Boeing and Airbus has been building aluminium aircraft for decades when they switched to composite designs for their respective 787 and A350 programmes. Whether that material change can be replicated for next-generation medium-haul aircraft, however, is unclear.
Long-haul aircraft are being produced at comparatively low rates. Out of 763 aircraft that Boeing delivered in 2017, the 737 family accounted for 529, or 72%. The US manufacturer handed over 136 Dreamliners (18%), with the rest of the deliveries split between the 777 (74 units), 747 (14) and 767 (10).
Boeing's two 787 final assembly lines in Seattle and North Charleston together produced, on average, 11 aircraft per month. That rate is to be increased to 14 Dreamliners – or 168 per year – from 2019.
Airbus delivered a total of 718 aircraft in 2017. The A320 family accounted for 558 (78%), with an average monthly output of 47. The European airframer meanwhile delivered 78 A350s, 67 A330s and 15 A380s last year. In May 2018, finance chief Harald Wilhelm said the manufacturer had achieved a target of producing 10 A350s a month.
The difference in volume indicates the scale of change that Airbus and Boeing would face if they were to adopt a composite airframe for a single-aisle aircraft – especially as the two manufacturers have plans to further increase narrowbody production.
Jose Sanchez, executive expert in composites at Airbus, confirms to FlightGlobal that there are no technical obstacles to building a single-aisle type with a composite airframe. The predominant composite material employed across the industry for large primary structures is carbonfibre pre-impregnated with resin (pre-preg). The carbonfibre is laid up in layers and assumes its final form through curing, under pressure and temperature, in an autoclave, and is thus categorised as a thermoset material.
Sanchez says that in-service experience with the material – which has been used for structures like empennages for decades – is "absolutely fantastic". But he acknowledges that simply expanding production capacity will not be enough to enable adoption of a mainly composite airframe for high-volume programmes, and that further development is required to make composite aerostructures manufacturing more economical.
"We are making some effort to further reduce the manufacturing costs [and] improve the productivity of the production," he says. Efforts are focused on reducing raw material costs which Sanchez describes as "far higher" than for aluminium, and on improving production processes. "With today's technology, we can manage. But we have to make a strong effort to reduce the costs," he says.
Miguel Castillo Acero, vice-president of technology development at Spanish aerostructures specialist Aernnova, estimates that the costs of producing composite aerostructures are 40-100% higher than for comparable metal components, depending on part complexity.
This is especially relevant to single-aisle aircraft, where profit margins tend to be slimmer than for long-haul jets.
Triumph Aerospace Structures vice-president of engineering Martin Perya thinks it would be "extremely difficult, if not impossible" to adopt a composite airframe for a single-aisle aircraft with today's carbonfibre technology. He says "high level" investment would be required to scale up production capacity with facilities featuring autoclaves, clean rooms and cold storage spaces, and that such infrastructure is "largely impractical for higher-volume production".
Perry notes that autoclave processing is "the single most cycle-time-affecting process in the value chain". He adds: "The economic viability [of potential capacity expansion] is very wearisome as the investment required would be significant and return on that investment is somewhat risky." He believes a "different approach" is needed as "there is a kind of limit that I think the industry is... hitting right now with that thermoset technology".
For today's carbonfibre structures, autoclave curing is necessary as "high pressure and high temperature... assure us of the quality and mechanical properties of the composite material", says Castillo Acero. "If we avoid the autoclave, [any alternative] process has to be robust enough to achieve acceptable... mechanical properties."
Castillo Acero does not suggest a complete departure from thermosets, but thinks the proportion of components requiring autoclave treatment might have to be reduced and more equipment cured via alternative methods. Out-of-autoclave curing is used for a limited range of parts today.
Meanwhile, manufacturers are exploring ways to improve current composite aerostructures' production and inspection. While much of the carbonfibre lay-up process has already been automated, the use of pre-preg stock has limits in terms of how much material can be deposited at a time. Suppliers are looking at ways of depositing dry fibres and separately adding resin, for example, through infusion or insertion of resin sheets between fibre layers.
Increased automation is another avenue to raise productivity in the short term. Today, manual work characterises much of the raw material handling and lay-up preparations, autoclave operations, and component rework and inspections after curing, notes Castillo Acero.
In the longer term, he foresees a step change in productivity with a shift towards greater use of thermoplastics. This material can be cured – in ovens or in heated press-moulding tools – within minutes, rather than the hours typically needed for thermosets in autoclaves.
Furthermore, thermoplastics tend to be more impact-resistant than thermosets. Additionally, it is possible to melt resin in thermoplastics through heat and weld components together with a structurally reliable joint, which could facilitate weight savings in larger assemblies.
GKN Aerospace, through its Dutch aerostructures division formerly known as Fokker Technologies, supplies thermoplastic leading-edge parts for the A380 and control surfaces for Gulfstream G650 business jets. Russ Dunn, GKN's senior vice-president for engineering, technology and quality, says thermoplastics have traditionally been used for lighter-loaded structures, while thermosets are suitable for large, complex primary structures, as they can be optimised for high static strength.
However, GKN is exploring the use of thermoplastics for "major structural components" – including fuselage skins, empennages and wing components – and is making "very good progress", says Dunn.
Perya describes thermoplastics as "almost the holy grail" of aerospace materials. Looking back at how employment of thermosets has grown over past decades, he expresses surprise that the aerospace industry did not concentrate earlier on opportunities offered by thermoplastics. He thinks the manufacturers were slow to adopt the material after they had spent time and effort on development of thermoset composites.
"You had such a huge investment, they [manufacturers] were trying to get a return on that investment and the time to get everybody comfortable [with using] thermosets," Perya says. But in order to achieve the "next big step" in lowering costs and raising productivity, he says: "We need to move away from that technology to thermoplastic-type technologies."
As Boeing evaluates options for its proposed New Mid-market Airplane (NMA), it is arguably under greater pressure than rival Airbus to decide whether to adopt a composite design for an all-new aircraft. Marketing vice-president Randy Tinseth said in March that the airframer would use on the NMA "proven and understood" technologies rather than radical new ones, and that the manufacturer plans "no big technology push as we saw on [the] 787".
While the NMA is an exploratory project that has yet to be launched, Boeing has discussed its vision of service entry circa 2025. The company declined to comment for this article.
If Boeing or Airbus were to switch to a composite design for a next-generation single-aisle aircraft, manufacturers would need to establish a new industrial production system that is capable of accommodating the huge output of today's 737 and A320 assembly lines, which have been established over several decades and have grown their production gradually over that time. This might be a bigger technological and industrial challenge than the previous shift from metal aircraft to composite designs for the 787 and A350.
While the two twinjets represent Boeing and Airbus's latest technology, Dunn thinks the aerospace industry is still on a "journey of learning" and has not yet utilised the "full potential of composite structures". He foresees that improvements in the deposition of composite material will deliver efficiencies in the manufacturing process and aircraft design.
"The better control we can get of our manufacturing processes and our inspection technologies, the more conservativisms we can remove and... the more cost-effective the products will be," he says, adding: "My view is that a bigger journey is left to be done, and there is more opportunity for improvement on the next generation of composite aircraft."
But he notes: "For every improvement that is made on the composites side, there is work ongoing on the metallic side... Whether [composites] will make the better business case than the alternative [for a future single-aisle aircraft] depends on how quickly and how strongly the opportunities on the metallic side come through."