Chapter 4
In comfortable second position (PDF – 183 kB)
4. IN COMFORTABLE SECOND POSITION
About A350
Volta-face
A balanced approach
More composite please
All-composite please
A bit early
Tell us the truth
A hybrid approach
Panel versus Barrel
The fuselage
Manufacturing
Assembly
Cockpit fuselage
Inspection and repair
All-composite wings
On safety
Lighting strike protection system
Design freeze
Likewise deHavilland in the early 1950’s, Boeing is now frontrunner with the development of a new generation of aircraft. Again material behaviour and not aircraft performance will be decisive. Likewise Boeing in the late 1950’s, Airbus is now in comfortable second position with its A350 and watches developments with the 787 at Boeing closely 7) and this will surely pay off – as it did for Boeing with the 707 some half a century ago – ample time for Airbus to rethink composites and focus on aluminium reinforced composites in particular – Airbus has Glare in its portfolio a non-beatable advantage over Boeing.
The market can be unpredictable and somehow airlines expressed their belief in the light and the strong of composite – the 787 gained orders in a way never seen before in aviation history. When the problems with the A380 surfaced Airbus found itself suddenly in exact desperate position they manoeuvred Boeing into only a couple of years ago. Taken by surprise – ‘It will take us about 10 years to catch up with Boeing in terms of development and efficiency’ 17) – and with limited recourses available, Airbus managed to scratch together 3.5 billion Euro and October 6th 2005, Airbus launched the program for the development of the A350 – to enter service 2010 114). But it would never come that far, radical changes were already in the pipeline.
Volta-face
When Boeing announced the all-composite 7E7, Airbus regarded the project initially just ‘a PR marketing threat’ 109) back in 2003, then a ’hype’ 110) in 2004 and by 2005 Airbus and Boeing engineers clashed about composite fuselage safety 14). But it worked out differently and now it was to Airbus to ponder about a response to keep at pace with Boeing. That became the A350 115 – not all but half composite – that is, with composite wings but aluminium fuselage ‘of course we don’t say that there is no advantage for the performance of composites, but we maintain that when all real-world constraints are considered, you’ll certainly decrease the performance of composites [for fuselages] to a level which is just marginal’ 111). Not well received by the market Airbus turned volta-face also for all-composite fuselage and introduced the A350 XWB in 2006, with X ‘for Extra Wide Body, Extra Comfort, Extra Efficiency and going the Extra mile for customers’ 112) – to be precise, 31 cm (12 inch) extra wide body to end up 5 in (13 cm) wider than the 787 – or as a Boeing adept puts it ‘each passenger gets an extra 0.72 cm space, about the width of a pencil’ 113). That may be so, but A350 XWB presents now the widest fuselage in its category.
A balanced approach
The design of the A350 was based on the A330 115) – a balanced approach 134) conservative on composites, typically for Airbus at that time. The wings were newly designed, largely out of composites carrying also the landing gear, while sharing the same fuselage cross-section as its predecessor but now out of ‘aluminium lithium rather than Glare’ 116) – remarkably and probably not so clever. Airbus rejected a composite fuselage because their analyses showed that most incidents of ground impacts on the structure occur on the lower fuselage 139). In total the structure was about 60% out of advanced materials – that is, 39% composites, 21% aluminum-lithium, 11% aluminum, 9% titanium, 14% steel, and 6% others – that were to provide the equivalent of almost 8 tons weight reduction. According Airbus – ‘the A350’s fuel costs would be 4% to 7% less per passenger than on the competing 787 models’ 117) – but that counts only when the plane is full. Airbus expected the A350 to turn out to be one of their next innovative hits – not so this time. As explained before, the A350 was not well received – but would probably have been very well received when introduced in 2009.
More composite please
Airbus kept firm at first, but was clearly taken by surprise when the design drew fierce public criticism and managers had apparently difficulties to make up their minds. An executive went public ‘I would expect to see that as the final design’ X 108) and the very same day, another place, another executive went public too – ‘I want to note that Airbus listens to its customers ….We are ready to make extra effort to respond to their expectations’ 109). Understandably a bit A380 trauma, 787 fever, a little pressure form key customers, some week knees and soon the problem was no longer about technical issues but how to get together the €10 billion to please their customers with an all-composite design. At the Farnborough Air show, July 17th 2006, Airbus announced that after ample consideration they had decided for a redesign of the A350, now with the fuselage also out of composites 119).
That became the A350 XWB, more than half – 52% – out of composites with aluminium for the fuselage frames, floor beams and gear bays and titanium for the nose section, landing gear, engine pylons and attachments. Typical for Airbus, the engineers did not adapt the barrel concept applied for the 787 nor Glare that was already successfully used for the A380, but decided for a more conservative panel design out of plain composite 3). The reason for this might be that ‘a method of manufacturing a unitary seamless section of an aircraft fuselage’ has been patented by Boeing 346), the reason might also be that engineers deemed a panel structure just more efficient – and there might also be another strategy behind this decision as will be discussed later.
All-composite please
Already more than half out of composites, airline executives still complained – ‘it has to do better’ 120) – all-composite please – meaning that a barrel design was to be preferred. Again, ample discussion and Airbus decided to give in a little bit and to construct the fuselage frames also out of composite – a decision ‘officially’ taken for ‘simplification of maintenance’ 121). But those composite frames are to be provided with aluminum strips to maintain electrical continuity in the fuselage 122) – it appears that at Airbus lightning strike protection is high on the agenda – however, crashworthiness might be negatively affected. The fuselage crossbeams remained metallic – with an Airbus spokeswoman quick to add that ‘These could also be switched to composite, we’re still running trade-off studies’ 123). Composite crossbeams would eliminate the need for corrosion inspection but do not contribute to significant weight saving – but would cause serious interference with the electrical continuity 122) and further affect crashworthiness. The nose was adapted more like the A380. Although customers kept pressing hard for the barrel approach 125), Airbus remained stubborn on this issue – the plan to use large composite fuselage panels for the main fuselage skin was not changed ‘we are definitely sticking to that’ 124) – a solid fuselage had been considered but rejected 130). The customers were now a bit more satisfied – ‘They listened and, from what we can see of the A350 now, it is a potentially good aircraft and it matches the 787 offering from Boeing’ 126) – Airbus could proceed with the design.
A bit early
Airbus claims that ‘The A350 XWB offers 2% lower Empty Weight (MWE) per Seat, 6% lower Block Fuel per seat, 8% lower Cash Operating Cost (COC) over the B787 Dreamliner’ 3) bases on 30% fuel efficiency. A bit ambiguous – may be – but certainly confusing since both planes have yet to fly and are alleged to be significant overweight. Airbus is confident that the first A350 will be delivered in the summer of 2013 – ‘the A350 is on track….we have sold 478 so far and it is out of the question that we can be late’ 144). ‘I think we will be right on time. I’m hoping even a bit early’ 128). Design freeze milestone due for late 2008 was reached early 2009 129).
Tell us the truth
Airbus admits that ‘Previous programs and the A380 in particular taught us some tough lessons’ 145) – add to that the problems with the A400M. Sure valuable lessons have been learned 148) and Airbus learns also from partners that are involved in the 787 program and are also were signed up to work on the A350. When such ‘double partner’ is quoted in the press ‘the company said firewalls will be in place that the A350 and the 787 work is kept separate and that there in no transfer of technology to manufacturing the composite fuselage of their respective jets’ 152) will never hold in practice. Lessons learned from the A400M are most important but also worrisome since engineers haven’t been able yet to resolve the problems. A most encouraging development at Airbus is that engineers are again encouraged to speak out ‘we’ve changed the mindset of our people. They tell us the truth. They warn of us problems. We know what’s going on because people know we expect the truth.’ 145) – which means a return to engineering driven management and this might have far reaching consequences for the A350 as will be discussed later.
A hybrid approach
Working out the configuration of the A350 engineers at Airbus had the advantage that they had far more experience with composites in service than their colleagues at Boeing. They had encountered serious problems with composites in the past and were way ahead with the composed design of the A380 – including large scale application of both plain composites and aluminium reinforced composites – and worked already for years on the composite wings of the A400M. The design of the A350 evolved over a longer period of time when composites were gradually increased – definitely better thought through then when Boeing decided for all-composite.
Most important, being in second position Airbus has Boeing at their inquisitive service being able to watch developments closely – which they did 7).
Airbus rejected the solid barrel approach – snubbed ‘old fashioned’ 127) – and indeed the proposed panel design looks well thought through 122). But likewise happened with the 787 also here problems will surface once the design becomes more detailed – but it is clear that panels provide the engineers far more flexibility. The weight issue is however essential. The target weight, set for 113,5 tonnes, has already been increased with 3 tonnes 390) which means a more than 1% fuel penalty 398) but much larger overweight of 8000 kg / (17,600 lbs) has been reported 8). Likewise the 787, the all-composite A350 is similar low on damage tolerance – the change from composite barrels to composite panels contributes here in marginal way only but the design allows for better lightning strike protection.
Panel versus Barrel
Contrary to claims from Boeing, a barrel construction is not necessary stronger than one with panels – but panels do provide much more freedom with design. Manufacturing of panels is far less complicated than with solid barrels where the design is confined because of limitations with available manufacturing technology. The disadvantage is that a panel structure requires more joints and jointing that can add to the weight in significant way. With the design of the A350 only general details have been disclosed so far and it is understood that engineers are still working hard to come to terms with the target weight. Reason that only a general comparison of both structures can be discussed.
Fuselage of the A350 is in three sections – forward (13m / 42.6 ft) centre (18m / 19.7 ft) and aft (16m / 52.5 ft) – with the cross section constant over a rather long distance. Engineers decided for a circular cross section because this enables to construct each section out of four long composite skin panels only – top, bottom and two side panels – with the long panels reducing the number of circumferential joints while the longitudinal joints contribute to the fuselage bending strength. Type and level of loading show strong and changing fluctuations throughout the fuselage and the panel design allows for relative easy local optimization through thickness and fibre architecture – combining optimum local performance with the lowest overall weight for structural performance. With barrels thickness can also be varied but to limited extend only since this leads to an uneven inner surface, which means that it becomes much more difficult to remove the mandrel after curing – moreover, available circumferential fibre placement technology allows for only limited degree of optimisation through fibre architecture.
Manufacturing – mould, lay up, curing, autoclaving and demoulding – of panels is far less complicated than with mandrel wound structures and allows for more extensive automation. More strict tolerances can be met, residual stresses can be better contained, panels are less susceptible to damage during manufacturing and inspection for quality control is far easier than with barrels. Panels are also better suited for placing of inserts for doors, windows and other parts, limiting cutting work afterwards – also fuselage frames can be integrated and edges can be designed for less complicated joining along the longitudinal and circumferential joints and in ways that the panels can be fastened to the frames with much less fastening points – which makes it possible to save on weight in significant way. It will be very interesting to see how Airbus engineers exploit these features.
Assembly is different in that with barrels the frames are placed and fixedly attached afterwards and with panels it is the other way around in that these have to be ‘banged’ to the superstructure that has to be put in place first – in a way similar to aluminium panels. Comparison comes essentially to the time needed for these procedures and this depends largely on the number of fasteners that have to be placed, to what extend fastening can be automated and the weight saving that is achieved. Drilling of the fastener holes is definitely much easier with panels and can be fully automated. Handling is also much easier with less chance for damage. Complete prefabrication of sections for easy assembly is of course possible with both systems.
Cockpit fuselage section is understood still to be constructed from aluminium lithium – despite its limited impact behaviour – certainly a challenge to resist bird strikes and hail impact. A one-piece fibre construction is still being considered but not favoured by the engineers – ‘If we went for a composite structure we’d have to reinforce the area above the cockpit with titanium which is expensive’ 123). The rear fuselage section will be composite in a way similar to the A380.
Inspection and repair of panels is definitely much easier with production but remains a problem in service, certainly so since damage can involve also the longitudinal joints. Only when damage is beyond repair it is easier to replace panels than barrels, but is still quite an undertaking.
All-composite wings have advanced design that – according Airbus – will make the A350 faster, more efficient and quieter. The wing area of 442 sq m (4,740 sq ft) is the largest ever applied for a single-deck wide body aircraft – 20% larger than the A330 with a wing span of 64.7 m (213 ft), about 4.5 m (15 ft) larger 3). The 777 has about the same wing area and wing span than the A350. The 787-8 has much smaller wings – wing area is 3501 cq ft (225,6 sq m) and wing span 192.5 ft (58,6 m) – but these wings are presently redesigned that is expected to increase the wing size. With the A350 the wings have been aerodynamic optimized to reduce drag and increase the load through the application of high lift devices – the necessary strengthening counts for part of the increase in weight mentioned before. These wings enable the aircraft to match the Mach 0.85 cruise speed of the A380.
On safety the A350 appears to score better than the 787 – but that is still on paper. Real world results have to be awaited. As pointed out before, with both 787 and A350 the fuselage and the wings are equally vulnerable to delamination and deterioration but the A350 might provide better crashworthiness – likewise the 787 toxic flammability remains a serious problem.
Lighting strike protection system
Much attention has been paid to lighting strike protection. Next to the fuel tanks also the electrical and electronic systems have to be protected and shielded – as was indicated before modern electronics applied in aircraft – in huge quantity – might prove to be more vulnerable than the fuel tanks. It can be questioned whether an all-composite structure leaves enough metal for proper grounding. This is apparently realized by Airbus. The panels are fastened to the composite frames which are provided with aluminium strips, that together with the metallic crossbeams create a metal network that provides the structure electrical continuity, important for lightning strike protection and grounding. Also the metallic ribs in the wings to which the composite wing panels are riveted are a continuous part of the electrical structural network. To provide a return electrical path, the metal network will incorporate all existing metal parts in the fuselage – seat rails, aluminum nose structure, lower frames and so on. Whether this is enough has to be awaited – effective physical testing is here virtually impossible – which adds another safety risk.
Whether the electrical structural network that is created this way provides a level of lighting strike protection equal to an aluminum fuselage has to be awaited – but most probably not. This is essentially a theoretical approach never tried before and cannot be verified by physical testing in reliable way. The system has to be able to channel and disperse electrical and electromagnetic fields uninterrupted – that is in perfect continuous way – from any point where lighting attaches throughout the aircraft structure towards specific points from where the lighting continues to the ground. With the A350, the electrical structural network seems much better thought through than with the 787 – but also here fasteners must be spark free fitted and encapsulated and grounded when deemed necessary, the composite skin must incorporate a conducting foil or mesh and the fuel tanks are to be provided with an inerting system.
Design freeze
It is reported that the A350 reached Milestone 5 – M5 or Maturity Gate (MG) 5 – early January 2009 23), about according schedule. In Airbus jargon M5 refers to the ‘major’ review for detailed design freeze – it is assumed that overweight is what M5 is all about for the moment. Officially ‘the review confirmed that the program can now go ahead with further specific design work’ 23) – but Airbus isn’t discussing what adjustments it may have to make to the original plans to meet the specified target weight, saying ‘it must brief airline customers first’ 23). This indicates that the design is far from fully frozen.
It will be interesting to await design details. For the moment engineers are still facing the haunting task to bring down the weight – as indicated before overweight of 8000 kg has been reported 8.) that has not been contradicted by Airbus ‘since the design is not frozen we can’t really comment’ 140). Some would say, an impossible task – but one may wonder whether Airbus is not pursuing a completely different approach with the panels – more on that later.
and designed by 