This paper describes an investigation into the application of the boundary element method (BEM) for analyses of aircraft structures. A new BEM using a multi-region formulation of plate bending and plane stress integral equations is developed. The method is used to study the behaviour of X-core and other similar sections subjected to out-of-plane loadings. Also investigated are behaviour of crack growth, which may be present on the skin. Comparison was made with finite element solutions for uncracked structures to show the accuracy and efficiency of proposed method.