This research to the best of our knowledge is the first paper to quantify airline network robustness in the presence reversible capacity of legs and alternative flights. In this study, we try to recognize the critical legs via changing the functional capacity offlights. Besides, we attempt to gauge the behavior of the flight network via shifting of leg capacities proposing a new leg cost function. In addition, we indicate how to capture the robustness of airline network in the case of variable flight capacities. Relative Total Cost Indices have been used to assess air network robustness in the case of behavior associated with both User-Optimization and System-Optimization. In this article from the different point of view, the variability of passenger's route preferences is the main subject. This paper may shed light on the robustness of networks in real life not only for the particular case of airlines but also for systems sharing similar topological properties. The paper presents a numerical case study with real data from an airline in Turkey for illustration purposes.