Composite aircraft may hide dangerous flaws

时间:2019-02-27 05:03:01166网络整理admin

By Paul Marks (Image: TSB) (Image: TSB) A standard test used to assess the safety of carbon-fibre composite airliners can be dangerously ineffective, according to air-safety investigators in Canada. In a report published on 22 November, the Canadian Transportation Safety Board (TSB) says an aircraft can pass the “tap test”, even though its composites actually have small flaws. Tap tests are part of routine servicing. A ground engineer listens for a change of pitch as they tap a composite surface like a tailplane or rudder. This is supposed to reveal gaps where layers of composite have come apart, but the TSB says small gaps can go undetected and later grow. These gaps can suddenly grow in size and potentially endanger an aircraft, the board says. It is working with the aviation industry’s global International Maintenance Review Board to review the current maintenance procedures. The TSB report follows a two-year investigation into a March 2005 incident in which an Air Transat Airbus A310 airliner lost nearly all of its 8.2-metre (27-foot) carbon composite rudder (see pictures) on a flight from Varadero, Cuba, to Quebec City, Canada. Following a loud bang that reverberated through the aircraft, the plane was immediately pitched into a “Dutch roll”, in which it repeatedly rolled its wings up and down while simultaneously swinging its tail side to side. By losing altitude, the pilots managed to bring the roll under control and land safely back in Varadero, with the whole episode only causing a minor back injury to one person. TSB tests in flight simulators showed that a Dutch roll could be induced if the rudder somehow separated from the plane at high altitude. But how could that happen? Environmental conditions on the night of the flight – weather, temperature and turbulence – had been nothing out of the ordinary, so it was unlikely any external forces had been to blame. So the investigators began to suspect that a major structural problem had caused the rudder to disintegrate. Could gaps in the tail’s composite surface have been missed during recent services? They took already damaged composite specimens – some from other A310 rudders – and placed them in a vacuum chamber. The samples experienced pressure changes simulating changing altitude during many simulated flights. “The areas of the damage almost doubled instantly,” the report says. “The rapid propagation event was explosively loud and violent.” The explosion even damaged their test chamber. The TSB say this explains the loud bang heard by the crew. The investigators concluded that a gap in an inner composite panel somehow passed tap tests. After many cycles of changing pressure the gap grew and finally exploded. The rudder lost strength and stiffness, started “fluttering” and broke up. “The manufacturer’s recommended inspection program for the aircraft was not adequate to detect all rudder defects,” says the report, saying there is no way to tell how old the damage was. The news that composite maintenance needs fresh attention comes as the aviation industry is preparing to launch passenger aircraft with fuselages made entirely from composites. US firm Boeing plans to launch its composite 787 Dreamliner in 2008, followed by Airbus’s A350 in 2012. Both companies use tap tests in their maintenance programs. The TSB hopes its findings will inspire an urgent search for more reliable procedures, like “twanging” composites with lasers to produce high resolution ultrasound echoes. Aviation – Learn more in our comprehensive special report. More on these topics: