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Second layer cracking of the C-14 lower inner wing spanwise slice has been identified by the C-141 system program office as the life-limiting feature of the C-141 aircraft. The current inspection method is bolt hole eddy current (BHEC) that detects second layer fatigue cracks that are 0.070 inch or longer. ALthough the BHEC has the detection capability, this method requires the removal of 3,700 fasteners in the lower wing splice joints. In addition, the BHEC inspection effort requires 9,000 man-hours per aircraft and is performed every five years under the programmed depot maintenance. Ultrasonic inspection of two-layer structures can be accomplished if sealant is located between the faying surfaces to conduct the ultrasonic signal from one layer to the next. Such an ultrasonic inspection has been performed and validated under a FAA-sponsored, technology-transfer task on DC-9 wing-box tee caps. On C-141 aircraft structures, recent proof-of-concept studies have shown that an ultrasonic inspection process has the potential to reliably detect 0.050 inch or smaller second layer cracks in simulated spanwise joint test specimens. Automation of the inspection process improves the reliability of the inspection and accomplishes the aircraft inspection in 200 man-hours per aircraft. Implementation of such an ultrasonic method results in a 45:1 reduction in the manpower as compared to the BHEC inspection and returns the aircraft to service much sooner. This paper describes a methodology for developing a prototype inspection process. Involved in that process is the design, fabrication, and testing of a prototype scanner for multi-channel, real-time data collection and display of A-scan, B-scan, C-scan ultrasonic data formats. Functional testing of the inspection process is described on fatigue crack test articles in the laboratory. Typical ultrasonic image results and a description of a probability-of-detection study will be presented.
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