Airport operations safety assessment with the use of colored Petri nets

Numerous operations connected with beginning and ending of air transportation process are executed at the airport. Each of these may be important for the safety of air traffic. However, due to the severity, the most important are taking off and landing operations carried out on runways. In many airports, there exists a system of two intersecting runways. In this case, usually one of the runways is used for takeoffs while the other one for landings. This allows increasing the capacity of the airport. Existing procedures are designed to ensure the safety of this process. However, the presence of traffic disturbances (for instance weather-related) and mistakes made by air traffic controllers and pilots could lead to a collision at the intersection of runways. The aim of this study is to estimate the probability of a collision at the intersection of runways with regard to disturbances in the procedures carried out. For this purpose there has been created a model of air traffic on intersecting runways, which are used alternately for takeoffs and landings. This model was developed as a hierarchical colored Petri net. Defined and included in the model are several groups of potential disturbances as well as the dynamics of the movement of aircraft in landing and take-off phases. Using this model, numerous simulation experiments were conducted thank to which the safety of traffic on intersecting runways has been assessed. The developed software tool allowed for determining the effectiveness of protective measures, such as, for instance, the use of additional technical means that allow for detecting the aircraft passing the runways intersection. The model and the tool has been implemented for Warsaw Chopin Airport, having the discussed structure of runways. The configuration of two intersecting runways, however, has a serious drawback. It is the existence of common point of different air traffic streams. In the case of a human error this can lead to simultaneous execution of the takeoff and landing operations and as a result—a collision with catastrophic consequences. It is worth noting that the biggest disaster in the history of civil aviation, which took place on the island of Tenerife in 1977 happened at the airport, which has just such a structure. And that fact was not without a significance for the accident causes (Netherlands Aviation Safety Board 1978). The error referred to may be, for instance, the Air Traffic Controller (ATC) clearance which will permit simultaneous takeoff and landing. It can also be committed by the aircraft crew, which can misread the permission obtained from the controller and as a result perform the procedure for which in fact did not have a permission. The facts discussed here provoke the question that is the research subject of this work. To what extent the applicable procedures and technical safety barriers make the airport resistant to aircraft collision in the case, when there are two intersecting runways in simultaneous use? This question is particularly relevant against potential human errors, adverse weather conditions and traffic disturbances.