The C-141 System Program Directorate (SPD) at Warner Robins Air Logistics Center (WR-ALC) has identified second-layer cracking of C-141 lower inner-wing spanwise splice-joints as the life limiting feature of the C-141 aircraft. Presently, splice-joint nondestructive inspection (NDI) is accomplished with an eddy current surface scan, repeated at field-level every 120 days. The surface inspection detects only surface breaking cracks, which dictates the short inspection interval. An existing bolt hole eddy current (BHEC) inspection technique is capable of detecting smaller second-layer splice-joint cracks, although the expense and aircraft downtime associated with the BHEC technique are unacceptable to the C-141 users. Including fastener removal and reinstallation operations, BHEC inspection of the C-141 splice-joints is estimated to require 9,000 man-hours per aircraft. The C-141 SPD requested the WR-ALC Materials Analysis Team (TIEDM) to develop an alternate, improved inspection method for the splice-joint. The C-141 SPD goal is to conduct a one-time, field-level fleet inspection and subsequently establish a five- year, depot-level inspection interval. TIEDM determined the best alternative inspection method available, with potential to meet the specific C-141 SPD requirements, was an automated ultrasonic scanning technique.
A prototype inspection system was designed and manufactured by SAIC and is currently undergoing acceptance testing. Laboratory and on-aircraft testing to date have proven the prototype system has a detection threshold potential of (0.050 inch crack lengths in the C-141 splice-joint second layer. A Probability-of-Detection (POD) Study has been designed to formally quantify the inspection reliability of the prototype process. POD data will allow the C-141 SPD to confidently determine future inspection intervals and requirements. The new automated ultrasonic scanning process has the potential to significantly reduce splice-joint inspection labor costs as compared to the costs of BHEC inspection.
This paper describes the inspection development process, including feasibility studies, design and fabrication of prototype, functional testing, POD testing, and Air Force implementation. Typical ultrasonic C-scan images will be presented, and the POD test plan will be discussed.