A350 XWB ready to rock
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By Max Kingsley-Jones
Airbus will undertake an intense development programme of the A350 XWB over the next 24 months, the likes of which it has not seen for two decades. Between now and mid-2011, when final assembly begins, the A350 engineering teams must complete the detailed design lead variant, the -900, and prove the carbonfibre production plan for construction to begin, while firming up the baseline specification for the two derivatives. Not since it introduced the A330/A340 family of twin and quadjets in 1993 has the airframer undertaken such ambitious multi-variant parallel development.
"At the end of this year, we will have both technology readiness and manufacturing readiness in place to start production of the first aircraft parts," says Didier Evrard, head of the A350 XWB programme.
"We are developing this aircraft faster and putting in place a lot of new enablers in developing a new industrial vision, a new way of doing business with our suppliers and of doing customisation with our customers."
© Airbus |
Airbus has accumulated more than 480 firm orders from 30 customers for the predominantly carbonfibre twinjet. The recent order from Kingfisher Airlines for five XWBs was the last migration of customers from the original A330-based A350 "Mk1".
Development of the A350-900 has so far proceeded on track since the XWB programme received its industrial launch in December 2006. It reached the important "maturity gate (MG) 5" milestone on schedule late last year, which represented the detailed definition freeze.
"MG5 is one of the most important gates in our new development process," says Evrard. "It meant we had defined all the detailed aero-lines for the fuselage, wings and empennage. We now have a clear way ahead targeting the entry-into-service of the A350-900 in mid-2013 and a first flight in quarter one 2012."
One landmark of MG5 was the confirmation of the A350's revised nose design with six-window cockpit glazing (see graphic) and definitive shape of the wing and winglet. Previously, Airbus's official imagery of the twinjet showed the original concept's more dramatic four-pane "visor" windscreen.
Other elements of MG5 included freezing the systems architecture ("this means all the equipment is at its [correct] location and properly connected with each other", says Evrard) and cabin design (except the rear galley configuration, which was resolved in April).
"Industrially, MG5 is important because from here we start committing to detailed design at all component levels," says Evrard, which enables long-lead items, tooling and jig production to be launched. "It meant we could empower the teams to deliver the components later."
Evrard adds that until MG5, the programme had been run in a "very centralised way from a central plateau in Toulouse".
The definitive rear galley layout, which has been agreed after customers rejected Airbus's proposed "D" shape because of concerns about workspace and turnaround time, "is more of a V shape", says Evrard. "It has the number of chilled trolleys they want and customers are happier with the serviceability."
Because the composite production effort requires very early commitment to tools manufacturers, Evrard says Airbus is providing regular data to tools builders to ensure development runs in parallel with detailed design. "We have regular data drops, so by the end of the year we will have the tools in place to start manufacturing parts for the first aircraft."
The A350's first metal will be cut in the last quarter of this year on the horizontal cruciform, followed by the first lay-up of a production fuselage panel. "We will start assembling and equipping the major components for the first A350-900 in early 2010 for delivery to the Toulouse final assembly line in early 2011."
One minor change incorporated at MG5, but only recently revealed, was a slight bump in the nominal thrust ratings for the A350 XWB's Rolls-Royce Trent XWB engines to counter the 2.2t empty weight and 3t maximum-take-off-weight increases disclosed last year.
WEIGHT GROWTH
At the time, A350 chief engineer Gordon McConnell said the weight growth would not necessitate additional engine thrust, except "at some specific airfields", but Evrard now confirms the thrusts were tweaked at definition freeze "following the weight increase".
Nominal thrust ratings for the -800, -900 and -1000 variants have each had a 1,000lb (4.5kN) increase to 75,000lb, 84,000lb and 93,000lb, respectively. Evrard says the change has had a "very marginal" impact on fuel burn and operating cost assumptions. The change partly reverses an adjustment two years ago when the Trent's nominal thrust values were reduced for each variant by between 1,000lb and 4,000lb.
With the supplier selection virtually complete, an almost two-year system and cabin development effort has been under way since the turn of the year. This is due to be completed in late 2010, by which time about 15 months of integration testing will be carried out using "aircraft zero" and "cabin zero" development simulators along similar lines to the A380 programme.
Airbus is trying to reach an unprecedented level of system maturity before first flight using the digital mock-up and virtual and actual functionality testing.
"Maturity at first flight is one of the challenges, knowing that at this point we will already have a lot of aircraft and components in our factories," says Evrard. "We aim to reduce the number of modifications needed during flight-testing. This will enable the flying programme to focus on the certification effort."
Milestones in 2009 are reaching the next level of maturity for the baseline A350-900, which is imminent, and achieving MG5 for the smaller A350-800 by year-end, says Evrard."We had 'Maturity A' for the -900 at MG5 and we will reach 'Maturity B' at mid-year." Maturity B is achieved when the detailed design is finalised by all the design teams.
After the MG5 detailed definition freeze of the baseline -900, similar milestones must be achieved for the smaller -800 and -1000 stretch over the next two years. Service entry of these two variants is due to follow one and two years, respectively, after the A350-900.
The effort to reach MG5 for the A350-800 is "simpler", says Evrard becauseit is effectively a shrink of its big sister, with six frames removed forward of the wing and four aft.
However, the task is not quite so straightforward for the -1000 stretch, which is due to reach MG5 in April 2011.
Although all three variants incorporate the same basic wing planform, Evrard says the configuration for the larger variant will become clearer once the final validation of the -900 is completed. "The intention is for the -1000's wing to be as close as possible to the -900's, but this will depend on final windtunnel testing of the -900 at the end of the year. This will provide a very accurate basis to start the final trade-offs for the -1000."
The A350 has a sophisticated single-slotted trailing-edge high-lift system, which, for the first time in an airliner, includes the capability for differential flap settings between inner and outer sections, and variable camber(see graphic). Possible changes for the -1000, which with a maximum take-off weight of 298t is 10% heavier than the -900, could be as straightforward as revised flap settings or possibly more fundamental and incorporate, for example, a double-slotted flap arrangement.
DIFFERENTIAL FLAPS
The differential flap setting enables the centre of lift to be moved depending on the loading scenario - for example, at heavy weights it can be transferred inboard by setting the inner flap slightly further down. "This allows us to reduce the aircraft's weight," says Evrard.
Both inner and outer flaps can be moved together during the cruise "to enable us to optimise the wing's camber for each phase of the flight and use the polar of drag to its best", Evrardsays.
"Since the programme began, we have steadily increased the composite content," he adds. The share now stands at 53%.
"We have full composite wings - including the centre wing box and keel beam - and a full composite fuselage with just a few exceptions [see graphic right]. We've kept the flexibility of using metallic panels where composite doesn't deliver the weight benefit because of other reasons such as impact tolerance requirements - for bird or lightning strikes, for example."
The main metallic areas, such as those around the nose or the crossbeams in the floor of the passenger cabin, are either aluminium or aluminium lithium, although titanium is used in the cargo floor.
"Stringers and the vast majority of frames are carbonfibre, but some frames in very high load areas are titanium, such as in the centre fuselage, landing gear components, the engine pylon primary structure, and the bilge to meet crashworthiness requirements," says Evrard.
The adoption of carbonfibre for the fuselage structure has a number of benefits, such as reduced corrosion and fewer fatigue-related maintenance tasks, as well as enabling the cabin altitude to be lower, which improves cabin comfort, says Evrard. However, the design "also means a lot of changes" for the design and production philosophy.
An important task Airbus faces this year is completing validation of the production process, as well as the design, of the A350's carbonfibre airframe using its "demonstrators programme". Airbus must be absolutely sure it has developed a robust production process for the new twinjet, as it aims to ramp output up to a minimum of 10 aircraft a month (a higher rate than was ever achieved for the A330/A340), with a contingency to go to 13 a month depending on market conditions.
Unlike the single-section spun mandril lay-up philosophy Boeing has for the 787's carbonfibre fuselage, Airbus has adopted a more traditional build process for the A350 which involves the attachment of large carbonfibre fuselage shells to a composite frame.
© Airbus |
"The demonstrators programme was used to bridge between the virtual world of the digital mock-up and the real world," says Evrard. "It has been very important to make the designers and manufacturing engineering people working together along with all the factories, right from the start."
BARREL DEMONSTRATORS
Under the programme, Airbus plants in Germany and France have built several fuselage barrel demonstrators, the most recent one- produced in Hamburg and dubbed "section 1b" -is described by Evrard as a "second generation" of barrel. "It will support the certification process and demonstrate the efficiency of use of common tools and processes."
Evrard says the completion of barrel 1b testing will provide updated information "to feed the last part of our specific design and to feed the next versions". Results from the demonstrator programme will be sent to all sites and partners as a baseline for production.
The barrels will also be used this year to demonstrate and validate the "electrical structure network" (ESN) Airbus has designed into the carbonfibre fuselage to provide the electrical conductivity "you get for free with an aluminium fuselage".
The ESN uses existing metallic components (seat rails and some fuselage frames, for example) that are supplemented by aluminium strips attached to carbonfibre fuselage frames to provide return electrical paths.
With the A350's design, systems and production plans in place, Airbus has created an outline plan for the 15-month flight-test programme that will see five aircraft fly 3,000h, beginning in the first quarter of 2012. "We plan to put [all] the aircraft into the programme within a couple of months," says Evrard.
Although 15 months have been allocated to the flight-test programme, Evrard says the true duration could be as little as 12 months.
The second and fifth aircraft will have furnished cabins, and the sixth will be the first delivered to a customer, he says. "We want to use the second aircraft to rehearse our cabin definition freeze process."The fifth A350 will operate "early long flight" demonstrations with Airbus staff as passengers, he adds.
There is an urgency to get a furnished aircraft flying because Airbus wants to examine how it reacts to the "different behaviour" of the carbonfibre fuselage compared with a metallic structure, says Evrard. Areas to be investigated early in the flight-test programme are cabin noise levels and sound damping, and the satisfactory functioning of the ESN.
The next two years are critical if Airbus is to avoid a repeat of the A380's production dramas. The airframer knows that it must not blow this chance to capitalise on the delays suffered by the A350's US rival, which has closed the timing gap between the Dreamliner and the XWB.
AIRBUS FOCUSES ON INTERGRATION AND MATURITY
"Simple and mature" is the way that A350 programme chief Didier Evrard describes the A350's systems. "We've decided to go for a lot of reuse and are focusing on integration and maturity up front."
Flight controls use the "2H2E" (two hydraulic and two electric) architecture developed for the A380, including the dual 345bar (5,000lb/in2) hydraulic system. "This involves the reuse of the 'EHA' [electrohydrostatic actuators] and EBHA hydraulic actuators with electrical back-up, but optimised for the A350," says Evrard.
The fuel system is "very simple", with only three tanks - two wing and one centre -in order to reduce the amount of equipment.
The electrical system is "focused on dispatch" with each engine driving two variable frequency generators supplemented by an auxiliary power unit system and a ram air turbine for emergencies.
© AirbusThe flightdeck is simplified compared with the A330/A340 |
The air system, which is evolved from earlier Airbus aircraft, is also simplified for improved reliability, with "the focus on integration up front", says Evrard. "We allocated this big work package incorporating all the air systems to one supplier - Honeywell."
The carbonfibre wing structure has prompted Airbus to adopt a different landing gear configuration to earlier aircraft. Rather than being bolted to the rear spar - which would have created problems with local thickness of wingskins to take the load - each A350 main landing gear leg is attached to the rear wing spar at the front and to a gear beam at the rear and is equipped with a double side-stay to distribute the load better.
The A350's integrated modular avionics Ethernet package reuses earlier principles, says Evrard, but with more efficiency. "There are 23 aircraft functions with only 21 computers, meaning a more efficient allocation of computational and memory resources."
The A350 flightdeck is simplified compared with the A330/A340, featuring six large (15in/38cm) interchangeable displays that Airbus expects will offer "80% savings" in spares and direct maintenance costs, compared with the airframer's current widebody twinjet/quadjet family.
The flightdeck will offer many new features, for example required navigation performance 0.1 and continuous descent approach capability that optimises the path of the aircraft from cruise to touchdown, providing noise and environmental benefits. The "brake to vacate" autobrake function under development for the A380 will also be standard.
