An investigation of efficient approximation methods for computationally expensive objective functions in aeronautic multidisciplinary design and multi objective optimisation is presented. Several approximation methods based on curve fitting using polynomials and artificial neural networks are considered. A comparison of these approximation methods in terms of the achieved quality and accuracy and the required computational cost is presented. The approximation models have been successfully applied in a preliminary design and multi objective optimisation study of a blended wing body aircraft.