Improvement of the launch phase of a jet powered Unmanned Aerial Vehicle (UAV) with Jet Assisted Take Off (JATO), has been the subject of attention in the UAV industry. Use of flight simulation tools reduces the risk and required some amount of flight testing for complex aerospace systems. Full nonlinear equations of motion are used to study and simulate this maneuver and three case studies of their application to UAV launch phase problems are presented. Attempt was made to explain some aspects that were not definite in the test by simulation. The result of the examination was satisfactory. The second and third examples involve the flight test of the UAV. These two applications are typical of launch phase problems. The second example demonstrated a good applicability of this technique to improve and increase the stability of the UAV during launch. In the third example, the UAV in the presence of headwind showed that simulation and real test had a good coincidence.