The European air traffic management (ATM) systems handles approximately 26,000 flights daily. Forecasts indicate that the European air traffic levels going to be double by 2020. In addition, European ATM costs an additional 2-3 billion every year, compared to other similar systems in the world.1 Taking into account this scenario, European ATM systems have to look for solutions for accommodates the increasing air traffic flows in the future airspaces, whilst cutting costs and improving its performance. One of these solutions came with the initiative of organizing airspace into functional blocks, according to traffic flows rather than to national borders. Such a project was not possible without common rules and procedures at European level.1 The establishment of Functional Airspace blocks (FAB) shall enable optimum use of airspace taking into account air traffic flows. In the same way, FABs initiatives contain a new philosophy for design the airspace, the new airspace structures include application of Flexible Use of Airspace (FUA) concept, Free Route Airspace (FRA) blocks, night direct routes implementation, etc. Additionally, FAB blocks look for cooperation or fusion between Air Navigation Services providers (ANPs), all this with the goal of reduce air navigation services costs for users. Particularly, Free Route Airspace concept offers enormous benefits to airspace users, because permits to select the user-preferred trajectories with the optimal flight profile, enabling flights to operate outside a pre-defined route structure, resulting in shortest routes and big distances saving for airlines. Furthermore, free route blocks will significantly impact in ATC systems, changing the way for establish aircraft separation in these new airspace structures. For all these reasons, this thesis will study the functionality of the future Southwest Functional Airspace Block (SW FAB), specially the implementation of the Free Route Airspace concept, focused in the long term phase (2020) that includes Santiago-Asturias, Lisbon, Santa Maria Oceanic and Canarias airspaces in one free route airspace block.2 Objectives 1.-Evaluate the Free Route Airspace block in the future Southwest Functional Airspace Block in terms of flight efficiency (fuel,CO2,NM), ATC workload****. 2.-Study the interaction between Free Route Airspace and conventional airspace divided in sectors (flying with RNAV). 3.-Explore the future systems and procedures for traffic separation in Free route airspace. Methodology As was explained before, this thesis pretend to make an analysis of the free route airspace in the future SW FAB, for achieves this goal it will be necessary, firstly to explain the main concepts and background of: Flexible Use of Airspace (FUA), Functional Airspace Block (FAB) and Free Route Airspace (FRA), and others concepts. In the same way, it will be necessary to review the phases of the future SW (Spain-Portugal) FAB main plan; all concepts will help to settle the base for the future analysis in the thesis. Secondly, this thesis will propose and simulate the future Free route airspace in the SW FAB. The simulation pretends to evaluate the advantages and possible problems of this new way of flight. Thirdly, this project will include some interviews from ATC professionals; these interviews pretend to see how will impact the FRA (Free route airspace) in the future, additionally those interviews can provide different points of view and needs in the ATM (Air traffic management) system. Finally this master thesis will include an exhaustive analysis of results (concepts, simulation, interviews, etc.) and a presentation of conclusions