Aircraft engineers know a lot about how these cracks progress and keep an eye on them in routine overhauls. Nevertheless, they can be difficult and costly to find. Apart from careful visual inspection, techniques like X-rays and ultrasonic probes are also used.
Now a British company has come up with a low-cost way of monitoring cracks in aircraft, even while they are airborne. The trick that Ultra Electronics uses is to listen for them with a system called Asis. It does this by fitting small piezoelectric acoustic sensors to parts of the structure to detect the particular frequency of noise caused by a crack in aircaft-grade aluminium. When first set up, Asis is calibrated to the acoustic signature of the aircraft.
The system can point engineers to where cracks are occurring because the sensors also record the precise moment it is “heard”. As the sound ripples through the structure it arrives at different sensors at different times, which can be used to work out the location and severity of the crack. Once on the ground, a touch-screen device a bit like an iPad is plugged into the system and shows where any cracks are on a three-dimensional image of the aircraft.
(Source: The Economist, August 2010)