MacRobertson Miller Airlines Flight 1750

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 1⅝ 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.