This paper presents an Optimal Control Problem (OCP) tailored to assess the system performance of a rigid wing Airborne Wind Energy (AWE) pumping system prior to real flight tests. The proposed method is validated against experimental data obtained using a small prototype designed by the Dutch company Ampyx Power B.V. Within such analysis, it is shown that a non-optimized closed-loop flight control system can generate power to within 10% of the open-loop optimal flight trajectories. Additionally, results demonstrate that the system efficiency does not depend on the trajectory topology, though a significant change in terms of trajectory shape is observed for different wind speeds as a result of trajectory constraints. (C) 2019 Elsevier Ltd. All rights reserved.