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.
[3] 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.
[6] MacRobertson Miller Airlines Flight 1750, a Vickers Viscount registered VH-RMQ, took off from Perth Airport at 08:36 am local time.
[2][4][7] 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.
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.
[4][8] 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.
Each bush was an interference fit in the hole to improve fatigue resistance and substantially increase the retirement life of the inner lower boom.
[Note 2][9][12][13] 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.
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.
[15][16] 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.
[7][17] 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 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 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.
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.
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.
[27] The bores of three bushes had been scored with a drill, possibly while maintenance personnel were trying to align three of the holes sufficiently to be able to insert the attachment bolts.
This evidence convinced British Aircraft Corporation and the UK Air Registration Board that the inner lower boom did not possess the fatigue endurance originally intended so the precautionary life of 7,000 flights was made permanent.
[6] Two civil aviation accidents caused 29 fatalities each – the 1950 Australian National Airways Douglas DC-4 crash and the 1960 Trans Australia Airlines Flight 538.