The paper presents an innovative approach for the functional specification of complex and highly integrated aircraft control systems, such as the Environmental Control System (ECS), by applying model based specification methods. Complexity and effectiveness of modern ECS have significantly increased during the last few years along with development of new technologies and innovations in control engineering as well as digital data distribution and processing. Efficient management of cabin air flows on the one hand makes the ECS more energy-saving and on the other hand more complex with regard to its functionality and interaction with other interfaced aircraft systems. Numerous data interfaces to other systems and a high degree of automation are typical for a modern ECS.The aircraft manufacturer specifies the entire ECS functions and its interactions within the aircraft. Following the conventional approach, a textual functional specification is written containing requirements for automatic control and monitoring for all ECS functions and interactions with other systems. Based on that, subcontracted suppliers develop the ECS hard- and software. Due to the large number and complexity of functions the textual functional specification is difficult to fully validate and may suffer from misinterpretation by the system supplier, inconsistencies and undetected errors before first prototypes are ready to run on a system test bench.This very often leads to iterative, expensive and time consuming soft- and hardware development cycles. It is shown in the paper how ECS functions can be more effectively specified using model based functional specification techniques, which have been derived from software engineering. With these techniques, the textual specification is replaced by an executable specification model. This enables the aircraft manufacturer to consistently specify the ECS in a uniformly structured manner with fit-for-purpose tools and the ability to simulate and to validate its specified functions and interfaces before prototypes are tested on a system test bench. Finally, the paper gives the benefits of this approach for an ECS development from a project management perspective. These are shorter development times and reduced number of errors in system control resulting in reduced development costs.