*Review* **Towards Safe and Efficient Unmanned Aircraft System Operations: Literature Review of Digital Twins' Applications and European Union Regulatory Compliance**

**Elham Fakhraian 1,2,\* , Ivana Semanjski 1,2 , Silvio Semanjski <sup>3</sup> and El-Houssaine Aghezzaf 1,2**


<sup>3</sup> SEAL Aeronautica S.L., 08860 Barcelona, Spain; silvio.semanjski@sealaero.com

**\*** Correspondence: elham.fakhraian@ugent.be

**Abstract:** Unmanned aerial system/unmanned aircraft system (UAS) operations have increased exponentially in recent years. With the creation of new air mobility concepts, industries use cuttingedge technology to create unmanned aerial vehicles (UAVs) for various applications. Due to the popularity and use of advanced technology in this relatively new and rapidly evolving context, a regulatory framework to ensure safe operations is essential. To reflect the several ongoing initiatives and new developments in the domain of European Union (EU) regulatory frameworks at various levels, the increasing needs, developments in, and potential uses of UAVs, particularly in the context of research and innovation, a systematic overview is carried out in this paper. We review the development of UAV regulation in the European Union. The issue of how to implement this new and evolving regulation in UAS operations is also tackled. The digital twin (DT)'s ability to design, build, and analyze procedures makes it one potential way to assist the certification process. DTs are time- and cost-efficient tools to assist the certification process, since they enable engineers to inspect, analyze, and integrate designs as well as express concerns immediately; however, it is fair to state that DT implementation in UASs for certification and regulation is not discussed in-depth in the literature. This paper underlines the significance of UAS DTs in the certification process to provide a solid foundation for future studies.

**Keywords:** digital twin (DT); unmanned aerial system/unmanned aircraft system (UAS); unmanned aerial vehicle (UAV); drone; urban air mobility (UAM); advanced air mobility (AAM); European Union (EU) regulation; regulatory framework
