AIRCRAFT built from hefty quantities of carbon-fibre composites rather than aluminium will be taking to the skies in the next two years. The high-tech lightweight materials should help airlines slash their fuel costs, which they see as vital at a time when so many airlines are struggling to make money.
But while there may be a compelling economic case for the new aircraft, composites present a unique set of engineering challenges to designers and certification authorities, who have to pronounce the planes fit to fly. That鈥檚 because if composites fail, they do so in a very different way from the aluminium alloys used in today鈥檚 aircraft, and so have to be tested in different ways.
The high-composite-content aircraft are the 250-seat Boeing 787 Dreamliner and the giant 800-seat Airbus A380. The first 787s to be built will be made from around 50 per cent composite materials by weight. The 787 is planned to start flying in 2007 and go into commercial service in 2008. The huge A380, which made its maiden flight at the end of April, is 25 per cent composite and is due to enter passenger service in late 2006.
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Composites are not new to aviation. Since the late 1960s, aircraft components have been constructed using composites designed to handle the tension, compression, shear or torsion forces that different aircraft parts are subjected to. First were wing leading edges, followed by trailing edges, tail fins, rudders, tailplanes and flaps. A watershed came in 2002, when Airbus Industrie of Toulouse, France, flew a new version of its A340 long-range jet fitted with a composite rear pressure bulkhead, a critical (but usually very heavy) structure that seals the pressurised fuselage.
None of these steps was taken lightly. 鈥淏efore we used a composite tail fin on the A310 in 1985, we had undertaken 10 years of research to assure its safety,鈥� says Roland Thevenin, chief structures engineer at Airbus.
The 787 and the A380 contain many more critical structures built from composites than previous designs, including the fuselage鈥檚 central 鈥渨ing box鈥� 鈥� to which the wings are attached and which contains the central fuel tanks. 鈥淭he 787 is the biggest leap forward of the pair, as its entire pressurised hull will be made of composites,鈥� says Simon Waite, an airframe surveyor with the UK Civil Aviation Authority and a damage tolerance expert at MIL-17, a global aviation body that monitors composite standards.
Composites are made of a polymer resin reinforced with carbon or glass fibres. Built up in tens of layers, they can be used to make components that are lighter, corrosion-proof and often stronger than their metal equivalents. The biggest threat to composite parts is that factors like ageing or stress might cause the layers to begin to separate, or 鈥渄elaminate鈥�, and so weaken the material.
Once a plane is in service, airlines look for evidence of delamination by subjecting planes to regular visual inspections. Defects, even deep down in the laminate, caused by stress from severe turbulence, say, or a minor collision with a ground vehicle, show up as wrinkles on the surface of a composite. 鈥淣inety per cent of these inspections are visual because it鈥檚 the cheapest option for an airline,鈥� Waite says.
But not all damage to composites can be detected this way. A class of defect known as 鈥渂arely visible impact damage鈥� is causing the most concern. Up to 60 per cent of the surface blemish on a composite can recover elastically, Waite says, potentially disguising the scale of any underlying damage.
鈥淚nspectability of composites in service is a problem,鈥� Waite says. 鈥淎nd in terms of our understanding of how composite defects grow, we are where we were with metal cracks some 40 to 50 years ago.鈥�
The way cracks propagate when metals become fatigued is well understood, and engineers know what to look for. Composites break down in a far more complex way. The polymer matrix can suffer multiple cracks, which can be temporarily stopped by fibres until more stress sets them off again. So failure can be sudden, Waite says.
鈥淭he Boeing 787 is a big leap forward, as its entire pressurised hull will be made of composites鈥�
Because so much of the 787 and A380 will be made from composites, the European Aviation Safety Agency and the US Federal Aviation Administration are requiring the aircraft makers to design and manufacture their composites in a way that ensures defects cannot grow beyond a safe level. 鈥淭hey have to demonstrate a 鈥榥o growth鈥� situation for defects in sample critical parts before an aircraft is certified,鈥� Waite says.
To do this, manufacturers deliberately damage the most safety-critical components, such as a tail-fin attachment bracket, and then prove that under tens of thousands of cycles of hot, cold, wet and dry conditions, any defect does not exceed safety limits. Getting it right in the design phase is the way to ensure that composite components are safe even if a defect is missed during in-service checks, Waite says.
With this approach to pre-manufacturing testing in place, both Thevenin and his opposite number at Boeing, Justin Hale, deputy chief mechanic for the 787, are confident that today鈥檚 in-service test regimes are more than adequate to assess the new planes.
However, some pilots in the US are not entirely reassured, and their concerns were voiced by first officer David Hayes of the US pilot鈥檚 union, the Air Line Pilots Association, in its in-house journal, Air Line Pilot (March 2005, p 24). 鈥淲e are concerned about the capability of airlines to conduct non-destructive testing of these composite materials during in-service maintenance,鈥� says Hayes, who heads ALPA鈥檚 A380 Project Team. 鈥淚f an airplane is hit by a catering truck, what has been damaged? Some of these composites are fairly exotic and require sophisticated imaging techniques to reveal defects.鈥�
鈥淚n terms of our understanding of how composite defects grow, we are where we were with metal cracks some 50 years ago鈥�
Marc Fernandez, an airframes expert with the Transportation Safety Board of Canada in Montreal, says that new in-service composite test techniques and equipment, such as thermographic cameras to reveal water ingress, are being developed in labs including Canada鈥檚 National Research Centres. But he adds it is too early to say if such measures will be necessary.
