*2.1. Research Methodology*

To answer the research question of how DT can assist the certification process, we provide a detailed and comprehensive analysis of the state of the art through the following source databases: Google Scholar, Scopus, Springer, Science Direct, and the European Union Aviation Safety Agency. We instigated a data search by combining the keywords "Unmanned Aerial Vehicle", "Unmanned Aircraft System", "Unmanned Aerial System", "UAV", "UAS", and "drone" in combination with "digital twins", "DT", "certification", "regulation", "European Union (EU) regulation", "regulatory framework", "Urban Air Mobility", "UAM", "Advanced Air Mobility", and "AAM". In the literature search, we identified relevant articles according to the title and context of the study. A total of 121 references, which were best-aligned with the scope and objectives of our research, were selected, of which 20 articles were directly relevant to the scope of DT applications for UASs. The results sections of the selected references were analyzed to gain valuable insights in this domain.

The first step to answering the identified research questions is to investigate the existing and upcoming European regulatory framework for UASs and to understand the concept, methodologies, and applications of DTs.
