Bonded repairs for C-5A fuselage crown cracking

This chapter describes the application of Glare repairs to a USAF C-5A transport. One of the problems in the ageing C-5A fleet is the multiple small cracks in the upper aft-crown section of the fuselage skin. These cracks are possibly caused due to the usage of stress-corrosion sensitive 7079-T6 aluminium. The crown section experiences significant longitudinal tensile bending in addition to biaxial tension due to internal pressurisation. Multiple short cracks (25 to 50 mm long) were found, see Figure 31.1, in the crown of several aircraft. In the autumn of 1995 two bonded Glare repairs were applied at fuselage stations 1700 and 1784 by a team of Wright Laboratory, San Antonio-Air Logistics Center, and United States Air Force Academy/Center for Structural Life Extension (CAStLE) personnel [1].The cracks are believed to have nucleated in the rivet holes of the 7079-T6 aluminium skins due to high fit-up stresses induced in manufacturing and the bending moment caused by the T-tail. Figure 31.1Crack Iocations in crown section of C-5A.