Our MRO Europe conference starts in Hamburg tomorrow, Sept. 22, and we're featuring a special section on AviationWeek.com on the event, including news, blogging and photos. That will go online later tonight. To kick things off we're posting some stories here from Overhaul & Maintenance magazine, including this one below from the Sept. 2009 issue.

Bill Burchell/Hamburg

The A350XWB's entry into service is scheduled for 2013, but Airbus targets system maturity before the first flight and expects repair schemes to reach a mature stage within the next two years.

--Airbus aims to achieve "an unprecedented level of system maturity" for its A350XWB twinjet well before the aircraft's first flight, according to Didier Evrard, who heads the A350XWB Program. "Maturity at first flight will be a major challenge because, at that point, lots of components for the aircraft will already be in production and repair schemes agreed," he says.

To accomplish this, Evrard says Airbus will use digital mock-ups, virtual testing and actual functionality testing to minimize modifications required during flight testing. "If we can achieve that, the flight-test program can focus on certification," he says.

The final shape and structure of the A350XWB is beginning to solidify following recent customer agreement of the detail definition freeze--dubbed 'maturity gate' (MG) 5--which was reached in December. With customer agreement, the fuselage and wing structures, all internal systems and the cabin architecture (with the exception of the rear galley) have been frozen.

"This was a very important point," says Evrard, "because from here we could start to commit to detailed design at component level. As a result, long lead-time items were launched together with the design and construction of tooling and jigs, while weights and performance remained unchanged."

Airbus' 3-D digital mock-up for the A350XWB will be used by all industrial partners involved in the program to harmonize processes, methods and tooling. This will include a common sizing tool as well as standardized machines, jigs and tools for composite component manufacture. Common training programs also will be used to standardize construction and repair skills.

The Intelligent Airframe

The A350XWB will feature what Evrard describes as an "intelligent airframe," a mainly composite structure interlaced with metallic elements to create a conductive electrical structures network (ESN). This interconnected network is designed to provide an electrical "earth" or "grounding" for the aircraft on the ground as well as low-resistance paths for lightning strikes and static charge dissipation.

"Using carbon in the fuselage, we have had to design some functions that would come for free with metallic materials," Evrard explains. "We've kept the flexibility of using metallic panels where composite does not deliver the weight benefit, for example, in areas susceptible to bird strike or lightning strike. In this case, they may be titanium, aluminium or aluminium-lithium."

Aluminium and titanium are used separately: titanium being used where stresses are high, such as the highly loaded frames in the centre fuselage, the door surroundings, parts of the landing gear and in the engine pylon primary structure--areas which also benefit from titanium's resistance to corrosion.

Cross-beams for the passenger cabin floor are aluminium, while titanium makes up the hold floor cross-beams. Additionally, cross-beam attachments and some frames in the "bilge" area are also titanium. That said, Evrard says that aluminium is still being considered for the hold floor cross-beams. Some fuselage half-frames are metal, and metal strips have been fitted to some of the composite frames.

"We didn't want the ESN to add weight," he says. "Where possible, we used existing metal components in the fuselage structure. Indeed, very few additional metal straps around the frames were required to optimize the level of connectivity and protection we wanted from the network." That being said, Evrard explains that incorporating metals in the right places has not been easy. Because carbon composite and aluminium have different rates of expansion, they are not compatible, he says, whereas titanium and carbon have similar expansion rates.

Multi-Panel Construction

To test this mixed construction, several fuselage section demonstrators have been built for the A350XWB program. These are full-size, 6-meter diameter modules built mainly from composite in what Airbus has dubbed a "multi-panel" concept. Unlike Boeing's 787, they are not one-piece composite barrels (although Evrard concedes this may be possible long term, say between 2015 and 2020).

Multi-panel construction comprises fuselage sections built from four composite panels: two long side panels joined by much shorter upper and lower panels. Airbus claims this method produces a much more accurate shape because tolerances can be adjusted as the panels are assembled, ensuring a perfect fit between each fuselage section.

Instead of being molded as part of each panel, "omega" profiled carbon fiber stringers are manufactured separately and bonded to the skin panels. But the fuselage frames and skin panels are not bonded together. These joins are made with a mix of thermoplastic clips and special fasteners.

"By concentrating on longitudinal joins in the fuselage we reduce the number of orbital junctions between sections, which require a lot more parts and potentially add weight," explains Evrard.

Weight also is saved in the window frames, which in metal normally have a "T" section profile. Carbon fiber construction allows this to be pared to a lighter "L" profile while retaining the same strength.

Repair Schemes

While the demonstrator barrels substantiate the design principles, establishment of repair principles runs in parallel. Evrard says hundreds of test pieces have been made to evaluate what his team calls "the effects of defects," which equates to what can be accepted during the manufacturing cycle and, eventually, what can be accepted when the aircraft is in service.

He says the next step is to develop the actual repair principles based on a range of solutions, depending on the type of damage sustained and the length of operation which needs to be flown with the damage in situ, or until repairs can be properly implemented. "We take these issues very seriously," stresses Evrard. "That's why we have customer focus groups already dealing with this issue."

That said, all repair schemes for the A350XWB are still under development, but Evrard insists they will all be determined prior to the aircraft's production.

Pressed when these repairs might be available, Evrard can't confirm. "Entry into service is 2013, so we have sufficient time to demonstrate repair schemes to the relevant authorities," he says. "But certainly, we expect the repair schemes we propose to have reached a mature stage within the next two years."

Common Wing

The wing is another major component detailed for maximum efficiency, not only through aerodynamic design but also through the systems integrated into it, which improve its characteristics.

"We wanted the wing to be capable of a Mach 0.85 cruise speed (like the A380) and still produce less drag, thereby saving fuel and increasing range," Evrard says. "Moreover, the same wing had to be common to all three models of the A350."

Its design also must deliver improvements at low speed, hence the use of advanced high lift devices on both leading and trailing edges. Evrard says a lot of computer modeling has been done to optimize the shape of attachments like the engine pylons, flap-tracks and belly fairings and to integrate the engine nacelle with the pylon.

To improve aerodynamics during the cruise phase, the flap system will be used to vary the wing camber, optimizing its camber for each phase of flight--not just to reduce drag, but also to alleviate loads on the wing structure.

Keeping It Simple The approach to aircraft systems on the A350XWB is "keep it simple" by focusing on large-scale integration, particularly for the flight controls, which use electrical back-up (like those on the A380). Similarly, hydraulic circuits have been optimized to just two circuits operating at 5,000 psi, while the fuel system has been cut to just three tanks to reduce the number of pumps and valves.

The electrical system follows this philosophy, too, using variable frequency generators on each engine for better reliability. Even the landing gears feature a simplified design to specifically reduce direct maintenance costs.

In line with this thinking, the A350XWB's cockpit features six identical, large, 15-in.-wide, interchangeable displays, providing less complexity and saving up to 80% of the spares and maintenance cost, compared to an A340, according to Airbus.

Although only Rolls-Royce's Trent XWB engine has been selected to power the A350 so far, Evrard says engine thrust ratings will be matched to each variant. For example, the engines will be rated at 75,000 lb.-thrust for the A350-800, 84,000 lb.-thrust for the -900, and 93,000 lb.-thrust for the -1000. Additionally, a common engine type should reduce the number of spares required, while new technology and materials should deliver better fuel efficiency and cut noise and emissions.