This paper presents the activities foreseen on the Leonardo Aircraft Division EIS (Entry In Service) 2020 derivative aircraft performed in the frame of the FP7 European research project TOICA (Thermal Overall Integrated Concept of Aircraft). On board air systems for conventional aircraft are fed by the bleed off-take which penalizes the amount of power available to the turbine of jet or turboprop engines. In order to minimize such operating penalties and optimize the energy efficiency of the overall aircraft, it is of major interest to support trade-offs at aircraft level including aircraft systems as early as possible in the development cycle. The study presents the Virtual Integrated Aircraft methodology and associated simulation activities relying on the system simulation platform LMS Imagine.Lab. This methodology is also relying on concept of flexible model and pyramid of models developed in the context of TOICA. Several aircraft configurations and ECS (Environmental Control System) packs architectures are studied and the different steps of the methodology are shown up to the trade between different aircraft configurations. From the very beginning of the design cycle, the requirements and the objectives for the air conditioning system are complemented with thermal requirements by performing thermal evaluations for the design points covering the aircraft operating conditions. Pre-sizing of every ECS pack component is achieved. Then, the resulting ECS packs numerical models are integrated with the engine bleed air system and the thermal model of the structure including the different heat loads, passengers and electronic equipment mainly. The integration is finally assessed over complete flight missions allowing to validate not only the steady state ECS performance, but also the transient pull up and pull down conditions as well as the overall engine bleed off-take and energy balance.