GKN perfects primary composite structure assembly process for new Airbus wing
With the wing of the already under close scrutiny before the first flying example has been put together, key suppliers are focusing more than ever on a steady production ramp-up and delivering to specification.
Despite the news earlier this year of delays to the delivery of wings for the first flight-test aircraft, as well as for the fatigue-test article, major A350 wing sub-assembly suppliers continue to accelerate their production processes. Although development issues with automatic drilling machines have been fixed, Airbus has acknowledged an expected related delay of around three months to first deliveries of the initial A350-900s in early 2014.
The European manufacturer is therefore looking to its complex supply chain for even sharper performance as it seeks to smooth out any remaining kinks in its production system. GKN Aerospace, which is responsible for the A350 aft composite spar and fixed trailing edge, has a pivotal role in the production flow, as it perfects new assembly concepts in a £170 million ($274 million) purpose-built fully automated facility near Bristol.
Five complete A350 wing sets, including static and fatigue units, have so far been delivered from GKN's Western Approach plant to Airbus UK's wing final assembly facility at Broughton, North Wales, where the complete wing is integrated. The company took over the lease for the site, originally designed to be a distribution center, in April 2009. “When GKN acquired the facility, it was an empty shell,” says Vice President Steve Colebrook, who adds that “we broke ground in May 2009 and laid foundations in July that year.”
The initiative traces its roots to 2008, when GKN launched an aggressive growth plan for its aerostructures work by acquiring Airbus UK's wing component manufacturing and assemblies operation in nearby Filton for £136 million. “It was aimed at keeping the wing center of excellence in the U.K.,” says Colebrook. “The A350 was originally due to be housed within that but because of its size and magnitude it was moved here.”
Currently employing almost 270, the workforce at the Western Approaches facility is expected to grow by another 60 or more as the rate increases on the A350 as well as for the composite wing on themilitary airlifter, major sections of which are also manufactured here.
The facility is made up of two main buildings, one of which is a climate-controlled, clean room area for the layup and fabrication of the large-scale composite structures, and the other an “industrial area” where the assemblies are completed. Although the composite manufacturing area is state-of-the-art with advanced automated fiber-placement (AFP) machines for the A350 and conventional automatic tape-laying for the A400M, it is in the assembly building where most of the production innovations are to be seen.
“This industrialization was a first for GKN,” says Colebrook. “Normally you'd take the spar to a jig, then take it out and continue to the next jig until it is built. With this one, the spar is loaded and locked into a bespoke jig. Then the jig moves down from station to station to hold the highest level of tolerance before we take it out and pop it into a final turn-over jig to deliver it to Airbus.”
“The key tolerance is the hinge line of the trailing edge,” says value stream manager Mark Howard. “We're making it to within +/- 0.15 mm [0.006 in.] over a roughly 30-meter-long [98-ft.] assembly,” he adds. Each of the two wings in the shipset is made up of three components; an inner, mid and outer spar section. Mounted on specially designed manually guided vehicles, the jigs containing the individual spar sections are picked up and moved through the production process. By bringing jigs to the machines, rather than the other way around, GKN is also reducing the amount of capital equipment required for the line, says Howard. Instead of four sets of static jigs per wing, “here we have just three, and two robots,” he adds.
The three sections combine to produce a trailing edge assembly 27 meters long and weighing about 1,800 kg (3,968 lb.) of which around 1,230 kg is made up of the carbon composite spar section. The inboard spar section, measuring 7.5 meters, weighs the most as it includes the spar root and attachment point for the main landing gear. Composite laminates made up of a Hexcel pre-impregnated carbon/epoxy are “up to 25-mm thick at the deepest end around the wing root and landing gear pivot fitting, and thinner, just under 5 mm, at wingtips,” says chief engineer Clint Diffin.
Assembly of each section begins when five-axis AFP machines, made by M. Torres of Spain, wind composite tows from up to 16 bobbins onto a Cytec-made rotating composite mandrel. The machine is programmed to lay varying thicknesses to tailor the structure for stress requirements and provide foundations for parts such as rib attachment points along the spar. “When we're at full rate production we will be up to 13 wingsets per month and using up to 20 tons of [Hexcel] composite material per month,” adds Diffin. Three AFP machines are in place, with a fourth in development.
“The A350 inner spar is the thickest we build. It undergoes a hard structural cure and is then prepared for the addition of sacrificial material before being cured again in a secondary cure. This gives us the opportunity to make sure the girth height is correct,” says Diffin, who explains the AFP process was adopted over tape laying because of the higher assembly rates and more complex shaping of the A350 wing.
Left and right spars are laid up simultaneously on the mandrel in a process that takes up to eight 24-hr. working days. The units are then “de-bulked” to remove any trapped air, and inspected. Advances made in the performance of machines and controls during the initial batch of wing shipsets means that less time will be required in future for this process, adds Diffin.
The formed parts are loaded onto an Invar tool for curing in the autoclave. A second autoclave is about to be commissioned as the rate rises. Once cured, parts go into one of two five-axis composite machining tools which combine waterjet and conventional cutting devices before undergoing non-destructive testing for porosity or inclusions. A third tool will be installed by the end of 2014 to match production demand.
The spars are then painted with primer, and metal rib posts are added before completion with trailing edge gear and flap fittings.
Three more wingsets are scheduled for delivery to Airbus UK by the end of the year.