## 1 Introduction
In [19], a model is developed to predict trajectories under adverse weather conditions. Advances like this would allow ATC to predict air traffic structure, and therefore to better understand air traffic complexity. Another area related to complexity is the systems used for Communication, Navigation, and Surveillance (CNS). As these systems advance technologically (i.e., the introduction of Automatic Dependent Surveillance-Broadcast (ADS-B) systems), a higher degree of security is achieved by facilitating the labour of ATCOs [20]. As the performance of CNS systems increases, the complexity of the airspace will be reduced. In addition to air traffic complexity being a topic of interest, the fundamental objective of Air Traffic Management (ATM) is to reduce complexity. To this end, some of the measures that the ATM are considering are as follows:  
* Advances in CNS systems. This allows for the optimization of air traffic structure. Several studies, such as [21], develop systems that try to optimise traffic through CNS systems. This would result in reduced complexity as air traffic is more structured and easier to control. * Development of technical and/or procedural enablers for ATCOs to control airspace more easily and with technological support tools. * Development of complexity indicators. Being able to measure and/or predict complexity will allow ATCOs to better understand and act on conflict areas, thereby reducing complexity. * Other factors discussed above, such as weather or war, are unpredictable. Even so, research is being carried out to reduce this uncertainty and mitigate the adverse effects on complexity. After this analysis, the conclusion is that air traffic complexity is a variable highly considered in ATM. Seeing that this topic is of great interest and that the advances made may be of great interest to air traffic management, it has been decided to carry out a bibliometric analysis to summarise the contributions in this field. Bibliometric analysis refers to the quantitative analysis of bibliometric material related to a research area, a specific topic, or a journal [22,23]. Bibliometric analysis is a technique that has been applied to different disciplines, such as medicine [24], finance [25], and even social media [26]. In aviation, bibliometric analysis is also a widely used technique. Bibliometric analyses have been carried out on topics such as airport service quality [27], the evolution of the different technological areas in aviation [28], or the aviation situation in the aftermath of the COVID-19 pandemic [29]. Being such a widespread and flexible tool, it is considered the best way to make a comprehensive study of the concept of complexity in aviation and the main contributions made in this regard. Therefore, this paper aims to analyse the main publications in the Web of Science and to find relationships between them. Another objective of this paper is to summarise and group the main contributions to deepen the definition of air traffic complexity. To this end, Section 2 describes the methodology used to carry out the study. Section 3 carries out the bibliometric analysis of the selected publications, and Section 4 analyses the contributions of the main publications on this topic. Finally, Section 5 presents the conclusions of the study.