Air industry drags its wings on fatigue

Aircraft manufacturers must do more to develop sensors that will pick up wear and tear in composite structures, writes Fiona Harvey.

It takes so little for people to lose confidence in air travel. Airlines know that more than anything their reputation rests on safety. Though everyday activities such as driving are much more dangerous, our perception of risk is such that most people tend to view flying as a more alarming prospect, and their fears are easily stoked by news of crashes.

These considerations put engineering firmly at the heart of the air industry. And for aeronautical engineers one of the core issues is how to protect aircraft materials against the problem of fatigue.

Fatigue makes a significant difference to the economics of the industry, as well as the safety record. Aircraft are expensive at the best of times, and ensuring that they last a decent time is a science in itself. But the techniques used for testing for fatigue have failed to keep up with the growing sophistication of the materials now being used in aircraft, and this mismatch has become a source of tension within the industry.

As the expansion of cheap travel continues apace, cost becomes an increasingly sensitive issue for airlines. Drawn into ever fiercer competition with one another by the rise of no-frills carriers like Ryanair and Easyjet, airlines need to cut wherever they can.

One way to examine an aircraft for fatigue remains old-fashioned visual inspection. However, as manufacturers turn increasingly to laminate materials, problems with the structures can occur between the outer and inner skin, and remain invisible to the naked eye. Wear and tear, or small impacts, can cause areas to delaminate, and different materials in the fabric can split away from each other, weakening the structure while the external layers appear unaffected.

Carbon fibre is being used to an ever greater extent in aircraft bodies. It will continue to be the material of choice given its advantages in lightness and strength over conventional materials, and the ongoing improvements in creating such composites.Testing them for wear and fatigue is time consuming, requiring aircraft to be grounded for long periods while parts are removed for ultrasound examination.

While the process certainly seems effective, it puts an inevitable strain on the finances of air companies. At present this is minimised, as inspections of composite aircraft structures are demanded only every five years. But many experts believe such infrequent inspections, which cut down on cost, can seriously affect safety, as damage to composites has been implicated in plane crashes. The tension between cost and safety looks to have resulted once more in an unsatisfactory compromise.

Are there alternatives to this process? Yes. A system of small actuators and sensors that could test the parts continuously and feed back information to a central system would allow testing to take place without lengthy ground visits. The feedback from these sensors could be periodically confirmed by ultrasound and manual checks, for extra safety. Yet systems like this, requiring embedded sensors, ironically can weaken the composites themselves.

A different system, which identifies invisible fatigue and damage to aircraft wings and fuselage, has been invented by Prof. Douglas Adams of Purdue University in the US. It uses techniques based on those of spiders, which send out waves to bounce off their prey and determine its size. Vibrating actuators and sensors around the edges of a part transmit high-frequency sound waves through the material, to be analysed by software.

So why is red tape holding up development of this safer and more effective method of testing aircraft materials for fatigue? Changes to aircraft to incorporate technology of this kind require government approval, and manufacturers must prove that a system of this kind can work effectively and will ultimately improve safety. This process is needlessly convoluted, however, and ought to be streamlined. New technologies come on track at a fast pace, but officialdom has failed to keep up, and its bureaucratic processes frustrate progress. As a result this technology will be used in unmanned military aircraft first.

This problem cannot be laid solely at the door of government. Industry must also help itself, both by lobbying government and showing greater enthusiasm to solve the problem. There is so much pressure on costs that any investment comes under close scrutiny, and installing such a system of sensors on all aircraft containing composite materials could be a lengthy and difficult process to get right. But only by taking a long-term view can aircraft manufacturers hope to solve this problem and ultimately save money. Fatigue is a problem that is not going away.