The primary aim of this study is to understand and implement in a computer program the basic theory underlying model order reduction of Finite Element (FE) structural models. More concretely, attention is focused on modal analysis via substructuring techniques, and the interest lies in comparing the performance of such substructuring methods with the standard modal analysis of linear elastodynamic FE models. The method is applied to 3 aeronautical structures, ranging in complexity from a simple truss structure to a complete airframe of a private jet aircraft. For the truss structure, all required operations are carried out with a Matlab code developed by the author. For the other two structures, the finite element information ( mass and stiffness matrices ) is retrieved from an open source FE software called KRATOS, and then processed by a Matlab script in order to determine the vibrational modes and the natural frequencies. It should be highlighted that the construction of the geometric models of the three tested airframes have been also developed, from scratch, by the author.