The process of design aircraft systems is becoming more and more complex, due to an increasing amount of requirements. Moreover, the knowledge on how to solve these complex design problems becomes less readily available, because of a decrease in availability of intellectual resources and reduced knowledge transfer opportunities. Aerospace companies need to capitalise on the knowledge available within their companies, in order to deal with the challenges of increasing complexity and competition. The research presented in this thesis contributes to tackling the above challenges. A knowledge based method for solving complex detailed design problems is presented. The process of setting-up a solution finding approach is discussed by means of a design problem in the detailed design of fibre metal laminate (FML) fuselage panels. The principles of knowledge based engineering (KBE) are used to setup a software application for finding solutions to the detailed design problem in FML. A method for solving complexity in design problems is presented. The method uses a structured approach of transforming the real world problem, via an expert view on the problem, to a mathematical model of the problem. Finally a solution finding strategy is tailored for the problem at hand, by combining available solution finding knowledge with expert problem solving knowledge from different knowledge domains.