Double pass retroreflection for corrosion detection in aircraft structures

An optical double-pass retroreflection surface inspection technique (D Sight) used for visualizing surface distortions, depressions, or protrusions has been adapted as a rapid, enhanced visual inspection technique inspection of large external aircraft surfaces. A project to fully characterize the double-pass technique indications of corrosion damage in lap splices is currently active. Over 200 large transport aircraft fuselage lap splice specimens have been collected. The double-pass technique has been developed and tested both in the laboratory and in the field. In laboratory tests lap splices retrieved from retired aircraft and subjected to accelerated corrosion and lap splices naturally corroded in-service were inspected with DAIS, eddy current, X-ray, and shadow moir\ue9 and subjected to tear down. It has been shown that the double-pass technique is capable of locating corrosion pillowing indicative of a thickness loss as low as 2 percent. The first field trial of the double-pass technique was based on two service bulletins requiring inspection of longitudinal and circumferential lap splices on the 737-200 aircraft from BS 259.5 to BS 1016. The DAIS 250C inspection, including analysis and report, took 36 work hours. The recommended technique in the service bulletin was close visual inspection and the time required according to the service bulletins was 278 work hours. Computer modelling of the bulging of fuselage skins due to corrosion and double-pass technique image simulation is also being conducted as a tool for inspection interpretation.NRC publication: Ye