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The TREE (Data-driven modelling of network-wide extension of the Tree of Reactionary delays in ECAC area) project proposes the use of a model to understand the European Air Transportation System response to airlines disruption management strategies linked to delays propagation.

The TREE project belongs to the 2nd WP-E Call for Projects in the SESAR programme.

Consortium Members: Ingeniería de Sistemas para la Defensa de España, S.A. (ISDEFE-Coordinator). Instituto de Física Interdisciplinar y Sistemas Complejos (IFISC – University of Balearic Islands UIB).


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 main cause which must be identified if effective action is to be taken.

TREE will characterize and analyse the propagation of reactionary delays taking into account the influence of the aircraft, crew and passenger links. To this end, the 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.

Problem statement

According 2011 CODA digest, reactionary delay means around 45 % of the total delay in the ECAC area, this situation would get worse for 2030. For this reason, it is considered paramount to have a model suited to predict the occurrence and propagation of reactionary delays that can be used as a benchmark for air transport network users (airlines, airport managers), and capable of implementing new measures or developing innovative strategies to take the best decisions from the overall ATM Network point of view. Previous research has been focused on the local effects of the delays in a hub or in a few important airports. Despite the obvious relevance of hubs, these are typically the airports better suited and equipped to deal with delays. The analysis of the vulnerability of the system must therefore be considered at a global scale. The results on each airport can then be extracted as a posteriori feature but cannot be the cornerstone of the study. The most suitable way to obtain network-wide models is to follow the tree of reactionary delays, This type of research has been so far carried out with a few selected hubs. However, it has never been implemented at a network-wide scale and especially not in Europe. These methods and models will be of great relevance to test and improve Air Traffic Management in areas such as scheduling, slot assignment or response to unexpected events, all of which rank within the objectives of SESAR and in particular of the WPE.

Project objectives and expected results


The aim of the project is to predict the occurrence of the reactionary delays in European Air Transportation network and characterize the propagation of these reactionary delays based on Complex System approaches. The delays are characterized by aircraft links, crew links and passenger links Passenger connections are to be implemented using market sector data, and will require a stochastic approach. Crew rotations will be considered when defining the modelling approach. New measures-strategies will be modelled to help Airspace Users and Network Manager on taking best decisions. Metrics already defined in Complex Networks Theory will be used with aim of quantifying the level of network Congestion.

Expected results

The work planned in TREE is of an interdisciplinary nature and as such can bring benefits for the research in Complex Systems as well as for the entire ATM industry. An outline of the main expected results goes as follows:

  • Cross-fertilization between disciplines with the introduction of a new conceptual framework and methods proceeding from the complexity concepts in ATM.
  • Development of new metrics able to quantify the extension of congestion in the air traffic network.
  • Application of these metrics to the ECAC area, which can help to improve the monitoring of congestion in Europe.
  • Introduction of innovative models to understand and forecast the propagation patterns of reactionary delays. Although the modelling could be generic, the resulting software will be specifically applied to ECAC.
  • Assessment of modelling capability for the forecasting and characterization the network-wide spreading of delays.
  • Analysis of the congestion models in a series of scenarios proposed in consultation with Stakeholders (ATM professionals, airlines, etc.).

Approach and methodology

Modeling approach

A literature review has been performed to search previous works with metrics and models aimed at characterizing delay propagation. New metrics will be defined inspired in Complex Networks Theory to quantify the level of network congestion. These metrics will be applied to the CODA data regarding delays in Europe. As for Air Transportation modelling, data-driven models will be developed with the aim of not only understanding real delay propagation patterns but also forecasting given enough information. Such “information” comprehends schedules and ground operational practices. Most of this can be obtained from CODA data but also interested Airlines will be involved in order to better understand which their typical operational rules are. For such a purpose questionnaires and interviews will be used. A third source of data will be the information on passenger connections, the connections between flights are the main cause behind the delay propagation at a network-wide scale. Once all the information needed is collected, a software model will be developed to simulate reactionary delay propagation in Europe. The modelling approach will be agent-based at the level of aircrafts.


Due to the complex nature of the system being modelled and to the macroscopic, statistical approach adopted by the model, validation is a little more challenging than usual. An ATM system is not apt to be put into a laboratory and measure the different operating conditions of one or several days of operations in a controlled environment. However, the way forward to attain validation in large socio-technical systems is to use part of the information available (data) to generate the model and then test the simulation predictions against the real outcome of the system in a different data set. This method can provide a certain level of confidence on the capacity of models to predict the effects that disruptions or different operation conditions may have on the performance of the system. Still realistic modelling requires real data and so the access to data becomes especially important for this project.

Experimental plan

Regarding the modelling scenarios and exercise plan definition, key stakeholders will be consulted to define plausible scenarios such as an increase in the traffic in the future years or large disruption in the European air traffic network. Such scenarios will be simulated with the model and results will be analysed.

This project has received funding from the SESAR Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 783287.