Development of a novel test rig for the evaluation of aircraft fuel tank sealants

Leaks from aircraft fuel tanks have always represented a problem for aircraft manufacturers, airline operators and maintenance crews. The integral fuel tanks within aircraft structures are typically located within the wings and they rely upon sealant materials to prevent leakage past joints and fasteners. However, the wing is designed as a structural member first and as a fuel tank second and there exist many potential leak paths for the fuel from these complex, highly loaded structures. Fuel leaks result in direct loss of fuel which may be dangerous, cause a loss in revenue due to aircraft being withdrawn from service and be difficult and expensive to repair. On top of this there are important health and safety issues involved in the repair of fuel tanks, for example, the Royal Australian Air Force's, F-lll Deseal Reseal Programme 1979 to 2000, where it was found that a significant number of RAAF personnel involved in the Deseal Reseal Programme were suffering from a variety of health problems. Current approaches to fuel tank sealant evaluation embrace immersion in a range of different fluids at different temperatures, of both bulk sealant samples and sealed joints. However, nearly all such tests are of a "static" nature and yet it is acknowledged that joint movement leads to leaks. Thus the missing component of testing is movement coupled with the other key variables. The aircraft industry has been searching for a relatively simple test method that can be used to evaluate sealed joint systems using realistic combinations of materials, joint geometries, imposed stresses and environmental conditions. The aim of this project was to do exactly this. A practical but realistic dynamic test, the Model Sealed System (MSS), was designed, made and evaluated. This unique mechanism consists of an axial stress machine into which fatigue, high and low temperatures and pressures can be programmed for automatic operation. A novel circular lap joint lies at the heart of the MSS in which test sealant is sandwiched between the circular coupons that are then assembled with aerospace fasteners and sealed. This joint configuration is representative of a wing skin butt-strap joint in a real aircraft. The MSS is easy to run, it accurately simulates real world dynamics and conditioning, and it provides results to qualify sealants in a more realistic manner than current testing methods provide. The MSS enables evaluation and comparative testing of sealant systems when used for interfay, fillet and overcoat applications. The information provided is complementary to that obtained from conventional small scale coupon testing; it is not seen as a substitute. Further work is required to refine the test variables and further data are required to provide confidence in the utility of the MSS. Development of the MSS was undertaken with the support of Airbus UK to ensure that the design, materials and all other variables met with the overall requirements of a commercial aircraft manufacturer. Airbus UK have a duplicate MSS of their own, installed by the author, from which they can obtain patterns of data for different combinations of materials and experimental variables