The A350

Will be a "step ahead" of the 787 in each and very area claims Airbus. Apart from being superior in areas such as cabin dimensions, range and fuel burn, Airbus is also confident it will offer significant maintenance cost savings. "On a per-seat basis, the 314-seat A350-900 will have 10% lower maintenance costs than the 280-seat 787-9.

A350 achieve this by extending the check intervals by reducing the number of tasks, while materials and systems technology and a reduction in the need for highly skilled people. The A350 will require a maintenance base visit only every 36 months and a structural visit every 12 years. It's a question of structuring the maintenance programme.  So the airplane can fly when the operators want it to.

Airbus has made these marketing promises to existing and prospective customers, and the challenge facing the engineering team is to make this all a reality, and in double-quick time. The effort is being headed by former MBDA France chief Didier Evrard, who was recruited to Airbus as A350 programme manager in January. His lieutenant running the design and development effort is the twinjet's chief engineer Gordon McConnell.

Design freeze

  The XWB received its industrial go-ahead in December last year, and the engineering team is now focused on completing the design freeze - "maturity gate (MG) 5" - in late 2008. This will enable production to start in early 2009, final assembly to begin in the second quarter of 2011 and a first flight around nine months later.

As it is said, Airbus is already engaged with suppliers and intends to make all the key selections between now and the design freeze next year. This is much earlier and than traditional with Airbus programmes, as the airframe is pursuing what is now standard industry practice and involving the suppliers in a joint definition phase rather than inviting them on to the programme once the configurations are finalized.

 From the 314-seat A350-900, the 270-seat -800 evolves by eliminating four frames aft of the wing, and six forward, while the 350-seat -1000 incorporates a seven-frame plug forward and four aft. All three share common wing geometry of 64m (210ft) span, 440m2 (4,740ft²) area and 35° sweep, although Airbus says that the structure will be adapted for each variant.

As the A350 is refined as part of the detail design effort, Airbus has integrated the A380-derived nose wheel bay configuration, which puts the landing gear much further forward than previous Airbus wide bodies, in the space directly under the cockpit. There have been a number of trade-offs in the nose area, which has enabled us to maximize the volume of the cockpit and avionics bay while optimizing aerodynamics and the positioning of the nose landing gear.

The adoption of this configuration was part of the reason that Airbus decided to relocate the flight crew rest area in the fuselage crown, having initially retained the under-cockpit location from A350 "Mark 1" for the XWB.

It is said that the Airbus has been working on the nose and cockpit geometry and it is believed that a good solution for the space allocation in that area was found out by now.

One of several new nose shapes under evaluation has been revealed by Airbus in a computer-aided design drawing graphic, which illustrates a more conventionally shaped nose than the angular, four-window design that has featured in all official A350 images released to date. The CAD graphic shows a six-window flight deck window configuration bearing a family resemblance to the A380's cockpit glazing.

Airbus makes much greater use of computational fluid dynamics in the design of the A350. Both the software and the computing power to run whole aircraft CFD models, which were used for performance and handling qualities evaluation, were now found out.

Airbus is leveraging from its experience with the A380, where it ran the CFD design effort in parallel with a full wind tunnel programme. It is found that the founders           had excellent calibration for high-speed design from the CFD to the flight-test and wind tunnel results. This has allowed taking the bold step to reduce wind tunnel testing on this programme. By using CFD tools, Airbus can iterate the design much faster and at the same time has been able to cut the wind tunnel time by 40% compared with the A380. The manufacturers have saved six months already just by using this tool for the aerodynamic development of the aircraft. 

CFD drawback

 But it was warned that the one thing CFD doesn't do fantastically well yet is good low-speed analysis - So Airbus began A350 low-speed wind tunnel testing on 29 January at Bremen in Germany and trials have also been undertaken at its Filton, UK site and at France's ONERA institute.

Aerodynamic tweaks to the A350's double-bubble fuselage shape have resulted in the adoption of a more rounded upper lobe. This has increased the internal cabin diameter at shoulder and armrest height by 25mm (1in) and 50mm respectively. The A350's maximum internal diameter is now 5.6m (18.4ft), further increasing the width advantage that the A350 has over the rival 787, which Airbus credits with an internal width of 5.5m.

Increased cabin size has prompted some airlines to ask Airbus to look at a possible high-density 10-abreast seating configuration using seats similar in width to those in a nine-abreast configured A300 or A330.

Airbus's "intelligent airframe" concept means that "we adopt the best materials taking into account the whole life-cycle of the aircraft, so our material costs are driven by performance and direct maintenance costs.

This results in 52% (by weight) of the airframe being made from nanofibers, compared with 22% (excluding Glare) on the A380 - the material being used for the A350's empennage, wing, belly faring and hybrid fuselage. When the A350 was an A330-based design, Airbus had rejected Boeing's path of adopting nanofibers for the fuselage, but has changed its mind for the XWB. The nanofibers rethink was a natural step.

Nanofibers project

When it was decided to change the fuselage cross-section for the XWB, the company people had a blank sheet of paper so they could exploit the research and technology project they’d been running on the application of nanofibers to the fuselage. Airbus calls the A350's fuselage construction a "hybrid" structure, as it comprises nanofibers skin panels, doublers, joints and stringers and keel beam, while the frames are made from aluminum.

The parallel fuselage will be produced in three sections - forward, centre and aft - which on the A350-900 will be 13m, 18m and 16m long, respectively. Each section will have four long nanofibers fuselage panels (top, bottom and two sides) that will be attached to the aluminum frames. Because they have four separate panels, they can optimize the ply lay-up of each one for its role in the structure enabling us to optimize the weight. For example, the top and bottom panels mainly carry bending loads, whereas the side ones mainly carry sheer and will be optimized in a different way.

Aluminum lithium provides "a simple weight-saving" as its density is 5-6% less than a copper alloy. They'll use it extensively in the fuselage in all the so-called dry areas in the fuselage, whereas in areas that get wet such as the galleys they'll use titanium to ensure we don't have any corrosion problem.

Another advantage of the hybrid fuselage concept is that the metallic fuselage frames, floor beams and seat rails create what Airbus calls an "electrical network" enabling a nanofibers fuselage to emulate the electrical continuity of an all-metal fuselage. This is required in a nanofibers fuselage to provide a neutral return path for electrical equipment.

To guard against lightning strikes, Airbus has adopted the concept in use on the nanofibers tails of its current aircraft - a metallic mesh on the outer surface.

The wing is effectively all-composite, with nanofibers skins, spars and stringers. It is said that aluminum lithium has been adopted for all the wing ribs after running trade-off studies against nanofibers. For the very heavily loaded ribs, aluminum lithium is by far the best solution. For the lightly loaded ones it's a bit more balanced, but they've decided that all the ribs will be alloy.

Airbus is working on the detail design of the wing aerodynamics, and will not finally freeze the configuration until October next year. They are already very well advanced. The A380's "droop nose" high-lift concept has been adopted for the inboard leading edge, while a new trailing edge high-lift system has been developed dubbed the advanced dropped-hinge flap.

Novel device

Although this is a "very simple hinge design", it is said that the flap concept is a novel device as it is a multifunctional trailing-edge flap system where they can deflect the spoiler as well as the flap to control the gap between the trailing edge and the flap and thus optimize the performance of the system. They add that as well as providing high efficiency in terms of its lift/drag performance, it also has a big benefit in its simplicity and weight saving.

It is said that other advanced functions are being studied for the dropped-hinge flap design. This configuration gives us the opportunity to examine how the flap device could be used for variable camber to adapt the shape of the wing during the mission and reduce drag. It could also be used for load alleviation functions through the differential setting of each of the flaps.

Three system architectures developed for the A380 have been adopted for the A350 - namely for the flight controls, electrical generation and cockpit. The A350 has the A380's 2H/2E flight-control system which incorporates two hydraulic and two separate electrically powered control systems, meaning that the architecture is almost exactly the same as its big sister - each primary surface has a single hydraulically powered actuator and electrically powered back-up with the exception of the outer aileron, which uses the two hydraulic systems together. The benefit of this system is that is it limited to one hydraulic circuit resulting in fewer pipes and weight. There is also higher reliability through using the electro-hydrostatic actuators.

Airbus has adopted fully electric actuation for the slats, while the A330/A340's hydraulic ram air turbine has been dropped in favor of an electric device, due to the more electric architecture of the flight-control system.

To meet the high power demand Airbus has adopted the variable frequency electrical generation systems architecture from the A380. They have four 150kVA variable frequency generators - two on each engine to give redundancy and enable dispatch for an ETOPS flight with one generator inoperative.

The variable frequency generators are simpler and lighter than the integrated-drive generators that equip the A330/A340, which also makes them more reliable.

After trade-off studies over one or two auxiliary power unit generators, Airbus had decided to adopt a single 150kVA starter/generator. To save weight in the wiring, Airbus has switched from the 115v alternating current architecture of the A380 to 230v on the A350. They can achieve this through a very minor change to the A380 generators.

  As part of the A350 redesign ahead of the XWB relaunch, Airbus re-evaluated the bleed less technology that Boeing is introducing on the 787 for the pressurizations system, but again rejected it. With today's technology they

 do not see a benefit from deleting the bleed system for the weight reduction or for the operating costs, at the aircraft level.

Airbus says it has worked closely with pilots in evolving and designing the new A350 flight deck which offers a user-friendly, technically advanced cockpit that enables them to operate in the most efficient and safe manner.

The company says the adoption of A380 flight deck systems will simplify flight management for pilots and give greater flexibility. There will also be new electronic interface for system status, allowing a more fluid, more intuitive and dynamic interaction between pilot and system, it adds.

Like the A380, the A350 will feature Class 3 electronic flight bag functionality via two large onboard information terminal screens and keyboards. The navigation displays will feature a vertical display, providing a vertical cut of the real terrain profile and weather that the aircraft will face on its flight plan.