22

[117] 2022 Other DTs Digital twin


A time frame of two decades was chosen for conducting the literature review, since the term "digital twin" was first introduced in 2010 [111]. However, in this section, as we discussed in the research methodology in Section 2.1, we only analyzed articles within the scope of DT applications for UASs. DT applications in UASs are relatively new, resulting in the majority of the relevant literature having been published within recent years. Although research on DTs in UAS applications has recently gained momentum, there remains a substantial amount of work to be undertaken toward the further exploration and understanding of the potential value and significance that DTs can bring to the field of UAS applications. Figure 11 provides a word cloud visualization that depicts the frequency of selected keywords (DT, UAV, AI, drone, UAS, certification, regulation, and VTOL) within publications related to the applications of DTs in UASs. These specific keywords were carefully selected during the research process, and the word cloud offers a concise representation of the pathway to the literature review scope.

**Figure 11.** Keyword analysis word cloud for DT applications in publication on UASs.

While the complexity of UASs is fast evolving, only 40% of the publications briefly mentioned certification and regulation when using DTs in UASs, and not many scientific literature efforts focus on the use of DT applications in UASs for certification and regulation, as shown in Figure 12. DTs facilitate designing, building, and analyzing procedures. DTs are very good and relatively time- as well as cost-efficient tools to assist the certification process, since they help engineers check, analyze, and integrate designs as well as express concerns instantly.

Autonomous (with the help of AI and without a pilot's intervention) UAVs are expected to conduct safe operations and cope with unforeseen conditions. As presented in Figure 13, half of the publications considering the use of DT applications in UASs mentioned autonomous flight operations, and 38% of these publications also discussed the use of AI, which leads to the key research question of how these operations can be safely

conducted. In UASs' EU operational scope, the EASA published the "EASA AI roadmap" as a human-centric approach to the safe use of AI in aviation.

**Figure 12.** Keyword analysis focused on certification and regulatory frameworks for DT applications in publication on UASs.

**Figure 13.** Keyword analysis focused on (**a**) autonomous flights and (**b**) the use of AI/ML/deep learning approaches for DT applications in publications on UASs. (**a**) Autonomous flights mentioned in publications. (**b**) AI/ML/deep learning approaches mentioned in publications.

#### **4. Discussion**

Flying cars and aerial transportation systems are some of the distinctive features of the future cities described in science fiction films and books. This is one of the basic concepts accepted by society when imagining the future, and with today's technological advancements we wonder if conducting safe automatic and autonomous flights for metropolitan areas is a few steps away in the near future. The establishment of a socially acceptable regulatory framework is necessary to transform this vision into a reality.

The regulatory framework for UASs in the European Union was fragmented before 2020, as shown in Figure 1, with each EU Member State being responsible for drones with a maximum take-off mass (MTOM) of less than 150 kg while the EASA was in charge of drones with an MTOM exceeding this weight. The transition to new regulations began in 2020, and the EASA is now responsible for drones of all weights and size. Nonetheless, this regulatory framework is still in its early stages, and further developments are expected.

Implementing this evolving regulatory framework presents a significant challenge in UAS operations. DTs can potentially offer a solution by facilitating the design, construction, and analysis processes. They are time- and cost-efficient tools to assist the certification process, since they help engineers check, analyze, and integrate designs as well as express concerns instantly. However, only a limited number of publications (40%) briefly mentioned certification and regulation when discussing the application of DTs in UASs, as shown in Figure 12. Therefore, efforts need to be carried out to emphasize the importance of DTs in assisting the certification process within UAS operations.

In Figure 13, it is notable that autonomous flight operations were mentioned in 50% of the papers examining the use of DTs in UASs, and AI applications were discussed in 38% of the publications. Autonomous and automatic UASs are expected to conduct safe operations in UAM. A quick comparison of autonomous and automatic flights can show that there is no human safety net present during an autonomous flight in the event of unforeseen circumstances. Therefore, precise regulations must be created in the context of AI to ensure autonomous flights are safely conducted. In 2020, the EASA also published the first guidance, the EASA AI roadmap, for the safe use of artificial intelligence in aviation. However, we are still a long way from the dream of having science fiction flying transport systems coming true, as the timeline outlined in the EASA AI roadmap document predicts the first approvals of AI in 2025.

The research subject of how to adapt UASs with UAM and regulation is studied in a wide range of the literature from across the world. However, the implementation of DTs in UASs for assisting the certification process and considering regulation, especially within the context of the EU regulatory framework, remains relatively unexplored. The concept of drone regulation, particularly in relation to EU legislation and the integration of UAM for cargo and passenger transport, is still relatively new. The development of regulations, as well as applying these regulations to UAS operational categories, requires the consideration of numerous criteria and parameters to ensure a robust level of safety and seamless flight operations. Moreover, due to safety concerns and ongoing regulation development, UAV autonomous flights are not currently being carried out in most European countries. The lack of literature and documents is inevitable in the early stages of a new concept's development. Consequently, one of the challenges lies in staying informed about evolving regulations and keeping track of the developments and changes that emerge in this field.

Overall, this paper highlights the necessity of further research on and the exploration of DT applications in UASs, particularly concerning certification and regulation. It is essential to recognize that DTs cannot function as standalone solutions but should be seamlessly integrated into a comprehensive framework that takes into account regulatory, technical, and operational aspects. The distinct advantage of employing DTs to facilitate UAS certification and regulation lies in their ability to create a digital replica of a physical system, enabling real-time validation and optimization. Nonetheless, this task is inherently complex and presents several challenges, such as the necessity of an accurate model of reality and handling a large amount of data from various sources. Furthermore, utilizing DTs to assist the certification process requires careful consideration of legal and regulatory requirements. It is crucial to address these challenges and associated complexities to pave the way for the successful implementation of UASs in UAM.

#### **5. Conclusions**

The popularity of UAV operations has increased, and new air mobility concepts have emerged over the past years. It is essential to develop regulations in this new technological context that effectively address the challenges and opportunities presented by UASs. There are various levels of ongoing activities and recent advances in UAS regulatory frameworks, especially in the domain of European Union (EU) regulations. Due to the growing demands, advancements, and possible applications of UASs, particularly in research and innovation, there is a need for a systematic overview. To bridge this gap, we present a comprehensive overview of the developed UAS regulations in the European Union and explore the concept

of DTs as well as their potential applications in the UAS domain. We aimed to conduct a systematic review to provide a structured methodology that synthesizes multiple studies to offer a comprehensive and unbiased assessment of DTs' applications in UASs with EU regulatory compliance. Despite limited scholarly focus on the implementation of DTs in UASs considering certification and regulation, we analyzed the existing literature to identify and emphasize the important trends and developments. The overall challenges and the importance of UAS DTs are highlighted to provide a robust foundation for future studies on UAS DTs and their compliance with the EU regulatory framework.

**Author Contributions:** Conceptualization, I.S.; methodology, E.F.; writing—original draft preparation, E.F.; writing—review and editing, S.S., I.S., E.-H.A. and E.F.; supervision, I.S. and E.-H.A.; project administration, I.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the European Union's Horizon 2020 research and innovation program (H2020-MG-3-6-2020 Research and Innovation Action "Towards sustainable urban air mobility"), grant number 101007134.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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