## 2 Materials and Methods  
The analysis of the references related to the complexity of air traffic has been carried out employing a bibliometric analysis. Following the structure of the relevant bibliometric analysis within the ATM [30], this study has been carried out with three fundamental objectives in mind. 1. Identify the main contributions to the field of air traffic complexity. 2. Map the 'intellectual structure' of the topic using co-citation and co-author analysis. 3. Analyse the relationships between the different articles to identify areas of interest and to better understand how research on this topic is evolving. Specifically, during this process, the following research questions will be addressed: * Is the study of air traffic complexity really relevant? * Which authors have published the most papers regarding the topic of air traffic complexity? * What have been the key issues studied related to air traffic complexity? * What are the main solutions proposed by academia to the complexity problem? * What are the weaknesses and strengths of the main complexity indicators? These questions, together with the objectives set, will guide the development of the bibliometric analysis. To answer these questions, a study was carried out based on the methodology of [31]. For this purpose, the steps followed in this paper are as follows, and are represented schematically in Figure 1.  
1. The first step is data collection and filtering of non-targeted articles. In this case, a search for the topics 'Air Traffic complexity' and 'Airspace complexity' was made. In this case, only the Web of Science (WoS) repository was used (Clarivate, London, UK), with a further manual filtering of the articles found to only study relevant articles. 2. Once the data have been filtered, a preliminary analysis is carried out. This analysis aims to answer the question \"Is the study of the complexity of air traffic really relevant?\". To achieve this, we studied how relevance in the topic has evolved over time in terms of the number of publications and citations. 3. A co-authorship and co-citation analysis is carried out. With these analyses, it is also expected to be able to identify which are the most prominent papers dealing with the complexity of aerospace. The question \"Which authors have studied air traffic complexity?\" will be answered. A co-authorship analysis by organisations will also be carried out to find out in a structured way where the knowledge on this topic is located. 4. In addition, an analysis of the keywords will be carried out. This analysis aims to detect the main areas of interest within the topic of complexity in aerospace in the hope of finding common lines of research among the papers analysed. Therefore, this analysis is carried out to answer the question \"What are the key topics studied related to the complexity of aerospace?\". 5. Once the previous analyses have been carried out, the main contributions on the topic can be identified and compared with each other in a more detailed way, being able to answer, \"What are the main solutions proposed by academia to the problem of complexity?\". This last study will be interesting as it will allow readers to have a detailed idea of what the main interests of the industry are on this topic, and what the solutions proposed by authors are to the problems of the industry related to this topic. It will also try to collect possible strengths and weaknesses of the complexity indicators. This will allow us to answer the question \"What are the strengths and weaknesses of the main complexity indicators?\" and to give guidance for the realisation of future complexity indicators. This process of bibliometric analysis is presented schematically in Figure 1, relating the steps of the methodology to the fulfilment of the objectives set out in this paper. Figure 1 also relates the steps of the methodology to the research questions posed. Figure 1: Description of the bibliometric analysis and relationship with the objectives.