For at least a decade the name "A350" was a phantom designation, an identity in search of an aircraft.
In that time, it had been used loosely to refer to a possible Airbus rival to the Boeing 747, before Toulouse caught 'eight fever' and branded its double-deck A380 accordingly.
The spur for the A350's development was the airline market's desire for a middle-market successor to the Boeing 767, a jet in the 250-seat category powered by highly efficient turbofans.
Boeing, which had failed to stir interest in a stretched 747 and whose brief dalliance with the Sonic Cruiser concept had an air of desperation, was suddenly enchanting customers with its 787.
Airbus began test-flying the A350-900 around seven years after programme launch
Airbus
Airbus's preoccupation with the A380 left it unprepared for the 787's mass appeal. In an effort to catch up, it hung the A350 tag on warmed-over versions of its A330, but struggled to convince customers that – despite a reworked wing and fresh engines – the end result was really as new as its number-change suggested. Boeing underlined the point by running a sarcastic campaign portraying frustrated Airbus designers hurriedly taping an "A350" sticker on a crudely hacked-up A330 model.
Belatedly acknowledging that Singapore Airlines, lessor ILFC and other top-tier clients would not be satisfied with anything less than an all-new design to counter the increasingly popular 787, Airbus peeled the A350 label off its revamped twinjet and rethought its strategy.
If there was a risk of the A350 brand becoming stale, the airframer put aside such concerns when it unveiled a new family concept in 2006 – although it opted to distinguish this larger aircraft by underlining the spaciousness afforded by its "extra-wide body".
Airbus envisaged the A350 XWB as a family of three – the keystone -900 flanked by a smaller -800 and a stretched -1000, spanning a 270- to 350-seat range – rather than the two previously proposed.
Three variants of the A350-900 have emerged, with maximum take-off weights at 268t, 272t and 275t. As of May, the aircraft is being marketed as a 322-seat jet with a range of 7,600nm (14,100km).
The -800 was effectively abandoned, ironically after Airbus opted to re-engine the A330, and pressure for greater -1000 performance led to a revamp in 2011 to raise maximum take-off weight from 298t to 308t and enhance range.
Just shy of seven years after it presented the XWB, Airbus started test-flying the A350-900, commencing a certification campaign that would prove remarkably smooth and enable the airframer to claw back some of the time it had surrendered to the 787 – which, to Airbus's benefit, had endured a far less straightforward path to service entry.
Airbus attributes the swiftness of the certification partly to a conservative decision to stay on the side of conventional design. The A350's power system features bleed-air architecture from its Rolls-Royce Trent XWB engines – less adventurous than the electrical complexity of the 787, which draws heavily on lithium battery power. Even the A350's planned incorporation of limited lithium power was initially shelved in favour of nickel-cadmium as – in the wake of regulatory uncertainty over lithium cells – Airbus adopted a devil-you-know attitude to keep the certification on track. Lithium will succeed nickel-cadmium on production A350s from around 2016.
Just over half of the A350 airframe is constructed of composite materials, which had been present only on the fringes of the A330 – its fin, cowls, fairings and control surfaces. Airbus opted for a simplified long carbonfibre four-panel structure for the fuselage, with the forward, centre, and aft sections comprising essentially a belly, a crown and two side shells. The long panels are attached longitudinally and reduce the need for circumferential joining, lightening the overall structure.
Dual-circuit hydraulics operating at 345bar (5,000psi) are based on the A380's system, while Airbus used a distributed conductive network – the "electrical structure network" – to provide the conditions required for correct system functioning within the composite fuselage.
Its wing surfaces and engine nacelles are built from monolithic carbonfibre with sandwich structures for the wing-root fairing, spoilers, ailerons and wing-tips. The large wing required a new manufacturing approach at Broughton, Airbus's UK wing centre, where the need to grapple with new automated drilling systems forced a three-month delay in service entry.
"Every section has been a challenge," says Didier Evrard, who led the A350 programme. "Sometimes we didn't have the technology in the beginning. Like for certifying the composite fuselage.
"Then we had to learn how to install systems in the composite fuselage – particularly the electrical systems. But again, it was a question of developing the right rules first, to make sure everything was safe, and then developing a solution against these rules."
Advanced production techniques for the A350 have included additive layer manufacturing, so-called 3D-printing, with over 1,000 such lightweight resin parts used in the initial production aircraft, US-Israeli specialist Stratasys indicates.
As the aircraft has been refined, says Evrard, the airframer has been able to reduce its initial over-cautiousness in the interpretation of its requirements, shedding unnecessarily extensive wiring protection, he says, or taking away surplus brackets. "We're getting rid of this complexity," he says.
Using a single three-dimensional digital master for the A350 "really has made a difference" to development and production, says Airbus chief operating officer Tom Williams. "The number of design query notes is significantly less," he says. "We don't see the same problems as on the A380 in terms of customisation."
Unlike the A330, the fuselage design gives the aircraft a parallel-wall cross-section between the forward and aft pairs of exit doors, to avoid compromising the cabin width at the rear.
The 5.6m (18.4ft) interior cabin, from which the XWB designation derives, is wider than the 787's and sufficient for a nine-abreast economy layout with 18in seats; the narrower A330 could only manage eight-abreast. Among the cabin enhancements are the larger passenger windows with an area of 145in2.
Airbus estimates that the turnaround time for the A350-900 will be around 61min, based on a nominal load of 315 passengers in a two-class layout with a 48-seat business cabin.
This assumes that two airbridges are used, one for each of the forward left-hand exits. The turnaround time is 2min longer than that of a 300-seat A330-300.
For the A350-1000, which will have typical seating capacity of 366, the airframer believes turnaround can be achieved in 70min.
Airbus resisted a composite nose, preferring the strength of aluminium alloy for the cockpit, and adopted design features of the A380.
The cockpit profile of the pre-XWB iteration had been refined in a bid to accommodate an underfloor crew rest area, freeing valuable revenue space. But Airbus reconsidered the nose for the XWB, adopting a distinctive aerodynamic profile closer to that of the A380 and using the space beneath the cockpit to mount the nose-gear further forward. The crew rest area instead was shifted to the forward crown above the passenger cabin.
Out went the four-window concept for the cockpit, replaced by a six-window arrangement similar to the A380's. Crew emergency exit would not be through the windows but instead via a roof hatch.
Carbonfibre wings are critical to the A350's economics, and the wing-tips – a blended upward-swept rake design – increase the span to almost 65m (213ft), matching that of the Boeing 777-300ER. The A350's wing, at 443m2, has 20% greater area than the A330's, combined with a 3˚ increased leading-edge sweep of 35˚ to nudge the aircraft's cruise speed upwards to Mach 0.85.
Aerodynamic enhancements to the wing, compared with that of the A330, include the use of an inboard droop-nose device on the leading edge. This was originally adopted for the A380, providing a relatively simple high-lift capability without the complexity of creating a slat mechanism suitable for the deep wing root.
Six slats are installed on the outboard leading edge, while the trailing edge features a simple single-slot flap layout that generates performance benefits through behavioural tweaks. Rather than extending in a line perpendicular to the wing sweep, the flaps deploy parallel to the slipstream to reduce drag. Airbus has also implemented a variable-camber capability, and the mechanism allows a differential setting between the inner and outer flaps. After retraction the flaps can be set at different rest positions that enables the wing loading to be better optimised to the aircraft's weight by shifting the centre of lift across the span.
There are seven spoilers on each of the A350's wings, compared with six on the A330's. To improve the wing's high-lift performance, Airbus has designed the spoilers to deflect slightly downwards during flap extension, smoothing the airfoil profile at the wing-flap interface.
With a maximum taxiing weight of just under 269t, the A350-900 is one of the heaviest airliners to use a four-wheel main bogie without the support of a centre gear, like the A340-600 or Boeing MD-11.
The lateral spacing of the main wheel is over 1.7m, compared with 1.4m for the A330, giving the A350 characteristically long axles.
While the A330's main-gear bogies are held in a trailing position by an articulating link, with the rear wheels low, those on the A350 are not.
The A330's main-gear assembly has a single side-stay for stability. But Airbus designed the main gear of the A350-900 with a double side-stay arrangement that enables better distribution of loads on the composite wing and avoids unnecessary reinforcement.
Within the cockpit, integrated modular avionics reduce electronic connections and complexity, while the layout presented to the pilots simplifies that of the A380 while retaining its capabilities.
Crucial to the programme has been the strategy of maintaining a common type rating with the A330 for pilot training. This approval enables crews who carry A330 qualifications to switch to the A350 with just a minimal differences-training course.
This eliminates the need for full-flight simulator operations and slashes the transition time to about a week.
Primary information display is on six identical screens – fewer than the 10 in the A380 but, at 15in, much larger than those of the A330. The size is intended to provide visual clarify as well as flexibility in the way data is presented to crews, notably through split-screen viewing and interchangeability.
Each of the two pilots has one screen for flight and navigation data and a second for the onboard information system – featuring charts and other aeronautical items, such as manuals and equipment lists, for quick access and paper document reduction. The screens are angled for cross-cockpit visibility and automatically reconfigure in the event of display failure. Pilots interact with the screen data via a keyboard cursor control unit in the pedestal.
Airbus has maintained its signatory side-stick control and non-back-driven thrust levers. There is also commonality in configuration management systems including the flap, speedbrake and landing gear controls, as well as in the overhead panel layout.
But while the layout and flight controls are designed to retain familiarity, Airbus has rethought features of the cockpit to simplify crews' workload and take advantage of technology introduced on the A380.
Features include a vertical display profile to give pilots greater situational awareness through a side-on view of the flightpath. Airbus has aimed for better information access through the electronic centralised aircraft monitoring interface, including revised checklist presentations and recommendations in the event of failures.
Singapore Airlines' first of 70 A350s is scheduled for delivery in the first quarter of 2016
Airbus
The airframer's brake-to-vacate system allows pilots to prepare for a specific runway exit while still on approach, using a combination of the auto-brake system and flight controls to slow the aircraft and reduce runway occupancy.
Part of the standard avionics package on the A350 is the runway overrun protection system that was originally approved for the A380 in 2009. The Airbus-developed system collates information on the approach runway condition and contrasts it with the weight and configuration of the aircraft to analyse the risk of an overrun.
If it calculates a mismatch the protection system can sound a warning to the crew, enabling them to execute a missed approach or take other precautions to ensure that the aircraft is able to stop safely in the available distance.
Mounted in the centre pedestal between the pilots' seats is an integrated radio-management system, like that on the A380, and the A350 also features refined datalink interfaces for air traffic control.
As well as providing the capability for precision navigation, the flight management system is designed to ease workload by enabling the crew to explore the performance impact of possible failure scenarios along the route. The A350's autopilot has also been enhanced to cope with small excursions outside of the normal flight envelope.
Airbus has provided flexibility that enables airlines to maintain differing electronic flightbag options, by allowing crews to use their own laptop computers to integrate with the aircraft's avionics systems.
While General Electric's GEnx engine had been poised to power the original A350, with Rolls-Royce subsequently offering the Trent 1700, emergence of the XWB spurred development of a new Rolls-Royce powerplant – one which would quietly edge GE off the programme.
The sixth production engine to bear the Trent designator, the Trent XWB was intended to deliver the expected 75,000-93,000lb-thrust (334-413kN) range covering all three A350 XWB airframes.
But the decision to raise the thrust of the A350-1000 – part of a redesign to increase range and broaden the type's appeal – resulted in an engineering divergence between the XWB-84, common to the A350-800 and -900, and the XWB-97 for the -1000.
Maintaining the three-shaft design of Trent predecessors, the XWB built on the technology of the Trent 900 and 1000, and became the largest of the family with a 3m (118in) fan comprising 22 titanium blades.
It has an eight-stage intermediate-pressure and a six-stage high-pressure compressor, notably incorporating bladed disk or "blisk" technology in the first three stages to save weight and improve aerodynamic efficiency.
"We learned a lot from the Trent 900 which was integrated into the XWB," says outgoing Rolls-Royce chief executive John Rishton.
"We've done everything better – there’s a better compressor, better materials, higher temperatures. Everything about it."
The high-pressure turbine is single-stage but the Trent XWB features a two-stage intermediate-pressure turbine, an architecture intended to avoid inefficiency in the intermediate section, and provide greater thrust with a lower fuel burn. Its six-stage low-pressure turbine is designed to be short and light, using semi-hollow blades.
Changes to the bearing load management system shifted the low-pressure system support forward, to the front bearing housing, compared with conventional Trent powerplants. Although heavier, the arrangement generates an improvement in fuel consumption.
Rolls-Royce has used composites for the rear fan case and employed a single-skin combustor casing to reduce weight.
The Trent XWB has been designed to meet stringent noise targets of QC1 for departures and QC0.5 for arrivals. Rishton says the initial two in-service aircraft for Qatar Airways have been "delivering what we'd expected" in terms of fuel burn. "Both aircraft and their engines are performing very well in service," he adds.
Qatar's lead aircraft had accumulated some 1,500h and more than 350 cycles by the end of May. Chief operating officer for civil large engines Simon Burr says the dispatch reliability for the Qatar Trent XWB reached 100% over the four months from the beginning of February. "It's gone through some interesting weather, sandstorms, and it's in great shape."
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The manufacturer is focused on preparing its XWB-97 for the A380 flying testbed. It will run in June and be delivered to Airbus in July. A 120h test campaign is scheduled to start in October.
Although the -1000 has higher thrust requirements, Rolls-Royce reasoned that the performance of the XWB-84 version, particularly the fuel consumption and cool operating temperatures, meant that the XWB-97 could be designed with an enlarged core but retain the -84's fan size.
However, internal changes will include an unshrouded high-pressure turbine. "We've done a lot of work on that, to control the clearances very closely," says Burr. "The performance of the turbine is great, we feel good about it. The fan is running a bit faster, [the core] a bit hotter – but it's not a 'hot rod', we're comfortable with the temperatures it's running at.”
Airbus is closely monitoring Qatar's early experience with the A350-900. "This is the period to learn all the remaining maturity," says Evrard. "It can be small things, linked to cabin use... things you can't see before you operate in an airline environment. We've deployed a significant team in Doha to capture all these findings."
Confident that the -900 is achieving a relatively trouble-free service entry, and with 21 -900s in final assembly at the end of May, Airbus is increasingly concentrating on the -1000's development.
Airbus officially lists the -1000 as a 366-seat aircraft and puts its range at around 8,000nm with maximum passenger payload. The airframer says it can accommodate up to 440 passengers in a high-density layout and its most recent airport compatibility data on the aircraft shows an optional "type C" exit between the aft main doors to meet evacuation requirements in certain configurations.
Despite the divergence created by its 2011 redesign, the -1000 still maintains strong commonality with the -900, with the primary differences – other than the 11-frame fuselage stretch and XWB-97 engines – comprising an extended trailing edge and six-wheel main gear.
But there are also smaller details that have been incorporated into the design. The -900's metallic door surrounds are composite on the -1000. It will feature electric landing gear door opening mechanisms, and a simplified installation of floor electrics. The aircraft also has an optional new aft galley layout option, and interior cabin studies are under way to achieve a 20-seat increase in accommodation by 2020.
"Full structure design maturity and systems installation architecture is complete," says head of A350 programme development Bruno Hernandez. "We've started production in all the plants."
Substantial component assembly has commenced, including the fixed trailing edge, pylon, centre wing-box, lateral junction panels and carbonfibre door surrounds. Entry to the final assembly line is scheduled for early 2016.
Evrard says Airbus tasked itself with the "highest development of maturity" on service entry for the A350, adding that he is "very positive as to what's been achieved" with the "Airline1" process – designed to mirror carrier operations before delivery.
"Within the supplier community, we have a lot of support," he adds. "The network of partnership built around this aircraft has not been easy to achieve, but it is a huge asset."
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Source: Flight International
