The term ‘Ageing aircraft’ indicates an aircraft structure that is about to reach its original design goal. At this stage, the light alloy structures used in commercial aircraft are susceptible to Widespread Fatigue Damage (WFD) and possibly other deteriorating effects, such as corrosion damage. One typical form of WFD is Multiple Site Damage (MSD), which refers to the simultaneous existence of multiple interacting fatigue cracks at different sites of the same structural component (Fig.1). In the presence of MSD, critical crack sizes are greatly reduced, thus decreasing the residual strength of the structure below critical levels [
] and leading to catastrophic failures due to the sudden cohesion of such interacting cracks [e.g.