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.