As Airbus A350 takes shape, can it avoid the A380's troubles?

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After a turbulent couple of years for the A350 XWB programme, Airbus finally finds itself in a relatively calm state. With the definition well advanced and on course for the year-end design freeze, the market has given Airbus's all-new twinjet a big "thumbs up" and, for the time being at least, the A350 is keeping to schedule.

The firm orderbook now stands at more than 370 aircraft from 23 customers and the tricky task of renegotiating contracts for the defunct, Mk1 A350 have been completed with only two firmly contracted customers not migrating to the new model.

a350 development schedule 
 

But it has not all been good news. Airbus recently had to concede that the aircraft has missed its empty weight target by 2.2t, requiring take-off weights to be increased by 3t, which has resulted in a 1% fuel-burn increase. Another issue is the final decision on the structure of the cabin floor crossbeams, with Airbus still evaluating whether to switch from aluminium lithium to carbonfibre.

Workshare And Production Plan

The production strategy for the XWB is also in a state of flux, as plans to divest certain plants to enable Airbus to place half of the aerostructures work outside the company are on hold. However, this has not prevented the airframer from finalising the workshare and production plan for the twinjet.

One dark cloud that will not go away is the question mark over a second engine supplier. While it is Airbus's preferred choice to offer a selection of engines, Rolls-Royce now has the monopoly with its Trent XWB family as discussions continue with General Electric, and possibly Pratt & Whitney, about participation in the programme.

But engineering-wise, the programme is "on track", says A350 chief engineer Gordon McConnell. "We've reached the point where we're launching the detailed design of the first long-lead-time items, and to do that we need to make sure we've frozen all the top-level aircraft and component requirements."

china airlines a350
 © Airbus

McConnell says that Airbus is running a "full requirements-based engineering process" on the A350 programme, "so we start at the top level and cascade these requirements right down into the components and the specifications for the suppliers. It's the first time we've been so rigorous in our application of requirements-based engineering."

The next major milestone comes at year-end, with the detailed definition freeze for the baseline A350-900. "This will mean we've frozen the configuration of the aircraft," McConnell says. "We're beginning the specific design or detail design of the parts, but it will start with a vengeance once we've frozen the configuration at the end of this year."

The first material for the aircraft will be cut in 2009 (Airbus points out that it may not be metal), and final assembly is due to start at the beginning of 2011, with first flight following in early 2012. Entry into service of the A350-900 is due in mid-2013 after an 18-month flight-test programme.

There are now more than 4,000 engineers within Airbus working on the A350, which McConnell says is a lot more than on previous aircraft for this stage of the programme. "We've front-loaded the programme deliberately because we want to have a very mature aircraft when we go to flight test so we don't have many changes," he says.

This should reduce the number of changes required after certification to enable a faster ramp-up during the flight-test programme when production of customer aircraft will be underway. "We've also selected our suppliers earlier than on previous programmes."

The earlier supplier selection is part of Airbus's strategy to follow the industry trend to involve companies in the design process sooner. "Once we've selected the suppliers, we immediately put in place a joint development phase and there are currently 21 JDPs running with system suppliers," says Francois Caudron, vice-president A350 customer and business development.

"These [JDPs] are typically around two to three months where Airbus and the suppliers work together to make sure there is a good common understanding of what is expected this allows us to align the tools and methodology," he says.

The suppliers are either working at the A350 programme headquarters in Toulouse or at the design offices, says McConnell. Just over half of the JDP plateaux (11) are being undertaken in Toulouse, five are in Hamburg, three in Bristol and two in Madrid.

The weight issue has come to light following the completion of the first detailed structural sizing for the A350. The first full set of calculated load data with input of windtunnel tests "allowed us to build our finite element models with our structural concept and then apply the loads to the models, and resulted in our global finite element model for the structure", says McConnell. The finite element model enables Airbus to understand the stress distribution in the fuselage shells.

a350 redesign
 © Airbus

Additional information has come from structural test specimens that Airbus has been building over the past year or so to validate the methods developed for the fuselage.

"This has allowed us to have the first really good sizing of the structure. Around 50% of the weight is now coming from the good sizing. It is the first time we've really had a bottom-up estimate of the weights of airframe components built from proper structural stress analysis," McConnell says.

Small Impact On Fuel Burn

Analysis of the global finite element model, along with input from systems suppliers that has provided a more accurate weight for the system equipment and installation, has established that the A350-900's manufacturer's weight empty is 2.2t greater than the 113.5t target and McConnell expects "similar deltas" for the -800 and -1000.

"We've increased the MTOW by 3t across the family to maintain the payload/range capability," says McConnell. The previously quoted MTOWs for the -800, -900 and -1000 were 245t, 265t and 295t, respectively.

As part of the weight changes Airbus is offering a 5t increase in the maximum zero fuel weight of the -1000 to boost structural payload capability by a similar amount.

McConnell says the weight growth will not affect take-off performance or create the need for additional engine thrust except "at some specific airfields where discussions have been held with R-R about where we'll need a percent or two more". However, the increase will have "a small impact on fuel burn of about a 1%", says McConnell, but emphasises that customers have been "quite understanding" and that there is "no issue" with performance guarantees. One element of the structural sizing that brought a surprise was the airframe's electrical structural network, says McConnell, for which "we had to add some weight which we didn't expect".

This network is Airbus's solution to the need to provide electrical continuity in the carbonfibre fuselage. "We get that for free on the metal fuselage," says McConnell.

The A350's electrical structural network uses all the existing metal parts in the fuselage (for example seat rails, aluminium nose structure and lower frames) supplemented by aluminium strips that are being added to the carbonfibre fuselage frames to provide a return electrical path. "We've also got some raceways in the upper fuselage to provide return current path and electro-magnetic shielding for the cable harnessing," says McConnell.

a350 electrical structure network

Weight And Complexity

Airbus still has to finalise the structural design of the floor crossbeams, which are specified as aluminium lithium but could be switched to carbonfibre. However, the metal beams form part of the electrical structural network, and solutions to provide electrical continuity with carbonfibre beams have "weight and complexity challenges". McConnell says this means that the switch would not generate a significant weight saving, but would eliminate the need for corrosion inspections of the crossbeams. "It will be a bit of a marginal decision, but we need to finalise this before the end of summer."

Aerodynamic changes have also contributed to the weight growth, with improvements to reduce drag "costing some increase in loads and therefore weight", says McConnell.

A large amount of the A350's aerodynamic evaluation has been performed using computational fluid dynamics, but the airframer has also completed 4,000h of windtunnel testing - three-quarters of which have been low-speed trials - since starting in September 2007.

Final check-out windtunnel tests are still more than a year away, says McConnell: "When we finalise the configuration at the end of this year we'll then build final windtunnel models for the frozen configuration and test these for a final check."

The A350 wing profile was frozen "some time ago" and engineers are now fine-tuning the aerodynamics: the span and area are unchanged since last year, at 64.7m (212.3ft) and 442m2 (4,758ft2), respectively, while sweep at the leading edge is 35°.

a350 wing characteristics

"We're down to the last few millimetres of change of aerofoil shape," says McConnell. "We've frozen the 'moveables planform' and the number of spoilers, which we've increased from six to seven and changed the chord slightly. The aero-lines will be frozen by the end of the summer."

The wing root joint geometry was the first part of the wing to be frozen - at the end of April - to allow the design of the aluminium alloy forgings at the root to be finalised. "These are big and heavy items, and they require a long lead-time to make the forging tooling for them," says McConnell.

The shape of the winglet has also been defined. This has "some rake and some 'blending', as well as some special things from Airbus patents" but the lines are not yet frozen.

The leading-edge devices are "more or less frozen" with six sealed slats and a single drop nose device inboard, while a streamwise motion for the outboard trailing-edge flap has been adopted to reduce drag.

"Usually the motion of the outboard flap is normal to the rear spar of the wing, which means that when it extends the flap fairing goes into wind and creates some drag," says McConnell.

Performance Benefits

This was the configuration originally adopted for the A350, but through "a fairly simple change" the outboard flap now extends streamwise, parallel to the fuselage datum, eliminating the drag penalty. "It will be a big benefit for take-off performance," says McConnell.

The A350's flaps are a very simple "drop-hinge" design with a single slot between the trailing edge of the spoiler and the leading edge of the flap, McConnell says. As the flap extends, the spoilers deflect downwards to control the gap and optimise the high lift performance of flap.

And for the first time on an Airbus, the flap system will have the capability for differential inner and outer flap settings as well as a variable camber function.

"We have a gearbox with a motor mounted between the outer and inner flap that enables us to differentially control the angle of the two flaps once they have been retracted," says McConnell. "We can change the centre of lift position for loads management for example, at heavy weights the inner flap can be set slightly down to move the centre of lift inboard and control the loads."

Flight-Control System

During the cruise, the system can give variable camber effect: "We can move both flaps together a small amount - either up or down - which allows us to tune the peak lift over drag to improve the performance." McConnell says that the flap functions will be controlled automatically by the flight-control system computers, which will sense data from the flight management system.

Tail surface sizes have been revised, with the tailplane being reduced in area from 92m2 to 85m2 and the fin growing from 49m2 to 51m2. "We've added a couple of square metres to the fin to maintain the minimum control speeds for take-off in the engine failure case," says McConnell.

Meanwhile, fine-tuning continues on the aerodynamic shape of the fuselage - the nose, the tail and the belly fairing. The latest screen-caps from the computer-aided design model show that the aircraft will look considerably different to the concept that Airbus uses for its official images, although an Emirates-released impression is a little closer to reality.

The A350-800 has gained two LD-3 container positions in the rear hold by rearranging the configuration and changing the cargo door latchings. "We had some lost space in the original design," says McConnell.

Following a cost/weight/reliability/performance trade-off study, Airbus has switched from the original hydraulic thrust-reverser actuation specification to electrical actuation, the method already in use on the A380. "It took us a while to get the balance between the cost and the performance right," says McConnell.

Another A380-derived system architecture is the hydraulic design, with Airbus opting for the 345 bar (5,000lb/in2) - rather than 207 bar of the older-generation Airbuses.

For the first time on a civil aircraft, Airbus has adopted digital signalling for the flight controls, switching from analogue to the 1553 mil standard digital bus that equips the A400M airlifter. "This allows us to save a lot of wires - normally we might have 32 wires going out to aileron actuator servo control, now we have six so we save a good bit of weight," says McConnell. Each servo control will have a common flight-control remote module to take the digital signals and convert them into commands, he adds.

Results from the "LINFaN" (low interior noise fan nozzle) tests conducted earlier this year on one of the Trent 500s powering Airbus's A340-600 development aircraft are being analysed, ahead of a decision to adopt the technology. "We're looking at the trade-offs of efficiency of the engine [ie fuel burn] and weight, to see if there is a benefit to reduce noise inside the cabin," says McConnell.

The configuration of the flightdeck with its Thales-supplied six-screen layout is being refined on a "Class 1" wooden mock-up in Toulouse. "This is being used for the ergonomics of the panel layout, seating position and so on," says McConnell.

The A350 product offering has been broadened with the introduction of a reduced weight version to allow airlines to benefit from lower charges when using the twinjet on medium-haul operations.

"From the start we'll offer the A350 with different take-off weights, and a 30t reduction is available that is suited to medium-haul operations for airlines that don't need the extra long range," says Caudron. The lower weight will be achieved through paperwork changes and cockpit placards and will not affect the aircraft's structure. "An engine derate of up to 40% is also available," says Caudron.

D-Shaped Galley

The 10-abreast high-density economy seating layout study for the A350 is now a firm offering, with one potential A350 customer interested in the layout, says Caudron. The minimum seat pitch for the A350 in nine-abreast layout is 29in (74cm), but this increases to 31in pitch for 10-abreast seating to avoid the need to reinforce the cabin floor for the higher density configuration. Another interior "novelty" is the adoption of a "D-shape" galley that Caudron says provides more comfort and space for the cabin crew and enables the trolley count to be increased from 17 to 21 units, compared with the earlier, more conventional "C-shape" configuration.

a350 gallery

Ambitious Programme

The new design also reduces turnaround time as it enables separate flow for full and empty trolleys during catering, and frees up space in the main cabin for an additional triple seat row. "We're going to have an operational validation using a mock-up with airlines as part of the customer focus group process," says Caudron.

Airbus knows that even though the pressure has lessened a little recently following Boeing's production woes with the 787, it still cannot afford a repeat of the A380's industrial catastrophe. While it is confident that the necessary steps have been taken to avoid this, many challenges lie ahead as it embarks on what is arguably Airbus's most ambitious programme ever.

Work On Schedule Despite Divestment Hitches

Although Airbus's production plans have hit some stormy weather with the failure of divestment plans for the plants in France and Germany, it says that the allocation of A350 work packages has proceeded on schedule and it aims to get most of the remainder concluded by the end of the year.

"We are on time with our supplier selection, both in terms of systems and structure," says Francois Caudron, vice-president A350 customer and business development. Ninety percent of the system vendors have been selected with "a good mixture between new suppliers bringing new technologies and reuse of A380 experience", he adds.

The business model for the A350 calls for the total aircraft value (excluding engines) to be split into thirds. This breaks down as 33% outsourced airframe work, 33% outsourced equipment, and the rest in-house equipment/structural build or assembly work. As part of this outsource policy, half of the all aerostructure work will be outside Airbus, once the abortive plant-divestment plan proceeds. Significantly, all contracts for the outsourced aerostructures work are dollar rather than euro-based, despite much of it staying in Europe. The same is true for most supplier contracts, including those with European suppliers.

"We have allocated half of the 50% of the 'buy' part of the structure," says Caudron.

a350 production flow

The six major fuselage aerostructure risk-sharing packages have been allocated, with the major outstanding allocations including the horizontal and vertical tailplanes, belly fairing, rear fuselage, tailcone, pylon components, wing moveables and wire harnesses. "The target is to get the majority of them to be allocated by the detailed design freeze at the end of the year," says Caudron.

Much of the fuselage work has in fact been allocated to existing Airbus plants in France and Germany that will eventually be divested, which are dubbed French and German "newcos" for the time being. The allocation is as follows:

  • French "newco" (St Nazaire-Ville and Meaulte plants) - nose section (three packages).
  • German "newco" (Varel, Nordenham and EADS Augsburg plants) - forward fuselage section (including assembly), carbonfibre side fuselage panels for the aft fuselage, floor grid and rear pressure bulkhead (two packages).
  • Spirit AeroSystems (North Carolina) - centre-section side and upper fuselage panels.

"The two 'newcos' will be created in France and Germany and owned by EADS," says Caudron. "The next step will be to open the capital of the shareholding to the public to meet the divestment target of Power8."

GKN will build the fixed trailing edge (and composite wing spars) at the former Airbus UK factory in Bristol that it is finalising the acquisition of. The selection process for the partner to build the fixed leading-edge assembly is at the final stage.Meanwhile, Stamford, Connecticut-based Hexcel has been contracted to supply the carbonfibre pre-preg raw material. The award runs to 2025, and is expected to generate revenues of $4-5 billion.

a350 business requirements 

Final assembly of the A350 will be undertaken in a new building to be constructed in Toulouse. Subassembly build allocation will follow similar national lines to existing Airbuses, with the German division building the aft fuselage (and German "newco" the forward fuselage), France the nose and centre fuselage/wing box, and the UK the wingbox (with equipping being undertaken in Bremen). Airbus's Spanish and German divisions are well established as composite "centres of competence", so they are allocated the carbonfibre wing covers (Illescas/Stade) upper/lower rear fuselage panels (Stade) and assembly of the carbonfibre tailplane (Getafe), fin (Stade) and full barrel carbonfibre tailcone (Getafe).

The sections will arrive by Beluga in Toulouse, where they will be united to create a complete aircraft using a new, streamlined final assembly concept. Airbus intends to build 18 aircraft in 2013, which will be the first year of full production and then ramp up through 83 units after two years and stabilise at 143 units annually from 2017.

"The fast ramp-up is achieved by the revised assembly process, which will reduce the time from fuselage join-up to delivery from four months on the A330/A340 to 2.5 months on the A350," says Caudron. "We'll do that by starting cabin furnishing very early in the process, in parallel with final assembly."

A330/A340s are built sequentially with structural assembly of the fuselage and wing and power-on being completed before interior installation and completion.

The process for the A350 will achieve power-on ahead of wing join-up, at which time cabin integration will begin in parallel with the remainder of the assembly process. This will enable all the major cabin monuments to be pre-positioned in the aircraft sections before they are joined, says Caudron.