Nondestructive testing (NDT) of new and aging aircraft structures is essential for flight safety. Inspection costs can be reduced by using an inspection technique with high sensitivity for small flaws. Of the many NDT methods being used in aircraft maintenance, eddy-current testing is well established, especially for layered structures. Nevertheless, some test tasks cannot be assured with conventional eddy current systems with sufficient sensitivity and dynamic range. Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive magnetic field sensors known to date. With the discovery of High Temperature Superconductors (HTS) ten years ago and the subsequent development of HTS SQUIDs requiring only cooling down to liquid nitrogen temperature, the greatest application barrier appears solvable. SQUID systems offer a high sensitivity at low excitation frequencies, permitting the detection of deeper flaws, and a high linearity, allowing quantitative evaluation of magnetic field maps from the investigated structure [1–3]. The potential of eddy current testing with HTS SQUIDs has previously been demonstrated for up to 5 cm deep-lying defects in stacks of aluminum sheets using a stationary axial SQUID gradiometer [4]. Kreutzbruck et al. [5] performed a direct comparison between a SQUID magnetometer system and a conventional eddy current testing unit (Elotest Bl of Rohmann GmbH), with a well defined saw cut in a plate of aircraft aluminum alloy hidden under a stack of flawless aluminum plates. They demonstrated an improvement in signal-to-noise ratio of approximately 150, when comparing the SQUID signature of the slot with the conventional system.</p
