A new technique that listens for cracks in ageing aircraft

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. …