Airbus's description of its A350 final assembly line as being similar to a Lego set might be somewhat simplistic, but it illustrates a crucial point: the airframer wants to ensure complexity is minimised as the structures come together in the end stages.
It is a plan that aims to avoid the mayhem of early A380 production, during which incomplete, out-of-sequence sections jammed the line. Airbus swears it has learned from that painful experience, but its confidence will be tested as it brings together the components for its first flying prototype, MSN1, with just a 12-month window in which to meet its maiden flight target.
The first A350-900 will take to the air "close to the middle of next year", says programme head Didier Evrard. Service entry is intended during the first half of 2014, but Evrard points out that this is likely to be "closer to the middle of 2014 than to the beginning".
This statement underlines the continuing pressure on schedule margins for the programme, which has been repeatedly labelled as "challenging" by both Airbus and its parent EADS. By adopting a risk-reduction strategy that prioritises component maturity, the manufacturer hopes that any deviations from the schedule - such as the three-month slip to starting final assembly - will be less costly than well-intentioned but ill-judged hastiness.
"It's all about discipline - how can we be strict about out-of-sequence work?" says Evrard, adding that, from MSN1 onwards, Airbus will have a strict "quality gate" procedure in place. He says that the airframer has an "obsession" with avoiding the production problems experienced by the Boeing 787.
Fifty days into final assembly, Airbus wheeled out its first static airframe, MSN5000, having mated the forward, central and aft fuselage sections at the Toulouse line point designated Station 50. After this junction the fuselage transfers to Station 40 for wing join.
"Everybody was pleased with the way this [fuselage fitting] came together," says Evrard. "This for us was a big satisfaction."
But he says that Airbus "still has to de-risk the wing and the junction of the wing". This is a particularly sensitive matter for the airframer following the discovery of cracking in brackets on the A380's hybrid wing ribs, which were intended to save weight on the type.
"We found out the hard way that we didn't know everything we should have known before making this decision," said outgoing Airbus chief Tom Enders, days before he stepped up to head EADS. "It was deeply irritating and disruptive, and certainly not good for the reputation of Airbus."
Although the A380 would survive, he added, the problem demonstrated the dilemma of innovation. "If you're too risk-averse, your competition overtakes you and you're dead," he said. "If you're [not], you're dead and then your competition overtakes you."
Evrard says there is a "lot of innovation, everywhere" on the A350, which will undergo tests on new customised rigs installed in the same building previously used for A380 tests. New computing tools are allowing more extensive modelling "in order to rehearse the [testing]", Evrard says. "This is the right way to go to de-risk the programme, knowing you're never 100% immune to something going wrong."
The military A400M was the "baseline in terms of knowledge" for the A350 wing, he says. But Evrard stresses that the manufacturer has analysed its experience with the A380, despite the design differences, to assess whether it can take into account any other de-risking - such as additional cycles - during A350 fatigue testing.
He adds that the airframer has built a vertical fin root-joint demonstrator, in order to conduct fatigue assessments ahead of the "real" fatigue testing of the tail structure.
Toulouse already houses two A350 simulators, but virtual modelling is used effectively as a third simulator, before systems reach the hardware and software stage. "This is one of the big changes we've made," says Evrard. "You can really debug a lot of things at the model level."
The innovation also extends to the physical structure, and commencement of assembly has enabled Airbus to verify its strategy of opting for carbonfibre panels over a barrel-type design for the A350.
"It was a big debate at the beginning of the programme - barrels or panels," says Evrard. "It's a little bit of a mix. I think we made the right choice. The industrial process shows we made the right choice with our panel concept."
Airbus is aiming to build MSN1 in parallel with the static airframe, and Evrard says the manufacturer is "on track" with system installation of the prototype.
Equipment installation for MSN1's cockpit is "very well advanced" at the St Nazaire facility, he says.
Drilling processes for the upper and lower wing covers of MSN1 have also started at the UK's Broughton plant, although Evrard admits that familiarity with the new technology being employed is taking time to accumulate.
Progress with the wing is slower than Airbus thought it would be initially, he says, because the process is "a little bit handicapped" by the drilling machinery. But he says that the system will enable the manufacturer to achieve the rates projected for A350 production ramp-up.
"The problem with an aircraft programme is you can't have the first aircraft without the [manufacturing machines]," he adds.
Airbus expects to begin final assembly of MSN1 in the summer - it has not narrowed this estimate - but Evrard says the horizontal stabiliser is "very close to achievement" and will arrive on the line in June-July. At the end of May the airframer also accepted the first set of doors for the flight-test aircraft.
A350 customer programme director Borna Vrdoljak says the assembly process is intended to be "much more efficient" than for conventional long-haul aircraft construction. The aim is to undertake structural, cabin and test activities in parallel - achieving fuselage join and early electrical power-on, and moving to wing join at the same time as cabin integration.
"In four years' time we're aiming for 10 aircraft per month," Vrdoljak says.
Evrard highlights the use of "mixed reality" technology to enable the digital mock-up to be compared with the actual structure on the assembly line, ensuring that detailed items, such as brackets, are in place. He admits the supply chain remains a source of concern, noting that a programme in development is "far less stable" than a product in routine production.
But Evrard adds that Airbus has "made a lot of progress" in addressing problems with the major structural sections, which set back the schedule for final assembly, although he concedes that the manufacturer is "not at the end of the journey".
Airbus has already stepped in to rescue German A350 parts provider PFW, and Evrard points out that the company is carefully watching the financial health of suppliers. "The problem for us is knowing how we can manage, or ease, their pain," he says.
Aside from the two simulators, several other large testing systems are running. The 'iron bird' rig in Toulouse is operational, while nose-gear and main-gear cycling has been carried out at the UK's Filton plant.
Testing is also under way for the high-lift system rig in Bremen, and for the air systems integration bench in Mexicali, Mexico, while cabin interiors work is being conducted at Hamburg.
The only powerplant under development for the A350 - the Rolls-Royce Trent XWB - started a year-long programme of airborne checks on an A380 on 18 February.
Five flight-test aircraft will be built for the A350 programme, two of which - MSN2 and MSN5 - will feature cabin installations. MSN2 will be fitted with 252 seats, comprising 42 in business class and 210 in economy class, says Evrard. He adds that a "key enabler" is pre-development of interior customisation packages for the various cabin zones.
Both MSN2 and MSN5 will be used for evacuation testing as well as assessment of long flights, route proving and extended twin-engined operations checks.
While EADS's choice of vocabulary regarding the programme has yet to prove completely reassuring, Evrard is nevertheless confident. "Every day something happens. There are always good - and less good - things every day," he says. "I think [the schedule is] feasible. But it's tight."