ON 1 NOVEMBER the first aircraft with a pressurised fuselage and wings made from carbon-fibre reinforced plastic (CFRP) flew its first passengers from Tokyo to Hiroshima. The All Nippon Airways Boeing 787's composite structure makes it around 15 per cent lighter than a typical aluminium-based plane of that size, increasing fuel efficiency and making aviation greener.
But the media hoopla over the flight disguised some worrying questions about the long-term safety of composite aircraft. On 20 October, the US Government Accountability Office (GAO) published a report which, while accepting that the 787 has been certified as airworthy, questions the ability of the US regulator, the Federal Aviation Administration, to ensure that inspectors are capable of assessing and repairing damage to composite structures over the long life of a plane.
"It is too early to fully assess the adequacy of FAA and industry efforts to address safety-related concerns and to build sufficient capacity to handle composite maintenance and repair: says the GAO.
Until now, only smaller, isolated pieces of secondary structure, such as tail fins and wing leading edges, have been made from composites. The GAO reviewed the scientific literature and interviewed engineers about the evidence underpinning the expansion of composite use to incorporate the whole fuselage. On damage and ageing issues it found the science wanting.
The GAO found that engineers don't know how such materials will behave when damaged, what such damage will look like, and how these factors change as the material ages. Because composite damage is hard to detect - indeed it can be effectively invisible - working out what risk a dent poses is difficult. Too few inspectors are being trained to diagnose such damage, the GAO report adds.
Boeing has no doubts. "We test, we analyse and we demonstrate that even in extreme conditions - which may never be experienced in a full life of service - the airplane is safe and durablethe firm said in a statement.
A composite is made by combining multiple layers of carbon fibres with an epoxy resin. It has a higher strength-to-weight ratio than alumin urn and resists corrosion. But it has different fatigue problems: it tends to snap, rather than bend or stretch over time like a metal.
Although the Boeing 787 is deemed safe, the GAO says regulators must focus on assessing composite damage in service. "The long-term ageing behaviour of these composite materials is indeed an unknown;' says Philip Irving, an aviation structures specialist at Cranfield University in the UK. "What is going to happen to these structures, which are often bonded as a single piece, in the 30-year lifetime of an aircraft?" Much is loovvn about metal, he says. "There is almost nothing equivalent published on composite-structure damage, visibility and growth - and the necessary research is still under wEy," he says.
Some of that research is being done by the Commercial Aircraft Composite Repair Committee, says Boeing, an industry-wide effort involving regulators and manufacturers, including Airbus (which is building its own composite fuselage plane, the A350). In the meantime, Irving says ground staff will need to wield one of their most powerful tools to track down damage in composite planes: "Their eyeballs."
SOURCE : NEW SCIENTIST NOVEMBER 2011