Improved Resealing Procedures For the Second Deseal/Reseal Program in RAAF F111 Aircraft Fuel Tanks.

摘要

A high temperature-resistant polyester sealant was originally employed to seal faying surface grooves in F111 fuselage fuel tanks as well as structural voids and aerodynamic surfaces. Chemical hydrolysis of this material was detected in March 1974 and led to its replacement by a polysulfide sealant in the late 70's and early 80's. Where possible, the polyester was removed, however much of it remained inaccessibly located between the overlapping panels. It was found that the continued hydrolysis of this residual polyester generated sufficient pressure to penetrate the polysulfide sealant resulting in fuel leaks as well as appearing externally in trails along the skin of the aircraft. To protect the polysulfide from penetration, a mechanical barrier was applied in the form of a layer of epoxy polyamide between the polysulfide and the polyester. This system was successful for some years, however the occurrence of fuel leaks had become unacceptable by 1987. All F111 aircraft suffered numerous fuel leaks over the period 1988-91 including a number of safety-of-flight (SOF) incidents. A detailed investigation was therefore requested to identify the cause(s) of sealant failures. This investigation found that the sealant and the underlying epoxy barrier coating could be manually peeled from the painted tank surface when prepared by the recommended methods and that two procedures were responsible for the poor adhesion. One of the procedures involved application of the barrier coating over incomplete cure priming paint resulting in solvent attack of the barrier. The second procedure, involving the use of a so-called titanate adhesion promoter , significantly reduced peel strength of the barrier coating to the paint.