The DLR Institute of Flight Guidance is one of the leading establishments in the field of air traffic management research worldwide. Research is conducted using different simulation models. The fast time simulation tools (e.g. Simmod, AirTOp, etc.) offer the possibility to examine various aspects of the air traffic in short evaluation cycles. Especially long lasting, complex traffic flows can be well analysed with the help of these tools. In contrast to the fast time simulation tools, the real time simulation facilities operated by the Institute allow the so-called human-in-the-loop research. For instance, the Apron- and Tower Simulator (ATS) emulates the air traffic controller’s working environment at the airport. Within this environment, the impact of new concepts on the controllers can be examined. The influence of different traffic scenarios, additional assistant systems, work organization and the design of workplaces on controllers’ task- and workload can be researched in this way. Inside the “Deutsches Zentrum fur Luft- und Raumfahrt” (DLR) project Remote Airport Traffic Control Center (RAiCe), an approach was developed to combine both types of the described simulation models. Therefore, the fast time simulation was enhanced with elements representing typical tasks of a controller (e.g. separation keeping or radio communication). By this approach, it was possible to preselect appropriate traffic scenarios and to determine special traffic events. At the ATS, air traffic controllers were confronted with the selected scenarios and events. Based on their performance, conclusions concerning different remote tower operation concepts were made. Within this paper, the cooperative use of fast and real time simulation is reviewed. A five step cooperative strategy is suggested to allow for efficient scenario design that enables the validation expert to design scenarios with specific traffic situations using a fast time simulation tool. Furthermore, harmonizing fast and real time simulation offers the possibility to use data of the fast time simulation as scenarios for real time simulations, and real time simulations can be used for the validation of fast time simulation models. Data of the simulations within RAiCe are introduced. The application of the cooperative use of fast and real time simulation for validation of future ATM concepts is indicated.