WHEN they were built, no one thought they would fly for so long. But fitted with new engines and avionics, aircraft can be kept going for a very long time. The average age of the world's airliners is more than ten years, with some passenger jets 25 years old or more. Military planes are more geriatric: the Sikorsky Black Hawk helicopter entered service 31 years ago and the Lockheed C-5 Galaxy 40 years back. Both are still going strong. Some Boeing KC-135 aerial-refuelling planes, which are based on the venerable 707, have been flying for over 50 years. Engineers reckon they could still be in the air when they are 80.

One thing that does ground old aircraft is the impending failure of their aluminium structure from metal fatigue. This begins in parts that are subjected to repeated strains, such as where the wings join the fuselage. Constant flexing of the structure concentrates stress, which leads to microscopic cracks. These cracks become more numerous and eventually large enough for the structure to fail. 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 aircraft-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.

Asis will be tried out later this year by America's Defence Advanced Research Projects Agency. A basic set-up to monitor the boom on a small helicopter (a particularly stress-prone part) starts at around $65,000, says Rob McDonald, Ultra Electronics' marketing director. As to the future, he expects the company will explore how Asis might also monitor carbon-composite materials, which are being used increasingly in new aircraft. Less is known about the long-term structural integrity of carbon fibre, but it is not prone to corrosion and is extremely tough, so with a bit of care and attention it might allow aircraft to fly even longer.