Validation of F-16 Wing Attachment Fitting bolts

During a modification programme, the Wing Attachment Fitting (WAF) cadmium-plated steel bolt assemblies from 33 F-16 aircraft were cleaned in a non-approved chemical solution, a paint stripper. This removed much of the cadmium plating, leading to in-service superficial rusting of some bolt ends and nuts. The OEM and chemical manufacturer became concerned about possible hydrogen embrittlement of the bolts owing to exposure to the paint stripper. The OEM recommended tensile and stress durability embrittlement testing of some of the bolts before continuing to fly with the rest. However, in the meantime all 33 aircraft were temporarily grounded. The Royal Netherlands Air Force (RNLAF) sent the bolts from five aircraft to the NLR, and 50 bolts were selected for tensile testing only. The results, statistical analysis, fractography, and a discussion of electrochemical and hydrogen diffusivity considerations and the “no failure in 200 h” criterion, led to the conclusion that the bolt failure strengths were unaffected by the paint stripper. The aircraft were cleared for further operation, whereby all the remaining WAF bolt assemblies were cleaned, protected with primer, and replaced at the latest during the next scheduled inspection.

► 33 F-16 aircraft grounded.
► Wing Attachment Fitting (WAF) bolts suspect after cleaning.
► Tensile testing, statistical analysis and electrochemical considerations validated the bolts.
► Aircraft cleared for further operation 1 week after starting the validation.