**1. Introduction**

In recent years, new innovative technologies, such as unmanned aerial vehicle (UAVs) and vertical take-off and landing (VTOL) aircraft, have led to the creation of new air mobility concepts [1]. UAVs operate in various sectors: agriculture, inspection, media, and entertainment. UAVs' operational and technological capabilities have evolved. They are expected to gain greater freedom of use and enter the area of commercial flights in the near future. Currently, most UAV civil operations are conducted in low-level uncontrolled or segregated controlled airspace due to safety concerns [2]. Operations in high-risk environments set higher requirements to overcome related risks: collisions with civil aircraft, injuries, and accidents due to UAV operation errors. The prevailing measures in UAS management necessitate the thorough consideration and addressing of concerns pertaining to scalability, compliance, cybersecurity, privacy, limitations in real-time monitoring, and the intricate regulatory landscape, which often entail significant investments of time and resources. One of the possible solutions is to leverage digital twin (DT) technology to map the physical space during UAV operation into the virtual space to assess the risk

**Citation:** Fakhraian, E.; Semanjski, I.; Semanjski, S.; Aghezzaf, E.-H. Towards Safe and Efficient Unmanned Aircraft System Operations: Literature Review of Digital Twins' Applications and European Union Regulatory Compliance. *Drones* **2023**, *7*, 478. https://doi.org/10.3390/ drones7070478

Academic Editor: Pablo Rodríguez-Gonzálvez

Received: 7 June 2023 Revised: 10 July 2023 Accepted: 18 July 2023 Published: 20 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

related to the operation beforehand. The utilization of DT has become an engaging subject today [3]. A DT is a virtual replica of real-world entities or processes. DTs develop models to simulate future scenarios and employ historical as well as real-time data to illustrate the past and present [4]. DTs can gain new and unexpectedly detailed insights into how machines and operations work in addition to how to improve them using sensors, costefficient and more secure data storage, powerful computers to analyze data, and artificial intelligence [5]. DT allows engineers to check, analyze, and integrate designs as well as express concerns immediately [6]. For example, DT helps anticipate when a machine may fail based on data analysis, which allows the boosting of productivity through preventive maintenance [7]. DTs' application is mainly grouped into the manufacturing [8], aviation, automotive, education and research [9], and healthcare and medicine fields [10]. DT technology is expected to change the "rules of the game" in aviation manufacturing in the future [11]. The aviation community is fostering an aspiration to offer air mobility as an alternative for everyday transportation needs, commonly known as urban air mobility (UAM) and advanced air mobility (AAM) [12]. AAM encompasses a broad concept that enables individuals to access on-demand air mobility, cargo and package delivery, healthcare applications, and emergency services through an interconnected multimodal transportation network [13]. Achieving this system necessitates the seamless integration of air traffic management systems, ground control systems, and communication networks to facilitate effective communication between AAM vehicles and ground systems to ensure safe and efficient operations. As a result, the aviation industry is actively working towards developing an innovative aerospace framework that promotes shared aerospace practices, ensuring the safety, sustainability, and efficiency of air traffic operations [14]. A wide range of literature has been published to explore operational strategies and expectations in the context of AAM [15–28]. Currently, NASA, in collaboration with the FAA, other federal partner agencies, industry, and academia, is actively engaged in research and development efforts to establish the infrastructure, information architecture, concepts of operation, operations management tools, software functions, and other functional components of AAM [29]. Nevertheless, several challenges have the potential to affect the growth of AAM. These challenges include autonomous flight capabilities, the availability of necessary infrastructure for take-off and landing, integration into existing airspace as well as other transportation modes, and competition with shared automated vehicles [30].

UAM, a subset of AAM, is anticipated to yield substantial economic benefits while posing notable developmental challenges. UAM necessitates the development of sophisticated urban-capable vehicles and the establishment of an airspace system capable of efficiently managing high-density operations [12]. According to the European Union Aviation Safety Agency (EASA), UAM is defined as "a new safe, secure and more sustainable air transportation system for passengers and cargo in urban environments, enabled by new technologies and integrated into multimodal transportation systems. The transportation is performed by electric aircraft taking off and landing vertically, remotely piloted or with a pilot on board" [31]. The EASA further predicts that, by 2030, approximately 340 million people residing in EU cities will experience UAM [31]. The concept of urban aerial transportation is not novel, as historical examples of UAM services date back to the 1940s [32]. A notable instance of these historical examples is New York Airways, which operated commercial helicopter-based passenger transport services from 1953 to 1979. However, due to a series of fatal accidents and crashes, New York Airways ultimately ceased operations and filed for bankruptcy. Although this particular chapter of urban aerial mobility concluded abruptly, modern-day congested metropolises have witnessed the resurgence of diverse helicopter transport services [33]. Similar to other transportation systems, UAM necessitates the establishment of infrastructure encompassing the physical ground infrastructure for vehicles as well as the implementation of digital technology and telecommunications for effective traffic management. An essential element for the successful introduction of UAM is the development of appropriate regulations, including the definition of certification standards and policies that govern UAM operations. Addressing these regulatory aspects

is crucial to ensure the safe and efficient integration of UAM into existing transportation frameworks [34].

A wide range of literature has been published to answer the research question of how to safely integrate unmanned aircraft systems (UASs) into UAM and AAM within the context of regulation. Studies have addressed key concerns about privacy, the operation of civilian drone regulations, and the social as well as ethical implications of this integration. Winkler et al. [35] highlighted the concerns and needs for privacy and the operation of civilian drone regulations. Clarke investigated the impacts of civilian drone regulation on behavioral privacy [36] and public safety [37]. Thomasen [38] evaluated the impact of robots (including drones) and their regulation on public spaces. In this paper, the authors also examined the technology's impacts on women's privacy and related regulations [39]. Merkert et al. [40] used a theoretical road pricing framework to analyze drone operators' willingness to pay for low-altitude airspace management (LAAM). West et al. [41] reviewed the public's opinions on drone policy. Li and Kim [42] studied the dynamics of local drone policy adoption in California. Nelson and Gorichanaz [43] investigated the emergence of drones and evolving regulation in 20 cities in Southern California. However, in the available literature and official documentation, there was no agreed and consolidated definition of UAM in Europe until recent years, when the EASA introduced the UAM concept as "The safe, secure and sustainable air mobility of passengers and cargo enabled by new generation technologies integrated into a multimodal transportation system conducted in to, within or out of urban environments" [1]. The EASA is also establishing a regulatory framework addressing the safety, security, and environmental aspects of UASs to ensure their acceptance and adoption by European citizens. Some elements of this regulatory framework have already been established; for example, Regulation (EU) 2019/947, Regulation (EU) 2019/945, Regulation (EU) 2021/664, Regulation (EU) 2021/665, and Regulation (EU) 2021/666 [1].

In parallel with the establishment of regulatory frameworks, the potential of [5,8–11] DT utilization in the aviation industry has been explored and documented in numerous pieces of the scientific literature [44–49]. DTs can be used in any stage of the aircraft life cycle [50–60], such as design, manufacturing, operations, and maintenance. DTs can also be implemented on components as well as systems [61–70] that provide a comprehensive view of an aircraft and its individual parts. It allows for monitoring and analysis at different levels, enabling engineers to assess the performance and health of specific components as well as understand the overall behavior and interactions within the system. Various research efforts have been conducted to use DT in UASs [3,71–90], addressing challenges and opportunities of UASs within this dynamic and evolving field. However, despite the significant discussion of DTs in the general aviation literature, especially in relation to manufacturing and maintenance, more effort and attention need to be devoted to the application of DTs in UASs [71].

Overall, the aviation industry is subjected to an international framework, yet it requires additional efforts to establish a similar framework for UAS operations [91]. Considering the strong ongoing developments in this domain, the approach to UAS certification does not evolve with the same dynamic [6], and the UAS European Union (EU) regulatory framework was fragmented before 2020, mainly considered in quite local and regional contexts. However, some significant steps have recently been made in this aspect, particularly since 2020, and the EU legal framework of UASs is undergoing changes to provide uniform regulation. One of the aims of this paper is to also bring these developments to the closer attention of the research community in order to support strongly evolving research efforts, as this aspect has so far been generally understated across the scientific literature. Understanding the appropriate operational category presented by the EASA for UASs helps to gain more insights into the requirements of authorizations and certification. However, when developing a product that requires regulatory certification, this is only one half of the matter. The separation between design and analysis activity is one of the critical gaps in the certification process. DTs facilitate engineering and manufacturing teams to design and

build products better and faster. It also helps them to check, analyze, and integrate designs as well as express concerns instantly [6]. This paper provides a comprehensive overview of the developed UAS regulation in the European Union provided by the EASA and examines the potential of DTs to assist the certification process. This paper aims to make a bridge between DTs, UASs, and the EU regulatory framework to present a reliable basis for future studies. The structure of the paper is as follows: Section 2 is structured into three subsections. The first subsection provides an overview of the research methodology. The second subsection introduces the current and upcoming European regulatory framework for UASs. The third subsection illustrates the concept and applications of DTs. Section 3 provides a valuable resource by analyzing the existing relevant literature and highlighting important trends as well as developments. Section 4 presents the links and potential to use DTs to assist the certification by drones' EU regulatory framework. forming an outlook for future studies and applications. Section 5 presents the conclusion, where some key lessons learned based on the existing body of literature are presented.

#### **2. Materials and Methods**

One of the essential steps toward determining the potential of DTs in the certification process is specifying the related regulation in the context of operational robustness and airworthiness. Airworthiness concerns the safety standards in all construction aspects: structural strength, safeguard provisions, design requirements relating to aerodynamics, performance, and electrical as well as hydraulic systems [92]. Robustness refers to the characteristic of mitigation measures resulting from combining the improvements in safety provided by mitigation measures and the levels of assurance as well as integrity in attaining the desired safety enhancement [93]. In general, international and national regulations are focused on safety. However, small drones avoid many of these requirements, as they pose fewer risks [91]. UAV operations are a relatively new concept and have significant potential in combination with new technologies, resulting in new applications (with their required regulations). DTs are also a relatively new concept accepted in various industries and have great potential for UAV operations. A DT is a description of a component, product, or system providing a series of interconnected relevant digital models containing engineering data, operation data, and behavior descriptions obtained from simulations. It can be modified as a real-world system can be developed through its life cycle. A DT is used to develop solutions that are applicable to actual systems in addition to describing the behavior. It can be applied to testing and simulation, enabling users to observe how new behaviors are exhibited and find answers to their problems [94].

In the legal context, it is essential to acknowledge and understand the distinct terminology used when referring to drones, as they may carry different legal implications. The term "drone" was first used in 1935 and is nowadays quite accepted by both the media and the general public [95]. Alongside "drone", the most frequently used terms are "unmanned aerial vehicle" (UAV) and "unmanned aircraft system/unmanned aerial system" (UAS). The terms "drone" and "unmanned aerial vehicle" (UAV) stand out as referring only to a flying platform (the airplane and its payload). The phrase "unmanned aerial system" (UAS) is the most well known term for an entire system (a flying platform and ground station). "Unmanned aircraft system" (UAS) is widely used by the Federal Aviation Administration (FAA), European Aviation Safety Agency (EASA), and International Civil Aviation Organization (ICAO). Hence, it is better to utilize the term "unmanned aircraft systems" when referring to UASs in this study. It is essential to utilize the correct terminology in order to deliver the concepts in the debate properly [95].

Official documents and legislations mainly use the terms "UAV" and "UAS". While professional drone users are familiar with these terms and use them, the terms "UAV" and "UAS" are less familiar to the public, especially when abbreviated [95]. People might therefore have few or no associations with these terms, so the term "drone" is occasionally used in conjunction with these terms for simpler demonstration in documents. In this

work, we make an effort to use the terminology accurately, considering the references to prevent misconception.

This section is divided into two subsections: The first subsection introduces the existing and upcoming European regulatory framework for UASs. The second subsection illustrates different DTs' methodologies.
