In new aircraft programs, systems’ functionality is increasingly becoming integrated into modular avionics. Controllers may not be delivered by the systems supplier so this trend creates a new interface between systems and controllers. A functional software specification is therefore needed to facilitate the building of the software by the controller supplier. In the case of an ECS system controller, the hardware was obtained from different suppliers and a software functional specification was needed for the controller supplier. To be able to design and verify the system functionality, an integrated ECS simulation model was created which coupled the thermodynamics of the aircraft and ECS system to the controller actions. The model also included functionality to simulate sensor noise and component failures. The thermodynamic model was created in Matlab/Simulink and consisted of a combination of direct programming as well as data on a Flowmaster model for the bleed system. The vapor cycle cooling system model was simulated within Matlab/Simulink using the Thermosys package. The controller side was modelled using compiled C++ controllers, thereby allowing rapid updates to functionality and configuration control. Component controllers were tested on a bleed air test rig where stable component control was shown. The integrated model allowed the integration of different level controllers in order to achieve the required functionality. Several flight profiles were created in order to be able to simulate both normal and abnormal conditions. After several iterations of the system functionality, a stable system response was obtained and the functionality was correctly specified. Some interesting observations were made regarding component behavior which were quickly addressed with the hardware suppliers thereby allowing further functionality optimization. Generally, the tool facilitated rapid development of the ECS control functionality.