Looking at different delay types, the so-called “reactionary delays” are identified as one of the largest contributor. These knock-on or ‘propagated delays’ do not seem to have a specific origin or cause. In fact, it is the duration of the delay what is transferred from a previous flight to the next one by the same or a different aircraft. Generally reactionary delays result from primary delays, but they have to be handled differently and are not to be seen as an individual delay cause. Reactionary delays have a great impact on air traffic performance. However, the research effort to understand and handle this kind of delays is in practice limited. While being critically important due to its contribution to the total cost of delay, it is the primary cause which must be identified if effective action is to be taken. The present project proposes to develop new concepts and methods to be implemented in software tools with the aim of describing in a realistic way the propagation of delays throughout European Air Transportation Network. Moreover, the model to be developed will permit an evaluation of the daily planning performance and the analysis of the impact of perturbations in the network. Additionally new metrics will be defined inspired in Complex Networks Theory to quantify the level of Air Transport Network congestion. These metrics of performance to be introduced will have different level of resolution, from local or airport based to regional or network-wide. Thus the model will allow to evaluate a day planning performance as well as to assess the impact of perturbations in the network. The model structure and its functionalities will also make possible the exploration of the limits of the present theories in Complex Networks regarding Air Transportation system stability and control of networked dynamic systems. The innovative methods and the odel to be developed will be of great relevance to test and improve Air Traffic Management in areas such as flight scheduling, slot assignment or response to nexpected events, all of which rank within the objectives of the SJU WPE theme “Mastering Complex Systems Safely”. The contact with scientific community, main stakeholders and industry is planned for almost the overall project lifecycle. Some of these contacts have been already carried out in an exploratory mode and the project team counts with a Letter of Support of a relevant Stakeholder.