This paper presents a novel methodology to analyze the takeoff phase of High Altitude Long Endurance (HALE) aircraft. Their lightweight structures and the resulting vulnerability to atmospheric disturbances makes operations around the ground, such as takeoff, specifically critical flight conditions. The thereby resulting necessity of a comprehensive verification and validation process is, however, constrained by significant amount of uncertainty in the underlying models due to the complex high-order dynamics originating from the highly flexible structure. This usually results in controller and takeoff safety assessments that require computationally intensive, simulation-based analyses on the full nonlinear aircraft model. To overcome this issue a lean finite-horizon robustness analysis based on the integral quadratic constraint framework is proposed to analyze HALE takeoffs. The study provides analytical upper bounds on the worst-case takeoff performance of the nonlinear model under wind disturbance and model uncertainty. The approach is demonstrated on the German Aerospace Center’s HALE platform.