MacRobertson Miller Airlines Flight 1750
Encyclopedia
On 31 December 1968 a Vickers Viscount
aircraft departed from Perth, Western Australia
for a flight of 724 nautical miles (1 341 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.
Flight 1750, a Vickers Viscount registered VH-RMQ, took off
from Perth Airport
at 08:36 am local time. On board were two pilots, two air hostesses and twenty-two passengers. The aircraft climbed to an altitude of 19000 feet (5,791.2 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 7000 feet (2,133.6 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.
on level rocky ground vegetated with spinifex grass and some stunted trees. Wreckage was spread over an area about 7750 feet (2,362.2 m) long and 2500 feet (762 m) wide.
Accident investigators immediately observed that half of the right wing, its outer engine and propeller were close together about 3000 feet (914.4 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 fracture
d in flight causing immediate separation of the half-wing from the rest of the aircraft.
) 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 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), chamfer
ed 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 chamfered end 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 of the bush 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.
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:
and entered airline service in Australia as VH-TVB. 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.
By 31 December 1968 the aircraft had made 25,336 flights and had flown for 31,746 hours. Since its previous complete overhaul it had made 6,429 flights and had flown for 7,188 hours.
and a cockpit voice recorder
. The flight data recorder functioned throughout the flight and continuously recorded the aircraft's pressure altitude
, indicated airspeed
, vertical acceleration and magnetic heading until the moment of impact with the ground. The cockpit voice recorder was slightly damaged in the impact and subsequent fire but there was no damage to the record of radio transmissions from the aircraft during the final 30 minutes of the flight. The record of ambient noise
in the cockpit was also preserved and revealed the precise moment at which the frequency and volume of noise increased suddenly.
to the principle of a safe-life
. Before a component reaches its safe-life it must be removed from the aircraft and retired. At the time of the accident the retirement life of the lower boom in the centre-section was 20,500 flights; the inner lower boom was 11,400 flights; and the outer lower boom was 19,000 flights. The retirement life of the spars in the horizontal tailplane
s and the vertical fin
was 30,000 flights.
The retirement life of the wing spar of a transport category
aeroplane type-certificated to the safe-life principle is based on a safety factor applied to data obtained from flight tests and information about properties of the spar material. 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. These safety factors were typical for this class of aeroplane. A 50% reduction of mean time to failure does not adequately explain why the inner lower boom in VH-RMQ should have failed before reaching its retirement life. In anticipation that the atmospheric gust spectrum in Australia may be more severe on the Viscount than the spectrum in some other climatic zones the gust spectrum was measured during 14,000 Viscount flights in Australia before 1961. 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.
The airworthiness
design requirements applicable to the Vickers Viscount and other safe-life transport category aeroplanes did not require retirement lives to be determined taking account of an unpredictable gross defect of the kind inflicted on the wing spar of VH-RMQ by insertion of the flared bush. Similarly, the airworthiness maintenance requirements did not require periodic inspection for fatigue cracking of the wing spars.
VH-RMQ was inspected by Ansett-ANA in May 1968, 922 flights prior to the accident, but it was not a requirement of this inspection that the wing structure be disassembled to allow access to the lower booms. Even if the wing had been disassembled it is unlikely the cracks radiating from the damaged bolt hole could have been detected.
On earlier Viscount aircraft replacement of the spar booms entailed fitting a new nacelle rear mounting assembly, the replacement part coming from the manufacturers with the spar attachment mounting holes undrilled, the drilling being performed by the maintenance personnel with the part in situ under the wing. However after considerable in-service experience with the aircraft design the manufacturer, BAC, found that it was permissible to re-use the removed nacelle rear assembly. Replacement of the removed component required accurate positioning of the nacelle assembly under the wing so that the mounting holes aligned with the ones in the wing spar boom before insertion of the bolts. On VH-RMQ evidence of an initial problem aligning the holes so that the bolts would pass through, and the subsequent attempts at resolving this issue, was thought to be the cause of the damaged bushing.
The replacement spar booms, like the early nacelle rear mountings, were supplied by the manufacturer undrilled, the maintenance personnel performing the necessary drilling for the nacelle mounting holes on-site.
Fatigue-failure of the wing of VH-RMQ immediately raised doubt about the validity of the retirement life of the inner lower boom in the Type 700 so British Aircraft Corporation and the UK Air Registration Board took the precaution of reducing the life from 11,400 flights to 7,000. This soon resulted in British Aircraft Corporation obtaining a number of inner lower booms with time in service greater than 7,000 flights. Nineteen of these retired booms were examined in detail. Sixteen contained minor fatigue cracks in different critical locations. The longest crack was 0.054 inch (1.37 mm) in a boom that had been in service for 8,194 flights. 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.
When this accident occurred the number of fatalities made it Australia's third worst civil aviation accident. It remains Australia's third worst civil aviation accident. Two civil aviation accidents caused 29 fatalities each - the ANA Skymaster Amana crash
in 1950 and Trans Australia Airlines Flight 538 in 1960.
Vickers Viscount
The Vickers Viscount was a British medium-range turboprop airliner first flown in 1948 by Vickers-Armstrongs, making it the first such aircraft to enter service in the world...
aircraft departed from Perth, Western Australia
Perth, Western Australia
Perth is the capital and largest city of the Australian state of Western Australia and the fourth most populous city in Australia. The Perth metropolitan area has an estimated population of almost 1,700,000....
for a flight of 724 nautical miles (1 341 km) to Port Hedland
Port Hedland, Western Australia
Port Hedland is the highest tonnage port in Australia and largest town in the Pilbara region of Western Australia, with a population of approximately 14,000 ....
. 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
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
to the wingtip, including the outer engine and its propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
, 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
British Aircraft Corporation
The British Aircraft Corporation was a British aircraft manufacturer formed from the government-pressured merger of English Electric Aviation Ltd., Vickers-Armstrongs , the Bristol Aeroplane Company and Hunting Aircraft in 1960. Bristol, English Electric and Vickers became "parents" of BAC with...
concluded that a mysterious action during maintenance
Aircraft maintenance
Aircraft maintenance is the overhaul, repair, inspection or modification of an aircraft or aircraft component.Maintenance includes the installation or removal of a component from an aircraft or aircraft subassembly, but does not include:...
led to extensive fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
cracking in the right wing spar
Spar (aviation)
In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles to the fuselage. The spar carries flight loads and the weight of the wings whilst on the ground...
. This accident remains the third worst in Australia's civil aviation history.
The flight
MacRobertson Miller AirlinesMacRobertson Miller Airlines
MacRobertson Miller Airlines Ltd. , callsign "Miller", IATA code "MV", was established in Australia in late 1927, by pilot Horrie Miller with the backing of chocolate millionaire Sir Macpherson Robertson....
Flight 1750, a Vickers Viscount registered VH-RMQ, took off
Takeoff
Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.For horizontal takeoff aircraft this usually involves starting with a transition from moving along the ground on a runway. For balloons, helicopters and some specialized fixed-wing aircraft , no...
from Perth Airport
Perth Airport
Perth Airport is an Australian domestic and international airport serving Perth, the capital and largest city of Western Australia. The airport itself is located in the suburb of Perth Airport....
at 08:36 am local time. On board were two pilots, two air hostesses and twenty-two passengers. The aircraft climbed to an altitude of 19000 feet (5,791.2 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 7000 feet (2,133.6 m) on descent to Port Hedland airport
Port Hedland International Airport
Port Hedland International Airport is an airport at Port Hedland, Western Australia. The airport is 14 km from Port Hedland and 11 km from South Hedland and is owned by the Town of Port Hedland Council. The airport has more than 250,000 passengers and 20,000 flights per year.It is...
. No further radio transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
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
Fuselage
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...
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.
Wreckage
The aircraft came to Earth on Indee StationStation (Australian agriculture)
Station is the term for a large Australian landholding used for livestock production. It corresponds to the North American term ranch or South American estancia...
on level rocky ground vegetated with spinifex grass and some stunted trees. Wreckage was spread over an area about 7750 feet (2,362.2 m) long and 2500 feet (762 m) wide.
Accident investigators immediately observed that half of the right wing, its outer engine and propeller were close together about 3000 feet (914.4 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 fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
d in flight causing immediate separation of the half-wing from the rest of the aircraft.
Investigation
Detailed investigation of the two fracture surfaces showed that metal fatigue had caused cracks to grow across the lower boom (or lower flangeI-beam
-beams, also known as H-beams, W-beams , rolled steel joist , or double-T are beams with an - or H-shaped cross-section. The horizontal elements of the "" are flanges, while the vertical element is the web...
) of the right wing’s main spar until they affected approximately 85% of the cross-sectional area
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
. 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 initiated at a bolt hole at Station 143, the rearmost of five bolt holes for attachment of the inner engine nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
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), chamfer
Chamfer
A chamfer is a beveled edge connecting two surfaces. If the surfaces are at right angles, the chamfer will typically be symmetrical at 45 degrees. A fillet is the rounding off of an interior corner. A rounding of an exterior corner is called a "round" or a "radius"."Chamfer" is a term commonly...
ed at one end, was pressed into each hole. Each bush was an interference fit
Interference fit
An interference fit, also known as a press fit or friction fit, is a fastening between two parts which is achieved by friction after the parts are pushed together, rather than by any other means of fastening...
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 chamfered end 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 of the bush 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.
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
Arithmetic mean
In mathematics and statistics, the arithmetic mean, often referred to as simply the mean or average when the context is clear, is a method to derive the central tendency of a sample space...
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.
Aircraft
The aircraft was a Vickers Viscount 720C manufactured in 1954 and assigned serial number 45. It was immediately acquired by Trans Australia AirlinesTrans Australia Airlines
Trans Australia Airlines or TAA, was one of the two major Australian domestic airlines between its inception in 1946 and its sale to Qantas in May 1996. During that period TAA played a major part in the development of the Australian air transport industry...
and entered airline service in Australia as VH-TVB. 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.
By 31 December 1968 the aircraft had made 25,336 flights and had flown for 31,746 hours. Since its previous complete overhaul it had made 6,429 flights and had flown for 7,188 hours.
Recorders
The aircraft was equipped with a flight data recorderFlight data recorder
A flight data recorder is an electronic device employed to record any instructions sent to any electronic systems on an aircraft. It is a device used to record specific aircraft performance parameters...
and a cockpit voice recorder
Cockpit voice recorder
A cockpit voice recorder , often referred to as a "black box", is a flight recorder used to record the audio environment in the flight deck of an aircraft for the purpose of investigation of accidents and incidents...
. The flight data recorder functioned throughout the flight and continuously recorded the aircraft's pressure altitude
Pressure altitude
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...
, indicated airspeed
Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
, vertical acceleration and magnetic heading until the moment of impact with the ground. The cockpit voice recorder was slightly damaged in the impact and subsequent fire but there was no damage to the record of radio transmissions from the aircraft during the final 30 minutes of the flight. The record of ambient noise
Ambient noise level
In atmospheric sounding and noise pollution, ambient noise level is the background sound pressure level at a given location, normally specified as a reference level to study a new intrusive sound source.Ambient sound levels are often measured in order to map sound conditions over a...
in the cockpit was also preserved and revealed the precise moment at which the frequency and volume of noise increased suddenly.
Safe-life design
The wing of the Vickers Viscount used a single main spar made up of a centre-section in the fuselage, two inner sections and two outer sections. The main spar comprised an upper boom, a shear web and a lower boom. The aircraft was designed and type-certificatedType certificate
A Type Certificate, is awarded by aviation regulating bodies to aerospace manufacturers after it has been established that the particular design of a civil aircraft, engine, or propeller has fulfilled the regulating bodies' current prevailing airworthiness requirements for the safe conduct of...
to the principle of a safe-life
Safe-life design
In safe-life design products are designed to survive a specific design life with a chosen reserve.The Safe-life design technique is employed in critical systems which are either very difficult to repair or may cause severe damage to life and property...
. Before a component reaches its safe-life it must be removed from the aircraft and retired. At the time of the accident the retirement life of the lower boom in the centre-section was 20,500 flights; the inner lower boom was 11,400 flights; and the outer lower boom was 19,000 flights. The retirement life of the spars in the horizontal tailplane
Tailplane
A tailplane, also known as horizontal stabilizer , is a small lifting surface located on the tail behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing aircraft such as helicopters and gyroplanes...
s and the vertical fin
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...
was 30,000 flights.
The retirement life of the wing spar of a transport category
Transport category
Transport category is a category of airworthiness applicable to large civil airplanes and large civil helicopters. Any aircraft's airworthiness category is shown on its airworthiness certificate...
aeroplane type-certificated to the safe-life principle is based on a safety factor applied to data obtained from flight tests and information about properties of the spar material. 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. These safety factors were typical for this class of aeroplane. A 50% reduction of mean time to failure does not adequately explain why the inner lower boom in VH-RMQ should have failed before reaching its retirement life. In anticipation that the atmospheric gust spectrum in Australia may be more severe on the Viscount than the spectrum in some other climatic zones the gust spectrum was measured during 14,000 Viscount flights in Australia before 1961. 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.
The airworthiness
Airworthiness
Airworthiness is a term used to describe whether an aircraft has been certified as suitable for safe flight. Certification is initially conferred by a Certificate of Airworthiness from a National Airworthiness Authority, and is maintained by performing required maintenance actions by a licensed...
design requirements applicable to the Vickers Viscount and other safe-life transport category aeroplanes did not require retirement lives to be determined taking account of an unpredictable gross defect of the kind inflicted on the wing spar of VH-RMQ by insertion of the flared bush. Similarly, the airworthiness maintenance requirements did not require periodic inspection for fatigue cracking of the wing spars.
VH-RMQ was inspected by Ansett-ANA in May 1968, 922 flights prior to the accident, but it was not a requirement of this inspection that the wing structure be disassembled to allow access to the lower booms. Even if the wing had been disassembled it is unlikely the cracks radiating from the damaged bolt hole could have been detected.
On earlier Viscount aircraft replacement of the spar booms entailed fitting a new nacelle rear mounting assembly, the replacement part coming from the manufacturers with the spar attachment mounting holes undrilled, the drilling being performed by the maintenance personnel with the part in situ under the wing. However after considerable in-service experience with the aircraft design the manufacturer, BAC, found that it was permissible to re-use the removed nacelle rear assembly. Replacement of the removed component required accurate positioning of the nacelle assembly under the wing so that the mounting holes aligned with the ones in the wing spar boom before insertion of the bolts. On VH-RMQ evidence of an initial problem aligning the holes so that the bolts would pass through, and the subsequent attempts at resolving this issue, was thought to be the cause of the damaged bushing.
The replacement spar booms, like the early nacelle rear mountings, were supplied by the manufacturer undrilled, the maintenance personnel performing the necessary drilling for the nacelle mounting holes on-site.
Aftermath
Immediately after the accident the Department of Civil Aviation temporarily grounded all Australian-registered Viscount Type 700 aircraft. The temporary grounding of Australian-registered Viscounts was eventually made permanent.Fatigue-failure of the wing of VH-RMQ immediately raised doubt about the validity of the retirement life of the inner lower boom in the Type 700 so British Aircraft Corporation and the UK Air Registration Board took the precaution of reducing the life from 11,400 flights to 7,000. This soon resulted in British Aircraft Corporation obtaining a number of inner lower booms with time in service greater than 7,000 flights. Nineteen of these retired booms were examined in detail. Sixteen contained minor fatigue cracks in different critical locations. The longest crack was 0.054 inch (1.37 mm) in a boom that had been in service for 8,194 flights. 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.
When this accident occurred the number of fatalities made it Australia's third worst civil aviation accident. It remains Australia's third worst civil aviation accident. Two civil aviation accidents caused 29 fatalities each - the ANA Skymaster Amana crash
ANA Skymaster Amana crash
The ANA Skymaster Amana crash was an aircraft crash which occurred near Perth, Western Australia on 26 June 1950. At 9:50pm, a Douglas DC-4 Skymaster aircraft named Amana, operated by Australian National Airways, departed Guildford aerodrome in Perth, Western Australia, heading for Adelaide...
in 1950 and Trans Australia Airlines Flight 538 in 1960.
See also
- Ansett-ANA Flight 149Ansett-ANA Flight 149On 22 September 1966 a Vickers Viscount aircraft departed from Mount Isa, Queensland, Australia for a 73 minute flight to Longreach. Forty-four minutes after takeoff a fire started in one of the engines. The crew were unable to extinguish the fire or feather the propeller so made an emergency...
- Vickers Viscount VH-RMI - Ansett-ANA Flight 325Ansett-ANA Flight 325On 30 November 1961 a Vickers Viscount aircraft departed from Sydney, Australia late in the afternoon for a flight of 128 nautical miles to Canberra. There were thunderstorms to the north and south of the airport. From about 9 minutes after takeoff the crew of Ansett-ANA Flight 325 did not...
- Vickers Viscount VH-TVC - List of disasters in Australia by death toll
- List of accidents and incidents involving commercial aircraft
- List of accidents and incidents involving airliners by location