Aircraft wheel testing with machine-cooled HTS SQUID gradiometer system

For eddy current detection of deep-lying flaws in large aircraft wheels, an automated airplane wheel inspection system using a HTS SQUID gradiometer sensor is being developed. Wheel drums made of aluminum alloys have to be tested frequently since they are subject to enormous dynamic loads and very high braking temperatures at landing. For economic reasons, testing should be performed from the outside without removing the inner ferromagnetic keys which fit the brake system. In order to operate the sensor in hostile environments such as airport maintenance hangars, a planar rf double hole SQUID gradiometer was used. SQUID cooling is performed by a closed cycle Joule-Thomson cryocooler, equipped with flexible plastic gas lines. The wheel testing is being performed on an automated test stand with the wheel slowly rotating and a robot with the SQUID enclosure scanning stepwise along the wheel axis. Additional signals due to inner cracks of 10 mm length, penetrating 25 percent of the 10 mm thick wall, are easily identifiable in the periodic signal background due to the presence of ferromagnetic keys. In comparative measurements, the prototype SQUID system clearly exhibited advantages over conventional techniques, with optimization reserve still at hand.