Transportation Safety Board of Canada
www.bst-tsb.gc.ca
TSB Feature Articles
This article has been reproduced with permission from
New Scientist © Reed Business Information.
Composite aircraft may hide dangerous flaws
22 November 2007
Paul Marks
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.
Exploding rudder
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.
Small gaps in the carbon-fibre composite of this plane's rudder were not spotted by routine checks and later exploded. Accident investigators say better maintenance tests are needed before composites are used for entire planes (Image: TSB)
After the accident only 5% of the 8.2-metre (27-foot) rudder remained – fortunately the plane landed without serious problems (Image: TSB)
Despite being pitched into a "Dutch roll", rolling its wings and wagging its tail, the aircraft landed safely, causing only a minor back injury to one person.
TSB tests in flight simulators showed that the Dutch roll happened if the rudder was missing. Yet weather conditions had been normal. The investigators started to suspect gaps in the tail's composite had 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.
Laser tests
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.