The topic of the thesis is framed within the activities of a project whose purpose is to convert a civil platform into a multi-role aircraft capable of long range maritime patrol mission, as well as anti-submarine and anti-surface warfare missions. Design and modification activities will require to evaluate (starting from an aerodynamic and engines mathematical model) the performances of both the unmodified and modified aircraft and to confront them with the end-user requirements. Within this context the main objective is to execute a performance analysis of both the unmodified and modified aircraft. Here are the milestones of the work: •??Mathematical model ??Run of simulations for the unmodified aircraft ??Run of simulations for the modified aircraft •??Evaluation of the performances of the unmodified aircraft •??Evaluation of the performances of the modified aircraft •??Evaluation of the obtained results for the modified aircraft against end-user requirements.