Design Improvement of the Longerons of a Trainer Aircraft towards better Corrosion Resistance

The upper and lower Longerons in the Pilatus PC-9(M) principle fuselage structure are made from AA2024-T3511 extruded shapes. This alloy product form is widely used in the aircraft industry, though it has shown poor resistance to exfoliation corrosion in the recent past. The ultimate ambition of this paper is to improve the Longeron corrosion resistance by design. Therefore, general exfoliation corrosion issues and recent advances in aluminium alloys and metallic fuselage design are reviewed. A good design approach is found to be a material supersedure through AA2124-T851 plate in conjunction with integrally machining. To reduce production costs, both upper Longerons are separated into a machined forward and a machined aft section which are connected by a bolted lap-joint. This joint is a major design change and thus invalidates the aircraft fatigue test. In this paper, we perform a joint fatigue analysis using FEM in combination with a strain-life method to predict the minimal crack initiation life and thus allow for FAR-23 certification without structural testing.