Red tape blocks fatigue inspection system

Red tape designed to protect air travellers could ironically delay the adoption of a technology to prevent air crashes.

The system, which identifies invisible fatigue and damage in the wings and fuselage, is instead likely to be used first by the military.

The acoustic technique, developed by Douglas Adams, assistant professor of mechanical engineering at Purdue University, Indiana, uses vibrations to identifyweakness deep within composite structures.

It is based on the methods used by spiders, which send out propagating waves which bounce off their prey to determine their size.

Now Adams fears that regulations designed to protect passengers may prevent his system from being adopted, despite its benefits.

He said: ‘We have been talking with Airbus and Boeing about the technology, as they have been developing systems that attempt the same thing. But the technology’s first application is likely to be for the military, probably monitoring armour,’ he said.

‘You practically need an act of Congress to put new instruments into a piece of equipment if consumers are affected. Building a commercial application wherepeople’s lives may be affected is very hard. However, the defence industry can use it for unmanned planes, giving it a chance to enter the market.’

Both the commercial aircraft industry and the military are increasingly replacing metal structures with a new generation of composite materials to build strong, lightweight planes that use less fuel.

But while metal dents when damaged, problems with composites are not as easy to see. Relatively small impacts or general wear and tear may cause the affected area to de-laminate as different materials in the fabric split away from each other, weakening the structure. Damage to the material’s core may be masked if the outer layers appear to be fine.

Experts, including several US pilots, have raised concerns about the inspection of composite aircraft structures – a visual inspection is required only once every five years. Concerns about the difficulty of identifying damage to composite components were raised after the November 2001 crash of an American Airlines flight in New York, which killed all 260 on board and five people on the ground. the crash has since been blamed on rudder movements that overloaded the Airbus plane’s composite tail, causing it to snap off.

The acoustic system acts as a structural health-monitoring tool that pinpoints damage undetectable to the eye that may result in a catastrophic failure when the material is placed under stress.

If the system spots any damage a simple cockpit light could warn the pilot if a plane was unfit to fly.

The technology consists of a series of vibrating actuators and sensors placed around the edges of a part. The actuators transmit high frequency soundwaves through the material, which bounce off any defects and scatter back towards the sound source where the sensors pick them up.

Any damage can be identified, along with its size, by analysing how the scatter is distributed.

The Purdue system can be tuned to look for damage in a specific direction and can cancel out interference from vibration sources such as engines.

If a transducer fails during operation, it can adapt to make best use of the remaining sensors.

The system is being considered for use within the fan assembly that will enable Lockheed Martin’s Joint Strike Fighter, currently under development for the US and UK armed forces, to take off and land vertically.

Previous attempts to monitor structural damage have relied on embedding a large number of sensors and actuators throughout the composite, weakening it. As the array is sparsely spread there are fewer parts that might be susceptible to failure, making it easier to maintain.

A spokesman for the Civil Aviation Authority said any new technology would have to provide evidence that it complied with current safety standards. ‘It might take time, but the advantages of the system would have to be weighed against passenger safety,’ he said.