The future of air transportation modeling and simulation represents a grand challenge for both the air transportation and the modeling and simulation communities. In addition to its size and scope, air transportation has many technical, operational, organizational, policy, security and legal aspects contributing to its complexity, yet must ensure an un-rivaled level of safety throughout any transition to next generation systems. Thus, modeling and simulation must address not only technical challenges in representing a system of this complexity, but also institutional realities that prevent full data and model sharing between the many entities involved in air transportation. We outline the attributes of a shared repository of data, models and computational tools required to meet this challenge, including the coordination and administrative processes required to manage it, and provide an example of its potential application.