Investigation Summary
The investigation focused on the fracture of the starboard inner main spar lower boom at Station 143, which was the rearmost of five bolt holes used for the attachment of the inner engine nacelle. This boom had a mandatory retirement life of 11,400 flights but failed after only 8,090 flights. Forensic analysis revealed that a cadmium-plated steel bush, which should have been an interference fit to enhance fatigue resistance, had been modified. At some point during maintenance, the bush was flared with a conical tool, making it 0.0038 inches oversize. When re-inserted, this flared end acted as a broach, scoring the anodized wall of the bolt hole and removing aluminum material. This scoring created stress concentrations that initiated fatigue cracks. These cracks eventually joined to affect 85% of the boom's cross-sectional area, leading to a sudden failure under normal flight loads. Despite an exhaustive search of maintenance records, investigators could not identify when or by whom the bush was flared and re-inserted, noting that such an action was contrary to all established engineering practices.
Final Conclusions
The Australian Department of Civil Aviation concluded in September 1969 that the accident was caused by the substantial reduction of the starboard inner main spar lower boom's fatigue endurance. This was due to the insertion of a flared bush at Station 143, which introduced a defect that the specified retirement life margin of safety could not accommodate. The investigation also prompted a global reassessment of the Viscount 700 series fatigue life.
Video Analysis
The Crash of MacRobertson Miller Airlines Flight 1750 — A detailed look at the structural failure of VH-RMQ and the maintenance mystery behind the flared bush that led to the loss of 26 lives.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
First Officer
Systems & Failure Modes
Main Spar Lower Boom
The primary load-bearing structural member of the wing, designed to withstand upward bending forces during flight.
Interference Fit Bushing
A steel sleeve pressed into bolt holes with a diameter slightly larger than the hole to induce compressive stress, which significantly improves the fatigue life of the aluminum spar.
Interesting Facts
- 01The aircraft was the first Australian Viscount to reach 30,000 flying hours.
- 02The fatigue crack affected 85% of the spar's cross-sectional area before the final failure.
- 03The failure occurred during a routine descent under approximately 1g steady flight loads.
- 04Wreckage was scattered over an area 7,750 feet long and 2,500 feet wide.
- 05The aircraft's manufacturer serial number (MSN) was 45.
- 06At the time, it was the third worst civil aviation accident in Australian history.
Safety Actions & Advisories
Retirement Life Reduction
The UK Air Registration Board and British Aircraft Corporation reduced the retirement life of the inner lower boom for Viscount 700 series aircraft from 11,400 flights to 7,000 flights. This precautionary reduction was later made permanent after inspections of 19 retired booms revealed fatigue cracks in 16 of them.
Fleet Grounding
The Australian Department of Civil Aviation initially grounded all Australian-registered Viscount Type 700 aircraft temporarily; this grounding was later made permanent for the type in Australia.
Known Controversies & Unanswered Questions
Public Inquiry Dispute
Reference ↗The Australian opposition spokesman for aviation, Charlie Jones, repeatedly challenged the government's decision not to hold a formal public court of inquiry into the accident.