## 1 Introduction  
Transport systems, along with other infrastructure, are fundamental elements of societies and economies. They guarantee a high level of mobility, which is key for market cohesion and for quality of life. Moreover, transport systems enable socioeconomic development and job creation [1]. The vision to democratize air mobility access for the global population has been a long-held aspiration of the aeronautical community [2]. With the increasing rate of gross domestic products (GPDs) across many countries and the decreasing cost of flight fares, air travel has become affordable for most people [3]. However, in recent years, the world has faced an ongoing mobility challenge in local and national transportation, and any improvements in efficiency, reliability, safety, and accessibility could lead to positive impacts on economic development, quality of life, and the environment [2]. Air transport systems provide a service to their customers from an origin location to a destination; therefore, the flight is an intangible good [4]. The global commercial deregulation and liberalization of air transport, which began in the United States at the end of the 1970s, resulted in numerous changes including the evolution of price competition, the emergence of low-cost airlines, a growing load factor, airport and airspace capacity problems, etc. [5]. In this context, mergers, alliances, bankruptcies, shifts in market shares, and the rise of new carriers became norms of the industry [6]. As constraints on routes and the number of flights as well as regulated tariff policies were removed, prices fell dramatically, especially on popular routes [5]. The aviation industry has become essential for the socioeconomic development of many countries [7]. Shared mobility services provide users with more efficient travel options and reduced environmental impacts as a result of lower emissions. This trend has led to a research interest to explore the third dimension: the skyscape [8]. Moreover, the rise of highly populated urban areas together with an increase in travel demands has led to frequent traffic jumps all over the world, and increasing the ground capacity is not considered feasible in many cities. Therefore, the vision of opening the third dimension, i.e., altitude, for urban and regional transportation has gained momentum in the last 5-10 years to explore \"on-demand\" air mobility (ODAM) [9]. The COVID-19 pandemic has led to global governmental restrictions that reduce and limit mass transportation options. The air mobility sector has been impacted severely, with a 75% decrease in all commercial flight activity by mid-April 2020, as compared to 2019. Furthermore, 60% of global oil demand has been associated with mobility and aviation [10]. Therefore, in the global energy crisis and climate change, the aviation sector is a significant player [11]. Aviation currently accounts for 12% of transport-related CO\({}_{2}\) emissions with a steadily increasing impact. In order to tackle this challenge, the European Commission and two U.S. government agencies, the International Air Transport Association (IATA) and the International Civil Aviation Organization (ICAO), formulated reduction targets such as a CO\({}_{2}\) reduction per passenger kilometer, NOx, and perceived noise. Considering the increasing amount of air travel, these goals are unlikely to be reached by improving existing aviation technology [11]. Thus, the aviation industry needs to accelerate fleet renewal, improve non-fuel cost management, improve fleet operations, and encourage the development of alternative fuel sources [3]. Expectations for continued growth, despite the global pandemic and coupled with environmental and commercial pressures, have placed the aviation industry in a challenging position. This has led to a growing interest in the design and certification of electrically powered aircraft as they offer better efficiency and have zero emissions [7].