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MacRobertson Miller Airlines Flight 1750

From Alfawiki
MacRobertson Miller Airlines
IndustryAviation
Headquarters
Western Australia
,
Australia
ParentAnsett-ANA


Template:EngvarB Template:Infobox Aircraft accident On the 31st of December, 1968, a Vickers Viscount departed from Perth, Western Australia, embarking on a journey covering 724 nautical miles (1341 km) destined for Port Hedland. The aircraft crashed approximately 28 nautical miles (52 km) before reaching its intended airport, resulting in the loss of all twenty-six individuals aboard.[1][2] During flight, more than half of the right wing—from beyond the inner engine to the wingtip—including the outer engine and its propeller detached and impacted the ground at a considerable distance from the main debris field.[3] Subsequent inquiries by the Australian Department of Civil Aviation alongside the British Aircraft Corporation revealed that an unusual maintenance procedure caused severe fatigue cracking within the right wing spar.[4][5] This tragedy ranks as the third deadliest accident in Australia's civil aviation records.[6]

Flight details

Flight 1750 operated by MacRobertson Miller Airlines, a Vickers Viscount registered as VH-RMQ, departed from Perth Airport at 08:36 am local time. The flight carried a total of twenty-six people: two pilots, two air hostesses, and twenty-two passengers.[Note 1] The aircraft ascended to a cruising altitude of 19,000 feet (5,800 m) for what was scheduled as a 189-minute journey.[2][4][7]

At approximately 11:34 am, the pilot communicated that they were descending through 7,000 feet (2,100 m), located about 30 nautical miles (56 km) from Port Hedland airport. No further radio contact was established following this message. Merely four seconds after this transmission ended, half of the right wing separated from the fuselage. The aircraft then impacted the terrain twenty-six seconds later.[7][8]

During the descent and subsequent fall, two witnesses observed the aircraft from distances of 4.5 miles (7.2 km) and 6.5 miles (10.5 km), although neither saw the actual crash due to obstructing elevated terrain. After no response to repeated radio calls from the crew, a Cessna 337 was dispatched from Port Hedland airport at 12:12 pm to search for the flight. Eleven minutes into its mission, the Cessna pilot reported spotting smoke and wreckage on fire. Ground teams reached the crash site an hour later and confirmed that all occupants had perished upon impact.[2][7]

Wreckage

The plane came down on Indee Station[1] situated on flat, rocky terrain covered with spinifex grass and sparse, stunted trees. Debris was scattered across an area approximately 7,750 feet (2,360 m) in length and 2,500 feet (760 m) in width.[4][8]

Upon arrival, investigators noted that half of the right wing, along with its outer engine and propeller, was located roughly 3,000 feet (910 m) away from the main crash site. This section of the wing had been forcibly embedded into the rocky surface by the impact. It was evident that the primary spar of the wing had fractured while airborne, causing the immediate detachment of the outer half-wing from the remainder of the aircraft.[3][7]

Investigation

A thorough examination of the fracture surfaces revealed that metal fatigue had propagated cracks across the lower flange (or lower boom) of the right wing’s main spar, compromising about 85% of its cross-sectional area. With such extensive damage to the lower flange, the wing lost its structural integrity, resulting in a sudden break and separation of the outer right wing from its inner section.[7][9]

The prescribed service life for the inner wing’s lower flange was 11,400 flights. VH-RMQ had received a pair of new inner lower flanges in 1964, which had accumulated only 8,090 flights at the time. The investigation focused on uncovering why failure occurred at just 70% of the expected retirement life.[9][10][11]

The critical fatigue crack originated at a bolt hole located at Station 143—the rearmost of five bolt holes securing the inner engine nacelle to the lower flange. These holes measured 7⁄8 inch (2.22 cm) in diameter and were anodised to prevent corrosion and wear. Each hole contained a cadmium-plated steel bushing measuring 1⁄1}{5}{8 inch (4.13 cm) in length with one end chamfered. The bushings were press-fitted into their holes to enhance fatigue resistance and significantly extend the lower flange’s service life.[Note 2][9][12][13]

Investigators found that some years before the crash, the bushing at Station 143 had been pushed upward so that its chamfer and 0.055-inch (1.40 mm) parallel-sided section extended beyond the upper surface of the flange. The exposed end was then struck with a conical tool inserted into its bore, which slightly flared this end and increased its external diameter by 0.0038 inch (0.097 mm).[14] Subsequently, this bushing was removed by pushing it upwards out of the hole and then reinserted from beneath. During reinsertion, its flared end broached away anodised material and some aluminium from the hole wall, scoring it and enlarging its diameter slightly so that an interference fit existed only at the flared end. This scoring combined with lack of proper interference fit left the inner lower flange susceptible to fatigue cracking at Station 143.[15][16]

Despite extensive efforts, investigators could not establish when, why, or by whom this bushing had been flared with a conical tool, removed, and reinserted in this manner. They found it difficult to conceive any situation where a skilled tradesperson would deliberately perform such actions.[7][17]

Around 5,000 flights after installation of new inner lower flanges in 1964, multiple fatigue cracks began appearing along both forward and rear edges of this bolt hole.[18] These cracks eventually merged into one extending forward from the hole’s front edge and another extending backward from its rear edge. Ultimately these fractures compromised approximately 85% of the cross-sectional area of the inner lower flange at Station 143.[19]

Seven weeks post-accident, Civil Aviation Minister Reg Swartz announced that metal fatigue was responsible for the crash and deemed a formal court inquiry unnecessary.[20] This decision faced opposition from aviation spokesman Charlie Jones.[21]

British Aircraft Corporation[Note 3] conducted several tests where bushings were intentionally flared using a conical tool and pressed into test pieces made from identical aluminium alloy as used in inner lower flanges. These samples underwent cyclic stress testing which demonstrated that removing the interference fit by inserting a flared bushing—like that found in VH-RMQ’s wreckage—significantly shortened mean fatigue life by up to approximately half.[22][23]

The Australian Department of Civil Aviation finalized their investigation in September 1969 with this conclusion:

The accident resulted from a substantial reduction in fatigue endurance of the starboard inner main spar lower boom due to insertion of a flared bushing at Station 143. The safety margin associated with specified retirement life did not guarantee achievement of full service life when such a defect was present.[4][24]

When presenting this report to Parliament later that month, Jones reiterated his call for a public inquiry into the crash.[25]

Aircraft

The aircraft involved was a Vickers Viscount 720C, constructed in 1954 and bearing the serial number 45. It was promptly acquired by Trans Australia Airlines and commenced service in Australia under the registration VH-TVB. In 1959, it was showcased at that year's Farnborough Airshow. Ownership changed in 1962 when it was sold to Ansett-ANA and re-registered as VH-RMQ. By September 1968, the aircraft had been relocated to Western Australia, operating under MacRobertson Miller Airlines, which had become a subsidiary of Ansett-ANA.

During its operational history, Trans Australia Airlines replaced both inner lower booms in 1958. Subsequently, Ansett-ANA carried out a similar replacement of these components in 1964. In February 1968, this aircraft became the first Viscount in Australia to surpass 30,000 flight hours. Its last maintenance inspection by Ansett-ANA occurred in May 1968, at which point it had completed 7,169 flights since the 1964 replacement of the lower booms. The aircraft completed an additional 922 flights before the accident.

By the end of 1968, VH-RMQ had accumulated a total of 25,336 flights and logged 31,746 flight hours. Since its most recent comprehensive overhaul, it had flown 6,429 flights and logged 7,188 hours.

Recorders

This aircraft was fitted with both a flight data recorder and a cockpit voice recorder. The flight data recorder operated continuously throughout the flight, capturing key parameters such as pressure altitude, indicated airspeed, vertical acceleration, and magnetic heading up until impact. Although the cockpit voice recorder sustained minor damage from the crash and ensuing fire, the recordings of radio communications during the last half-hour remained intact. Additionally, ambient cockpit noise was preserved, revealing the exact instant when noise frequency and intensity abruptly increased.

Safe-life design

The wing structure of the Vickers Viscount featured a single main spar composed of multiple sections: a center segment within the fuselage, two inner segments, and two outer segments. This main spar consisted of an upper boom, shear web, and lower boom. The design and certification of the aircraft adhered to the principles of a safe-life methodology. Components were required to be retired before exceeding their designated safe-life limits. At the time of this accident, retirement thresholds were set at 20,500 flights for the center section lower boom; 11,400 flights for the inner lower boom; and 19,000 flights for the outer lower boom. The spars within the horizontal tailplanes and vertical stabilizer had retirement lives established at 30,000 flights.

Retirement lives for wing spars on transport category aircraft certified under safe-life criteria are determined by applying safety factors to flight test results and material property data. For the Viscount’s inner lower boom with a life limit of 11,400 flights, safety factors of 3.5 were applied for ground-air-ground cycles and 5.0 for fatigue caused by atmospheric gusts—values consistent with industry standards for this class of aircraft. A mere halving of expected time to failure cannot sufficiently explain why VH-RMQ’s inner lower boom failed prior to reaching its retirement life. To address concerns that atmospheric gust conditions in Australia might be harsher than elsewhere, measurements were taken during approximately 14,000 Viscount flights within Australia before 1961. The Department of Civil Aviation accepted that these retirement lives were appropriate given Australian operational conditions.

The airworthiness standards applicable to the Vickers Viscount and other safe-life transport category airplanes did not mandate retirement lives be adjusted to account for unpredictable major defects such as those caused by insertion of a flared bush into VH-RMQ’s wing spar. Similarly, maintenance regulations did not require routine inspections specifically aimed at detecting fatigue cracks within wing spars.

Ansett-ANA conducted an inspection on VH-RMQ in May 1968—922 flights before the accident—but this did not include disassembly of wing structures to access lower booms. Even if disassembly had occurred, detecting cracks emanating from damaged bolt holes would have been improbable.

Early in the Viscount’s service life, replacing inner lower booms involved fitting new mountings for attaching the rear sections of the two inboard engine nacelles to these booms. Initially, new mount fittings were supplied without pre-drilled holes; holes were drilled during installation to ensure precise alignment between engine nacelle and wing. However, after extensive operational experience with boom replacements, British Aircraft Corporation modified procedures to permit reuse of existing nacelle rear mount fittings—assuming alignment between old holes and new bushes was sufficiently close.

When VH-RMQ received new inner lower booms in 1958, new mount fittings were installed; however, during another replacement in 1964, fittings originally installed in 1958 were reused. Examination of wreckage from VH-RMQ’s right wing revealed initial difficulties aligning five holes in these reused fittings with bushes on the new boom. Three bushes exhibited scoring consistent with drilling—likely caused by maintenance personnel attempting to align holes enough to insert attachment bolts. This drilling removed small amounts (.006 inch) from bush bores at Station 143 and other locations; attachment bolts measured 5/16 inch in diameter. It is probable that drilling through the bush at Station 143 disturbed its integrity and initiated damage leading ultimately to catastrophic failure of the hole wall.

Aftermath

Following the incident, the Department of Civil Aviation promptly imposed a temporary grounding on all Australian-registered Viscount Type 700 aircraft.[26] This provisional suspension was later converted into a permanent ban while investigations into the accident's cause were underway.[27]

The discovery of fatigue failure in the wing of VH-RMQ immediately cast doubt on the established retirement life of the inner lower boom in the Type 700 model. As a precautionary measure, British Aircraft Corporation along with the UK Air Registration Board (ARB) reduced the component’s service life from 11,400 to 7,000 flights.[4][10][28] Subsequently, British Aircraft Corporation acquired several inner lower booms that had exceeded this newly imposed limit. Detailed inspections were conducted on nineteen of these retired components, revealing that sixteen had minor fatigue cracks in various critical areas. The most extensive crack measured 0.054 inches (1.37 mm) in an inner lower boom that had logged 8,194 flights. This evidence led both British Aircraft Corporation and the UK Air Registration Board to conclude that the original fatigue life estimate was overly optimistic, thereby cementing the reduced limit of 7,000 flights as permanent.[10][18][28]

At the time of this tragedy, the death toll rendered it Australia's third deadliest civil aviation disaster—a ranking it maintains to this day.[6] Two other civil aviation accidents each resulted in 29 fatalities: the 1950 Australian National Airways Douglas DC-4 crash and Trans Australia Airlines Flight 538 in 1960.

See also

Notes

  1. ↑ Certain sources indicate there were twenty-one passengers and three air hostesses. Among those aboard was a trainee air hostess travelling solely as a passenger for familiarisation purposes; she held a passenger ticket issued by the airline and was not assigned any standard cabin duties.[2]
  2. ↑ The interference fit ranged between .0015 and .0029 inch in diameter.[9] The bush lengths varied slightly to accommodate tapering thicknesses of the lower flange.[9]
  3. ↑ The original manufacturer Vickers-Armstrongs merged into British Aircraft Corporation in 1960.

References

  1. ↑ 1.0 1.1 "26 Killed in Viscount Crash" The Canberra Times – 1 January 1969, p.1 (National Library of Australia) Retrieved 7 October 2013
  2. ↑ 2.0 2.1 2.2 2.3 Accident Investigation Report, Section 1.1
  3. ↑ 3.0 3.1 Accident Investigation Report, page 9
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Operational history of aircraft C/N 45 Retrieved 16 August 2011
  5. ↑ Aviation Safety Network Retrieved 16 August 2011
  6. ↑ 6.0 6.1 Ten Worst Aircraft Crashes in Australia Archived 19 March 2012 at the Wayback Machine Retrieved 16 August 2011
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Job 1992, p. 196
  8. ↑ 8.0 8.1 Accident Investigation Report, page 8
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Accident Investigation Report, page 11
  10. ↑ 10.0 10.1 10.2 Accident Investigation Report, page 26
  11. ↑ Accident Investigation Report, page 24
  12. ↑ Accident Investigation Report, page 19
  13. ↑ Accident Investigation Report, page 20
  14. ↑ Accident Investigation Report, page 13
  15. ↑ Accident Investigation Report, page 25
  16. ↑ Accident Investigation Report, page 16
  17. ↑ Accident Investigation Report, pages 24, 25
  18. ↑ 18.0 18.1 Accident Investigation Report, page 27
  19. ↑ Accident Investigation Report, page 23
  20. ↑ "No inquiry into air crash" The Canberra Times – 22 February 1969, p.3 (National Library of Australia) Retrieved 26 April 2014
  21. ↑ "Inquiry sought on air crash" The Canberra Times – 19 March 1969, p.11 (National Library of Australia) Retrieved 26 April 2014
  22. ↑ Accident Investigation Report, page 18
  23. ↑ Accident Investigation Report, Section 3.23
  24. ↑ Accident Investigation Report, Section 3.28
  25. ↑ "Crash was 'clear case of negligence'" The Canberra Times – 27 September 1969, p.11 (National Library of Australia) Retrieved 26 April 2014
  26. ↑ "Other disasters" The Canberra Times – 1 January 1969, p.1 (National Library of Australia) Retrieved 20 December 2013
  27. ↑ "Viscount 700 series to stay grounded" The Canberra Times – 26 September 1969, p.15 (National Library of Australia) Retrieved 26 April 2014
  28. ↑ 28.0 28.1 Cite error: Invalid <ref> tag; no text was provided for refs named p21

Bibliography

  • ACCIDENT INVESTIGATION REPORT by Air Safety Investigation Branch, Department of Civil Aviation (15MB file)
  • Lua error in Module:Citation/CS1/Configuration at line 2123: attempt to index field '?' (a nil value). Fyshwick, Australia. pp. 200. ISBN 1-875671-01-3
  • Operational history of aircraft C/N 45

External links

Template:Aviation accidents and incidents in 1968 Template:Aviation accidents and incidents in Australia

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