The primary means to detect corrosion in fuselage multi-layered structures, such as lap splices, is the visual inspection. This method is used because the metal lost to corrosion product forces the plates apart causing an air gap and an outer surface distortion between rivets (i.e., pillowing). The visual inspection does not provide a quantitative assessment of corrosion and is not a foolproof indication that the deformation is due to corrosion. Plate separation can exist as a result of poor quality control during manufacture or previous repairs. Detecting and characterizing corrosion in aging aircraft is currently one of the major problems in aircraft inspection, and according to the Federal Aviation Administration's (FAA) National Aging Aircraft Research Program, improvements must be made to existing inspection techniques and devices for more reliable corrosion detection capabilities.
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