pilot advised that the aircraft was 30 nautical miles (56 km) short of its destination and passing altitude 7,000 feet (2,100 m) on descent to Port Hedland airport. No further radio transmission was received from the aircraft. Four seconds after completion of this transmission half of the right wing separated from the aircraft. Twenty-six seconds later the aircraft fuselage struck the ground.
As the aircraft was falling two people are known to have observed it from distances of 4.5 miles (7.2 km) and 6.5 miles (10.5 km) but because of intervening high terrain neither saw it strike the ground. When the crew of the aircraft failed to respond to further radio calls a Cessna 337 aircraft was despatched from Port Hedland airport at 12:12 pm to investigate. Eleven minutes later the pilot of the Cessna reported sighting the burning wreckage. A ground party from Port Hedland arrived at the accident site an hour later and confirmed that none of the occupants had survived the impact.
The aircraft crashed on Indee Station on level rocky ground vegetated with spinifex grass and some stunted trees. Wreckage was spread over an area about 7,750 feet (2,360 m) long and 2,500 feet (760 m) wide.
Accident investigators immediately observed that half of the right wing, its outer engine and propeller were close together about 3,000 feet (910 m) from the main wreckage. The half-wing had been driven into the rocky ground by the impact but it was clear that the wing's main spar had fractured in flight causing immediate separation of the half-wing from the rest of the aircraft.
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 7⁄8 inch (2.22 cm) diameter and were anodised to resist wear and corrosion. A cadmium-plated steel bush of length 1+5⁄8 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 the absence of an interference fit left the inner lower boom vulnerable to developing fatigue cracks at Station 143.
Despite exhaustive investigation it was not possible to determine when, why, or by whom, the bush at Station 143 had been flared with a conical tool, removed and then re-inserted in the bolt hole. Investigators could not imagine circumstances in which a responsible tradesman would take these actions.
Approximately 5000 flights after new inner lower booms were installed in 1964, numerous fatigue cracks began to develop in both the forward and rear edges of the hole. These cracks eventually joined to form a single crack growing forwards from the forward edge of the hole, and a single crack growing rearwards from the rear edge of the hole. These two cracks grew to affect 85% of the cross-sectional area of the inner lower boom at Station 143.
Seven weeks after the accident the Minister for Civil Aviation, Reg Swartz, announced that the accident had been caused by metal fatigue and he did not consider it necessary to appoint a court to inquire into the accident. This position was challenged by the opposition spokesman for aviation, Charlie Jones.
British Aircraft Corporation carried out multiple tests in which a bush was slightly flared with a conical tool and pressed into a hole in a test piece of the same aluminium alloy as the inner lower boom. Each test piece was then subjected to alternating stresses. These tests showed that eliminating the interference fit by insertion of a flared bush identical to the one found in the wreckage of VH-RMQ substantially reduced the mean life to failure of the boom – possibly by as much as 50%.
The investigation by the Australian Department of Civil Aviation was completed in September 1969 and concluded:The cause of this accident was that the fatigue endurance of the starboard inner main spar lower boom was substantially reduced by the insertion of a flared bush at Station 143 when the margin of safety associated with the retirement life specified for such booms did not ensure that this boom would achieve its retirement life in the presence of such a defect.
When the Minister tabled the report in the Parliament in September 1969, Jones again called for a public inquiry.
The aircraft was a Vickers Viscount 720C manufactured in 1954 and assigned serial number 45. It was immediately acquired by Trans Australia Airlines and entered airline service in Australia as VH-TVB. In 1959 it appeared at that year's Farnborough Airshow. It was sold to Ansett-ANA in 1962 and re-registered as VH-RMQ. In September 1968 the aircraft was transferred to Western Australia and operated by MacRobertson Miller Airlines, by then a subsidiary of Ansett-ANA.
In 1958 the operator, Trans Australia Airlines, replaced both inner lower booms. In 1964 the new owner, Ansett-ANA, again replaced both inner lower booms. In February 1968 the aircraft became the first Australian Viscount to reach 30,000 flying hours. It was last inspected by Ansett-ANA in May 1968 when it had made 7,169 flights since the 1964 lower boom replacement. It made another 922 flights before the accident.