A post-design robustness assessment for the longitudinal flight control system of an oceanographic unmanned aerial vehicle (UAV) is presented in this paper. Two novel systematic approaches of generating the linear fractional transformation ( LFT ) model directly from nonlinear equations are proposed for this particular robustness analysis problem. The closed-loop system combined with each controller is used to determine combinations of aerodynamic parameters that result in worstcase performance. Classical simultaneous gain and phase margin stability metrics currently used in the aerospace industry are introduced for the certification of robustness of this uncertain multivariable system. The results show that the control system remains stable and achieves desired performance for all possible parameter variations over a specified range in both frequency domain and time domain.