Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe

BACHELOR THESIS TITLE: Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe DEGREE: Bachelor's Degree in Aerospace Systems Engineering AUTHOR: Guillem Machacón Lobato DIRECTOR: Jovana Kuljanin SUBMISSION DATE: 20/01/2025 TITLE: Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe DEGREE: Bachelor's Degree in Aerospace Systems Engineering AUTHOR: Guillem Machacón Lobato DIRECTOR: Jovana Kuljanin SUBMISSION DATE: 20/01/2025 Abstract The recent ban on ultra-short haul flights and their replacement by rail transport have been adopted in several European Union countries to reduce aviation's environmental footprint. However, the upcoming introduction of electric Vertical Take-Off and Landing (eVTOL) vehicles offers a potential way to reclaim these routes, heralding a paradigm shift in personal mobility. Airlines, though, face challenges in aircraft selection since eVTOLs differ significantly from the banned aircraft, primarily in terms of size and payload capacity. Therefore, in order to recover the previous traffic demand , the main objective of this study is to select the most convenient electrically powered aircraft for 2 out of the 5 currently banned routes, using a decision-making tool through an Analytical Hierarchical Process (AHP) which allows multi-criteria problems to be addressed. For this purpose, both the potential eVTOL candidates, which are capable of operating these air routes, and the criteria to be taken into account in the aircraft selection process will be defined. Finally, the feasibility of implementing eVTOL aircraft in the short term is discussed. A mi abuelo Luis, cuya memoria ilumina cada paso de este camino. Siempre en mi corazón y en cada logro. ACKNOWLEDGEMENTS This thesis is the culmination of an unforgettable stage full of experiences and learning. I would like to deeply thank my tutor Jovana Kuljanin for her endless willingness to help and guide me throughout our journey together. I will always remember your advice in my professional career. I want to express my most heartfelt gratitude to my parents Guillermo and Yolanda both for their unconditional effort and tireless work from the very beginning to help me achieve my goals. For being my strength at times when I did not believe and for giving me courage when I needed it most. We have made this journey together. My achievements are also yours. LIST OF CONTENTS 1 Introduction 1 2 Contextual settings of aviation 3 2.1 Current situation, challenges and objectives 3 2.2 Regulatory framework 5 2.3 Comparison between rail and air transport 7 3 Route Analysis 13 3.1 Currently banned routes across Europe 14 3.2 Potentially restricted routes in the near future 20 3.3 Out of scope, potentially restricted and banned routes comparison 23 4 The need for replacement of conventional aircraft 26 4.1 Introduction of eVTOLs as a sustainable solution 26 4.1.1 eVTOL development timeline 27 4.1.2 Technology and operational constraints 28 4.1.3 Design architecture 30 4.2 eVTOLs selection 31 5 Decision making process 38 5.1 Introduction to the AHP Process 38 5.1.1 Overview of AHP methodology 38 5.1.2 Relevance of AHP for eVTOL Selection 38 5.1.3 AHP methodological framework for the eVTOLs selection process 39 5.2 Criteria Definition and Pairwise Comparison 41 5.2.1 Criteria for eVTOL selection 42 5.2.2 Pairwise comparison matrices 42 5.2.3 Justifications for pairwise comparisons 43 5.2.3.1 Scenario 1: Vienna-Salzburg 43 5.2.3.2 Scenario 2: París-Orly-Bordeaux 44 5.3 AHP Calculations for Each Scenario 45 5.3.1 Scenario 1: Vienna–Salzburg 46 5.3.2 Scenario 2: Paris Orly-Bordeaux 47 6. Results and Analysis of the AHP Process 49 7. Application of AHP Results to eVTOL Selection 50 7.1 Normalized Quantitative Data for eVTOL Candidates 50 7.2 Weighted Scores for eVTOL Candidates 54 7.2.1 Ranked score list for Scenario 1 with 9 and 11 alternatives 55 7.2.2 Ranked score list for Scenario 2 with 3 alternatives 57 7.3 Comparison of eVTOL Rankings Across Scenarios 57 7.3.1 Comparison of eVTOL Rankings Across Scenario 1 57 7.3.2 Comparison of eVTOL Rankings Across Scenario 2 58 8. Viability of eVTOL implementation 59 8.1. Summary of AHP results 59 8.2. Accommodating Previous Traffic Demand 59 8.3. Certification challenges and infrastructure requirements 60 8.4 Economic Feasibility 61 9 Sustainability and social commitment 62 10 Conclusions and future work 63 Bibliography 64 LIST OF FIGURES Figure 2.1: Percentual change of days with heavy rain in 2050 [3] 4 Figure 2.2: Distribution of GHG emissions by transport means in Europe, 2018 [5] 5 Figure 2.3: Distribution of air transport emissions in Austria, according to the type of flight in 2018 [9] 7 Figure 2.4: Distribution of CO2 emissions by flight radius in 2019 and 2050 [12] 8 Figure 2.5: CO2 emissions from European, intra-European aviation and its potential of reduction in 2020 [13] 9 Figure 2.6: Comparison of connectivity between air and high-speed rail networks. (a) HSR network (source: OpenRailwayMap). (b) Air transport network (source: Sabre Airline Solutions) [15] 10 Figure 2.7: Speeds of railway lines between urban centres in Europe in 2019 [15] 11 Figure 2.8: Speed of rail connections between urban centers in the European Union, including region, population and route type, 2019 [15] 12 Figure 3.1: Number and proportion of flights in France by route type (2014-2023) [18]. 15 Figure 3.2: Representation of speeds on French railway lines at Montparnasse and Lyon stations [19] 15 Figure 3.3: Connectivity of Paris to Bordeaux, Nantes, Rennes and Lyon via TGV high-speed lines [19] 16 Figure 3.4: Flight distances (km) on air routes replaced by train alternative 18 Figure 3.5: Flight times (km) on currently banned routes substituted by train 18 Figure 3.6: Distribution of aircraft types in the set of banned air routes 19 Figure 3.7: Distribution of aircraft types in the set of banned routes 20 Figure 3.8: Distance between Montparnasse station and Paris-Orly and Paris-Charles de Gaulle airports (according to Google Maps). 21 Figure 3.9: Number of routes of less than 500 km of distance per country 22 Figure 3.10: Share of aircraft on routes with distances of less than 500 km. 23 Figure 3.11: Distribution of seating capacity on potentially constrained routes by 20-minute flight time interval 23 Figure 3.12: Monthly frequency distribution by aircraft type on routes below 500 km 24 Figure 3.13: Number of routes by airline and route status. 25 Figure 3.14: Comparison of seating capacity by route status: Out of reach, Potentially restricted and banned. 25 Figure 4.1 : Number of eVTOL aircraft announcements per year from 2014-2019 period [28]. 28 Figure 4.2: Electric-powered aviation market outlook (conservative view) [29] 29 Figure 4.3 : Results of the Roland Berger survey of aerospace and defence professionals [30]. 29 Figure 4.4: Relationship between battery specific energy and operating distance range for 1-passenger eVTOLs, based on gross take-off mass (GTOM) values and parameters with poor, standard and ideal performance values [32]. 30 Figure 4.5: Propulsion architectures for eVTOL aircraft [28] 31 Figure 4.6: EHang 216 eVTOL aircraft [33] 31 Figure 4.7: Aircraft eVTOL powered lift (a) Wisk Generation 5 (independent thrust) [34] (b) Jetoptera J-2000 (vector thrust) [35] 32 Figure 4.8: Unnamed eVTOL developed by Kelekona [37] 33 Figure 4.9: Illini Air Shuttle eVTOL from the University of Illinois [38] 33 Figure 4.10: Genesys X-2 from Skynet Project SRL [39] 34 Figure 4.11: Ace VTOL Trinity H2 [40] 34 Figure 4.12: Rotor X Aircraft eTransporter aircraft [41] 35 Figure 4.13: Joby Aviation S4 eVTOL aircraft [42] 35 Figure 4.14: Sastra University’ Mistral aircraft [43] 36 Figure 4.15: AutoFlight eVTOL Prosperity aircraft [44]. 36 Figure 4.16: (a) ET9 001 passenger eVTOL prototype before free flight test. (b) ET9 001 passenger eVTOL prototype top view, before tethered flight testing [45] 37 Figure 4.17: Bartini aircraft from Bartini Aero [46]. 37 Figure 4.18: eJet Aerospace's GJet Kóan [47]. 38 Figure 5.1: AHP Process hierarchy levels and structure definition 40 Figure 5.2: AHP Process hierarchy on Vienna-Salzburg route with SuperDecisions tool 41 Figure 5.3: Weights assigned to the criteria of Scenario 1 of the AHP process using the eigenvalue method. 48 Figure 5.4: Weights assigned to the criteria of scenario 2 of the AHP process using the eigenvalue method 49 Figure 7.1: (a) Ideal score list for Scenario 1 with 11 alternatives (b) Ideal score list for Scenario 1 with 9 alternatives by means of SuperDecisions tool. 57 Figure 7.2: Ranked score list for Scenario 2 with 3 alternatives by means of SuperDecisions tool. 58 LIST OF TABLES Table 3.1: Example of database structure for direct air routes according to: origin, destination, distance, duration, airline and aircraft type. 13 Table 3.2: Database structure on direct air routes according to: aircraft type, seat capacity, monthly frequency, domestic/international and route status. 13 Table 3.3: Road connections from Paris with speeds above 150 km/h [15] 17 Table 3.4: Pairs of banned routes with aircraft operating them 19 Table 3.5: Pairs of banned routes according to its type 20 Table 3.6: Monthly frequencies on banned route pairs 21 Table 3.7: Railway line speed between Lyon-Marseille [15]. 22 Table 3.8: Number of domestic and cross-border routes according to route status 26 Table 4.1: List of selected eVTOLs with its main parameters of study 38 Table 5.1: Saaty's fundamental scale for AHP process [48] 42 Table 5.2: Pairwise comparison matrix between criteria for scenario 1: Vienna - Salzburg 44 Table 5.3: Pairwise comparison matrix between criteria for Scenario 2: Paris-OrlyBordeaux 44 Table 5.4: Comparison and justification of ratings of the pairwise comparison matrices in both study scenarios 46 Table 5.5: Random consistency index (RI) for matrices of order n [48] 48 Table 6.1: Comparison of weights for each criterion in scenarios 1 and 2. 50 Table 7.1: Normalised and scaled values of the range criterion for each alternative (case of 11 alternatives) 53 Table 7.2: Normalised and scaled values for Scenario 1 for the five criteria across 11 alternatives. 54 Table 7.3: Normalised and scaled values for Scenario 1 for the five criteria across 9 alternatives. 55 Table 7.4: Normalised and scaled values for Scenario 2 for the five criteria across 3 alternatives. 55 Table 7.5 Ranked score list for Scenario 1 with 9 and 11 alternatives. 57 Table 7.6: Ranked score list for Scenario 2 with 3 alternatives. 58 Table 9.1: Sustainability matrix 63 Contextual settings of aviation 7 Figure 2.3: Distribution of air transport emissions in Austria, according to the type of flight in 2018 [9] Having described the regulatory framework and considering that air transport is going to be affected by rail transition, it makes sense to discuss and compare both modes of transport. A number of questions arise: Are the affected routes sufficiently polluting to generate a noticeable change in the reduction of gas emissions in Europe and bring it closer to the environmental targets? How prepared and adapted is the high-speed rail network for these countries to accommodate the passenger traffic on these restricted routes? 2.3 Comparison between rail and air transport In accordance with the regulations presented above, it is worth analysing the proposed implementation and substitution of air routes by rail in a number of cases. The analysis, based on the sustainability objectives set by the European Union's Green Deal, includes the distribution of emissions according to route radius and the emission reduction potential on these routes. Furthermore, the current state of the high-speed rail network in Europe will be presented in comparison with air transport as well as how many routes could be eligible within the law of flight substitution, according to variables such as average and maximum line speed. Firstly, it is necessary to define what kind of routes would be affected by the French measure in favour of rail transport. With the objective of defining the range of routes to be studied, the average speed (in maximum values) of rail lines in Europe must be specified. Knowing that direct train connections cannot exceed 2h 30m, the radius of routes exposed to this regulation is determined. A special report issued by the European Court of Auditors ‘European high-speed rail network: not a reality, but a fragmented and inefficient system’ states that trains run on average at 40% of the line's design speed. This data suggests that the highest average speeds on direct train connections vary from 200 km/h to 250 km/h [10]. Based on this, if the rail connection time cannot exceed 150 minutes and the average speed of the lines is between a minimum of 60 and 240 km/h, the type of direct air routes potentially influenced by regulation are those between 0-500 km. Eurocontrol defined short-radius routes as less than 1500 km [11]. Under this classification, we categorise routes under 500 km radius as ultra-short routes. 8 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe The following diagram (Figure 2.4) points out the distribution of flights in 2019 and 2050 according to radius and the directly related CO2 emissions under radius classification. The most revealing part of Eurocontrol's report addresses the range of flights that will be affected by the modal shift, as flights with a radius of less than 500 km accounted for 30% of the European total while producing only 6.1% of CO2 emissions in 2019. As we can see, flights over 3000 km distance share is 8% in 2019 and will account for 11% by 2050. Instead, they make up more than 50% of CO2 emissions in European territory, and therefore, the focus should be on decarbonising these long-haul flights in order to have a significant reduction in emissions [12]. Figure 2.4: Distribution of CO2 emissions by flight radius in 2019 and 2050 [12] As shown in Figure 2.5 below, in terms of total emissions as of 2020 from European aviation are quantified at more than 180 Mt CO2[13]. Although this is not relevant for the study, since emissions from both intra-European and extra-European flights (including international flights with origin or destination in Europe) are covered. Such data includes all countries of the European Union (EU-27), the United Kingdom, Norway, Iceland and Switzerland. Among the category of intra-European flights, flights over 1000 km emit 40 Mt of CO2. Since adapting these flights to the rail alternative is not realistic at this stage, no potential for reducing these emissions with the implementation of modal shift exists. For routes shorter than 1000 km (excluding origin or destination islands), emissions are 26 Mt CO2 and these are the potential best-case scenario emissions that could be reduced with the rail solution [13]. Summarising, and under the assumptions of: high-speed rail connections between major EU cities, modal shift to rail on all flights below 300 km where a rail connection exists, a 10% increase in speed between rail connections and the implementation of night trains, the potential of emission reduction from air transport to rail on intra-European flights below 1000 km (excluding islands) is negligible[13]. Contextual settings of aviation 9 As is clear from Figure 2.5 below, improving the connectivity of the high-speed rail network between major cities and replacing a large part of intra-European short-haul routes with rail will reduce between 4 and 7 Mt CO2. Comparatively, this represents between 6% and 11% of intra-European aviation emissions (taking into account routes with distances over 1000 km) and only 2% and 4% of European aviation emissions (EUR-27, UK, Norway, Iceland and Switzerland)[13]. Therefore, we can conclude that even if it is mandatory to adopt and replace this type of routes by train, this restriction will have a relevant impact on the short-haul market, at the expense of a negligible potential reduction of emissions released into the environment. Figure 2.5: CO2 emissions from European, intra-European aviation and its potential of reduction in 2020 [13] When defining the potential modal shift from air to rail, it is illustrative to show and compare the medium-high speed rail network with the air network. For this purpose, the extent of these will be recognised by providing information on which routes are likely to be substituted and which territories will be mostly affected. It is worth remembering that these routes must consist of direct flights served by an existing direct rail connection and, generally, the radius must be less than 500 km. In detail, Figure 2.6 illustrates a comparison between the high-speed rail network in Europe (on the left) and the air transport network (on the right). In Western Europe, although the network structure remains widely distributed throughout its territory, the high-speed network is densely concentrated in Germany, Spain, France, Italy as well as partially in Belgium, the Netherlands and Austria. As far as Eastern Europe is concerned, there is rarely any high-speed line, a fact that allows us to anticipate the difficulties in the transition to a modal shift in these countries. On the other hand, the air transport network is more far-reaching and enables connections between different countries, which is not the case with trains. Moreover, in addition to air routes which cross borders effortlessly, the presence of regional routes which connect more 10 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe inaccessible cities or those with more limited land infrastructure, such as island destinations, make air transport a more cross-cutting means of transport without limitations. Based on these facts, the study of the routes affected by the ban on air operations can be narrowed down more specifically. Due to the layout of the high-speed rail network and the range of these routes so as not to exceed the 150 minutes of direct train connection, most of these routes will represent ultra-short-haul European domestic routes. Figure 2.6: Comparison of connectivity between air and high-speed rail networks. (a) HSR network (source: OpenRailwayMap). (b) Air transport network (source: Sabre Airline Solutions) [14] Based on the differences between both of these networks, we can focus on rail transport. As shown in Figure 2.6 (a) above, the high-speed network is weaker than the low-medium-speed network. Among the total rail passenger kilometres travelled in the European Union, only 31% are high-speed related. Still, in countries such as France and Spain they represent almost 60% [15]. Figure 2.7 displays the speeds of rail connections between cities in the European Union with a minimum of 200,000 inhabitants and spaced 500 km or less apart. Out of the 1356 existing connections, only 3% of the lines have an average speed higher than 150 km/h [15]. This is also due to the fact that in territories where population density is high (e.g. Germany and France), railway network density is also high and distances between cities are shorter, which has an impact on line speed. Eastern Europe is clearly underdeveloped, as it does not have any rail connection with an average speed above the 150 km/h threshold, as opposed to the 60% of connections operating below 60 km/h [15]. For this reason, domestic flights of less than 500 km between the regions of Eastern countries are ruled out to be replaced by rail. Contextual settings of aviation 11 Figure 2.7: Speeds of railway lines between urban centres in Europe in 2019 [15] Taking into account the speeds of direct rail lines between city pairs at distances of less than 500 km, Figure 2.8 represents the percentage of train routes according to variables such as region, type of route and city population. It is noteworthy to mention that lines with average speed between 60 and 90 km/h represent almost 40% of the total, as opposed to 3% of routes whose speed is higher than 150 km/h. The dimension of city pairs without direct rail connection is minimal. In terms of regions, Eastern Europe cannot compete with South and Northeast Europe. The former, in addition to having 20% of routes with no rail connection, 90% of them are served by lines with an average speed of less than 90 km/h. South is highly influenced by Spain and Italy (AVE, also known as Alta Velocidad Española in Spain, Trenitalia and NTV in Italy) and Northeast by France and Germany. Higher percentages of high-speed routes are available in Southern Europe as the population density of its countries is lower than in the North. On the other hand, with higher density in the network, the distances between cities are shorter (as well as the route) and consequently the speed of the connection is lower. Sorting by route type in Europe, more city pairs (cities with 200,000 inhabitants within a range of 500 km) are not connected by rail on cross-border routes. In other words, on domestic routes between pairs of relevant urban destinations, the network is fully covered as opposed to when crossing the border into another country. Line speed differs in the sense that domestic connections have a lower proportion of low-speed lines than cross-border connections and a larger segment of high-speed routes covered by rail. It is also worth noting that the larger the population between the two urban centers, the greater the probability of finding a high-speed rail network. To conclude, as Figure 2.8 indicates, the most competitive conditions offered by rail transport compared to air transport in terms of speed and duration are national routes covered by rail, with at least one city of the pair 12 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe containing 500,000 inhabitants, located preferably in the southern or northeastern European region. Figure 2.8: Speed of rail connections between urban centers in the European Union, including region, population and route type, 2019 [15] This comparison between rail and air transport makes it possible to establish the criteria on the basis of which the routes restricted by the sustainability regulations will be studied and defined. It has been found that, considering the maximum duration threshold to be offered by the rail alternative (2h 30m), the only routes that can be replaced by the train are those with a radius of less than 500 km. These emit 6% of CO2 into the atmosphere and their reduction potential is limited, which indicates that this law does not propose an effective or easily applicable solution. Taking a look at the network of rail connections (see Figure 2.6), their status, range and average speeds of the routes, main flights which can be replaced by rail routes will be distributed in the South and Northeast of Europe (France, Germany, Spain, Belgium, the Netherlands, Austria, Italy) over ultra-short distances with national routes and between urban centers of considerable population. Even if the air transport sector's interests are concerned that environmental regulations must be complied in accordance with the European target of zero emissions by 2050, in order not to lose this ultra-short haul range market, the need for alternatives to conventional propulsion of current aircraft will be pursued. Such an alternative, which may offer decarbonization, may well be electric. Route Analysis 13 3 Route Analysis Both the objectives of the European Green Deal and the regulatory framework upon which the ban on flights with a direct rail connection within 150 minutes applies, have been defined. It is known that, for these flights, rail connections must have satisfactory timetables and a high frequency in order to be replaced. In addition, through the distribution of CO2 emissions by radius, it is recognised that the impact of this measure will be low. Such a footprint appears to be limited because the large proportion of emissions corresponding to European aviation are directly related to long-haul flights. Once the distribution of the European rail network has been represented, an analysis of the air routes will be carried out. These will be direct routes (not considering connecting flights), within European airspace and will be divided mainly into 3 categories. As mentioned above, the routes likely to be affected by environmental regulations are those with a radius of 500 km or less. Therefore, routes whose flight distance is greater than the 500 km threshold are referred to as ‘out of scope’ routes. Afterwards, routes which currently remain operational but fall within this range are considered as ‘potentially affected’ routes. Finally, routes replaced by rail are classified in the ‘currently banned’ category. Moreover, flights will be differentiated on the basis of route parameters such as: distance, flight duration, airlines and aircraft serving the route, seating capacity of the aircraft, and monthly frequency by route. Also, routes will be sorted in terms of aircraft classes and their origin-destination (i.e. domestic or international). The database structure created for the route analysis is organised as follows by accessing different sources, which will be mentioned later: Origin Destination Distance (km) Duration Airlines Aircraft LISBON (LIS) BRUSSELS (BRU) 1717,97 2h 41m ● Brussels Airlines; ● TAP Portugal A319; A320-100; A320-200; A320neo; A321neo Table 3.1: Example of database structure for direct air routes according to: origin, destination, distance, duration, airline and aircraft type. Table 3.2: Database structure on direct air routes according to: aircraft type, seat capacity, monthly frequency, domestic/international and route status. Aircraft Type Seat capacity Monthly frequency Domestic or International Out of scope, Potentially Restricted, Banned Narrowbody; Narrowbody;Narrowbody; Narrowbody;Narrowbody 142;180;180; 180;227 130 International Out of scope 14 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe In order to analyse intra-European direct routes, a total of 2532 air routes have been obtained from FlightConnections [16]. Routes were manually obtained as per October 2024 and displayed in a self-made database. Among all the direct intra-European air routes, commercial flights departing or arriving from an airport located on an island have been excluded. The reason why these routes have been excluded is because it would not be possible to substitute the route by train. As previously mentioned, connecting flights have been excluded since they are not affected by the modal shift regulation. At the same time, routes whose origin or destination (or even both) emanates from islands have also been discarded, as the possibility of implementing a rail link is not available. Under these considerations, islands-related routes concerning Spain, France, Italy, Malta, Greece and Ireland (island territory and not part of the Schengen area) have been discarded. For Sweden, Finland and Greece, which belong to the EU, and Norway (which is part of the Schengen area), only routes connected to urban centres and airports with the highest passenger traffic (Athens, Stockholm, Gothenburg, Oslo, Helsinki) have been taken into account. Exceptionally, commercial flights linked to the territory of Switzerland have been accounted for, even though it is a member of the Schengen area and not of the EU, given its geographical location. The country is located in the centre of Europe and may in the near future be able to offer rail connections to neighbouring countries such as Germany or Austria. Altogether, the 2532 direct routes analysed involve a total of 25 countries. Next, routes outside the scope of the investigation will be defined, as well as the ones that can potentially be restricted in the short-medium term and those that are currently banned. In this way, by identifying which routes are removed from European airspace, it will be possible to estimate the size of the affected market, including: loss of passenger traffic, most affected countries, airlines and aircraft. 3.1 Currently banned routes across Europe France currently has the most definite basis on this environmental issue across the European Union through the implementation of the climate and resilience law. For this reason, the impact on the territory's air services exists in 3 connections. Based on the current infrastructure offer on national railway networks and the conditions of the decree, the actual restricted connections where a switch between air and rail has been arranged are the following [17]: ● Paris-Orly (ORY) - Bordeaux (BOD) ● Paris-Orly (ORY) - Nantes (NTE) ● Paris-Orly (ORY) - Lyon (LYS) The restriction in France involves only 0.002% of the total number of flights in the country. A representation depicting the distribution of number of flights in France by route type (domestic affected, domestic unaffected and international) is shown as a graph (Figure 3.1) in IATA's recent publication, French domestic flight bans and carbon emissions reductions. Route Analysis 15 Figure 3.1: Number and proportion of flights in France by route type (2014-2023) [18]. These connections are served by the French high-speed TGV train service. Passengers on these flights travel to Montparnasse stations for trips to Bordeaux and Nantes, whilst for Lyon, routes originate at the corresponding station. Each route is domestic and, as Paris-Orly airport is close to the city centre, transfer to Montparnasse and Lyon stations is short. Therefore, travel time is less than 150 minutes including transfer to both stations and trip time. On the following representation in Figure 3.2, railway lines from Montparnasse and Lyon stations are shown, permitting a direct connection in replacement of ORY-BOD, ORY-NTE and ORY-LYS flights. The track connecting the centre of Paris to Lyon, although not being a high-speed line from the outset, speeds over 200 km/h are reached later in the journey (see Figure 3.2 and Table 3.3), making it possible to cover the distance under the time limit. Figure 3.2: Representation of speeds on French railway lines at Montparnasse and Lyon stations [19] Connectivity between routes from Paris-Orly to the cities of Nantes, Rennes, Lyon and Toulouse, among others, is visualised below. Most routes with a high-speed rail connection are located in the central and northern part of France, where a high density around Paris plays a 16 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe key role in the interconnection between the cities. Such a network has made it possible to implement the climate strategy and reduce reliance on short-haul domestic flights. Figure 3.3: Connectivity of Paris to Bordeaux, Nantes, Rennes and Lyon via TGV high-speed lines [19] With the report How fast are rail trips between EU cities and is rail faster than air? and based on data provided by the EU's urban and regional policy department, there are 12 direct rail connections from Paris with speeds above 150 km/h [15]. Among these are the three flights banned by the regulation given in red in Table 3.3. City A (larger) City B (smaller) Straight-line speed (km/h) Paris Bordeaux 239 Paris Strasbourg 225 Paris Tours 206 Paris Lyon 203 Paris Rennes 203 Paris Lille 190 Paris Brussels 190 Paris Nancy 186 Paris Karlsruhe 176 Paris Reims 169 Paris Nantes 167 Paris Grenoble 160 Table 3.3: Road connections from Paris with speeds above 150 km/h [15] Route Analysis 23 Figure 3.11: Distribution of seating capacity on potentially constrained routes by 20-minute flight time interval At the same time, the small proportion of widebody aircraft offers the highest monthly frequencies on specific routes where they operate jointly with narrowbodies. These include Barcelona-Madrid with 424 flights per month and Frankfurt-Munich with 286 trips per month. (Figure 3.12) Figure 3.12: Monthly frequency distribution by aircraft type on routes below 500 km 3.3 Out of scope, potentially restricted and banned routes comparison The comparative analysis between actual substituted routes and potentially restricted flights under 500 km is complemented by the so-called “out-of-scope” routes. This category, which 24 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe addresses flights with distances greater than 500 km, involves those flights that will rarely be under the exposure of any type of restriction in the short term. Using the information extracted from FlightConnections on flight distances and carriers, Figure 3.13 provides a comparison of the number of routes operated per airline according to the status of the route. On the list of 76 airlines, those with a total of less than 20 routes have been discarded. Among 25 countries and 2532 intra-European routes, the airline with the largest market share is Ryanair (939 routes) followed by Wizz Air (290, in total). As expected, the spectrum of routes outside the scope of forthcoming regulations occupies almost 90% of the total routes analyzed, versus the 9.8% of potentially restricted routes (radius less than 500 km) and barely 0.2% of substituted flights. Regarding potentially regulated routes, the fact that some airlines have a significant proportion of flights in relation to the total indicates that their operations are exposed to a regulation which would reduce the offer of intra-European routes. This is the case for Air Dolomiti, with 40% (14 out of 35) of routes at potential risk, 37% for Iberia (28 out of 76), 34% for ITA AirWays (13 out of 38) and 31% for Lufthansa (48 out of 156), which covers a large part of the domestic routes in its densely populated territory. The difficulties in distinguishing the banned routes are due to the low 0.2% which is made up of trips previously operated by Air France, Austrian Airlines and KLM. Figure 3.13: Number of routes by airline and route status. Then, Figure 3.14 compares the seating capacity according to the aforementioned route classification. First, the median seat capacity on potentially constrained routes is close to 150 seats, suggesting an average capacity associated with narrowbody aircraft. Unlike the out of scope routes, variability in this route category (ranges less than 500 km) is notable, as well as the appearance of outliers on routes with up to 350 seats offered. Seat capacity values on “out-of-scope” routes range from 160-190 and denote less variability and greater consistency. Banned routes seat capacity distribution shows a similar shape to the first group, which is due to the fact that the distances of the 5 restricted flights are also shorter than 500 km. Route Analysis 25 Figure 3.14: Comparison of seating capacity by route status: Out of reach, Potentially restricted and banned. A distinction is made in Table 3.8 between domestic and cross-border in the EU, translating into the following conclusion: For flights with a radius of less than 500 km, the proportion of domestic routes is greater than cross-border ones. Domestic Cross-Border Total Banned 4 1 5 Out of Scope 181 2048 2229 Potentially Restricted 160 138 298 Table 3.8: Number of domestic and cross-border routes according to route status 26 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe 4 The need for replacement of conventional aircraft With the goal of decarbonising the aviation industry without losing market share on ultra-short-radius substituted routes, the solution of replacing traditional propulsion with electric propulsion is emerging. As mentioned in the European Union's Green Deal, by 2050, vehicles are expected to run on sustainable energy sources such as electricity and hydrogen [1]. The latter type of propulsion, as will be discussed below, will prevail on short routes in the near-medium term, as will sustainable aviation fuels (SAF) for long-haul flights, at the expense of the use of fossil fuels. Within the scope of the implementation of electric aircraft in the civil aviation industry, airport configuration and energy infrastructure will need to be adapted to ensure both their energy security and support the operations of all types of aircraft and propulsion systems. In this section, electric vertical take-off and landing aircraft (eVTOLs) and their respective technology will be defined, as these will be the means of assessment of traffic recovery on the replaced route pairs (ORY-BOD, ORY-NTE, ORY-LYS, AMS-BRU and VIE-SZG). Also, the analysis will take into account the feasibility of short-term implementation according to the timeline of technological development of electric vehicles. Thus, it will be possible to forecast the time horizon regarding the entry of these electrically propelled vehicles. With all this information, according to the prediction on the entry forecast in the industry and the understanding of their functioning, key operational parameters of these aircraft that may allow or limit their use for journeys shorter than 500 km will be determined. Finally, having defined the operational parameters that are likely to condition their introduction to the market, a series of electric vertical take-off and landing aircraft will be selected to operate those pairs of routes that are currently performed by rail services. 4.1 Introduction of eVTOLs as a sustainable solution An electric vertical take-off and landing aircraft, also known as eVTOL, ‘implies an aircraft capable of carrying up to 5 people that may or may not include a pilot if operated fully autonomously, assuming an average of 91 kg (200 lbs) per person or equivalent payload and a range of up to 60 miles plus adequate reserve’ [27]. Therefore, the main difference between a conventional take-off and landing aircraft (CTOL) and an eVTOL is that the latter allows vertical take-off and landing by means of electric propulsion. In this sense, by not requiring conventional means (e.g., long runways, hangars), eVTOL aircraft are easier to apply in urban air mobility and allow operations in smaller spaces than conventional aircraft. Developing and applying this energy source in aviation aims to decarbonise air transport and promote urban air mobility (UAM). In this sense, a market analysis will be carried out on the current state of eVTOL aircraft, knowing the conceptual and developing models and contrasting a series of characteristics to assess the implementation of eVTOL vehicles on currently restricted routes. Despite their recent introduction into production, by the second quarter of 2022, more than 500 eVTOL aircraft concepts have been presented with a view towards flying and occupying a The need for replacement of conventional aircraft 27 relevant position in the industry in the short-medium term. As we will see below, although regulatory and technological constraints to market inclusion exist today, the cumulative distribution of eVTOL aircraft in the period of 2014-2019 has been exponential (see Figure 4.1). This fact, together with the market prospects for electrically powered aviation (see Section 4.1.1), demonstrates how the air transport industry's intention to decarbonise its operations and regain traffic on ultra-short-haul routes is to transition from conventional to electric propulsion. Figure 4.1 : Number of eVTOL aircraft announcements per year from 2014-2019 period [28]. 4.1.1 eVTOL development timeline While the idea of air transport consisting mainly of electric aircraft may be unrealistic, in practice, the transition to this energy source is not far from reaching its full realization. Moreover, the market outlook for electric aviation, with a view toward achieving zero emissions by 2050, is promising and shows that the solution of eVTOL aircraft on ultra-short routes is not theoretical but applicable in the near future. According to IATA's Aircraft Technology Roadmap to 2050 (Figure 4.2), in an optimistic scenario, the entry into service of hybrid aircraft with 10 to 15 seats would be around 2030, reaching capacities of 50 to 100 passengers on regional flights from 2030 to 2035 [29]. On the other hand, the entry of battery-powered aircraft could operate commercial flights with a seating capacity of more than 100 passengers and a range of 300 km from 2035 onwards. While at present air taxis with capacities of less than 5 passengers are the only ones available, progress in adoption will be conditioned by advances in battery technology (see section 4.1.2). 28 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe Figure 4.2: Electric-powered aviation market outlook (optimistic view) [29] On the other hand, according to the results of a 2018 survey among 40 industry professionals under an optimistic scenario (Figure 4.3), the entry into service of all-electric aircraft would be possible by 2025. [30] Figure 4.3 : Results of the Roland Berger survey of aerospace and defence professionals [30]. As a result, a number of existing eVTOL aircraft will further be selected in order to analyse the feasibility of their operation in the segment of routes banned under the emission reduction regulation. 4.1.2 Technology and operational constraints eVTOL aircraft are electrically propelled thanks to energy storage technologies, such as batteries. On the propulsion side, electric propulsion is powered by electric motors. These batteries are made up of cells which, together, consist of a cathode, an anode and an electrolyte [31]. Unlike conventional propulsion systems that store energy in the fuel, electric propulsion systems store energy in the battery to drive the electric motor. The need for replacement of conventional aircraft 29 The most widely used battery for electric aircraft and eVTOLs today is the lithium-ion battery because it offers the highest specific energy density values to date. Currently, lithium-ion batteries in use have a specific energy density of 250-300 Wh/kg. These values are fractional compared to the densities of fuels used in conventional aircraft up to 12000 Wh/kg [31]. For this reason, the specific energy density of batteries is one of the most constraining factors for the deployment of medium and long-range operations, since the distances that aircraft can operate will be conditioned by having as much energy and as little mass as possible to fly (Wh/kg). Such is the case in fact, that the relationship between battery specific energy density and operating distance range is presented in the report Performance Metrics Required of Next Generation Batteries to Electrify Vertical Takeoff and Landing (VTOL) Aircraft (Figure 4.4) [32]. It shows that the operating distance range increases linearly with increasing specific energy of the batteries. Also, for the same specific energy density values, the range increases with gross takeoff mass as long as the battery can support the weight increase. Figure 4.4: Relationship between battery specific energy and operating distance range for 1-passenger eVTOLs, based on gross take-off mass (GTOM) values and parameters with poor, standard and ideal performance values [32]. In this sense, with these values, propulsion is limited to small aircraft capable of operating distance ranges of less than 300 km. Therefore, these types of batteries currently allow urban air mobility with limited passenger capacities. On the other hand, the technical study on electric aviation in 2022 suggests that minimum specific energy densities of 500 Wh/kg will be required to operate regional and short-range flights [31]. Following this, the electric motor is responsible for the conversion of electrical energy into mechanical energy. The major benefit of these in comparison to the combustion engine is their high efficiency. Whereas combustion engines have efficiency values of less than 50%, electric motors are close to 95%[31]. In conclusion, one of the most restraining variables that will be encountered in the viability of implementing eVTOL aircraft on the banned series of routes will be the specific energy density of the batteries, as well as their weight. These factors, as mentioned above, condition both the operating range of the aircraft (in terms of distance) and the seating capacity. Seating capacity 30 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe can be constrained by the weight of the battery, because it affects its maximum gross take-off mass and consequently the type of aircraft that can be operated. Finally, the life cycle of the battery will also have to be taken into account. 4.1.3 Design architecture eVTOL aircraft can be classified into two categories: wingless eVTOL aircraft and eVTOL powered lift aircraft (with wings and active lift). This categorisation is represented in Figure 4.5 by dividing the aircraft types according to propulsion architectures. Figure 4.5: Propulsion architectures for eVTOL aircraft [28] Firstly, wingless aircraft are mostly multicopters. In these, rotors are used exclusively for lift and propulsion. Rotors provide the vertical lift, and at the same time allow horizontal displacement when the vehicle is tilted. Thus, multicopters do not have specific components for horizontal propulsion. An example of a multicopter eVTOL aircraft is the EHang 216 (see Figure 4.6). Figure 4.6: EHang 216 eVTOL aircraft [33] On the other hand, eVTOL aircraft with wings, known as powered lift, are also available. Winged aircraft can achieve higher cruise speeds than multicopters and consequently, allow higher payloads and longer distance routes to be operated. Independent thrust aircraft have dedicated rotors for vertical lift and a separate propulsion system specifically for horizontal displacement in the cruise phase. An example of this type is the Wisk Generation 5 as illustrated in Figure 4.7a. The need for replacement of conventional aircraft 31 Thrust vectored aircraft use a propulsion system that allows vertical lift and horizontal displacement. The Jetoptera J-2000 is an example of this category (see Figure 4.7b). Figure 4.7: Aircraft eVTOL powered lift (a) Wisk Generation 5 (independent thrust) [34] (b) Jetoptera J-2000 (vector thrust) [35] Lastly, there is the combined thrust category which, as shown in the propulsion architecture categorisation (Figure 4.5), combines thrust vectoring for some propulsion units, while others focus their performance solely on vertical lift. 4.2 eVTOLs selection Following the definition of the current growth in eVTOL aircraft, their technology and the possible architecture designs, a number of different aircraft will be presented below, all meeting a common requirement: batteries as a power source. While aircraft under development with hydrogen as a power source could also have been considered due to their net zero emissions, the aim of the study is to analyse the feasibility of implementation of these aircraft, ranking them according to different scenarios that will be presented later. Having said that, all the data has been extracted from Electric VTOL News [36] and the most relevant parameters to be taken into account for the selected aircraft will be showcased. Subsequently, based on these parameters, the most favourable candidates for a given banned route will be chosen. Next, we will proceed with a brief description of the 11 selected aircraft. Firstly, this eVTOL aircraft has no specific name and belongs to Kelekona’s company (see Figure 4.8). The Unnamed eVTOL is a passenger, cargo and can even be used for medical and military services. For passenger service, it will be pilot-operated, in contrast to the cargo service, which will be remotely piloted. It has a seating capacity of 40 passengers (excluding the pilot) and a maximum payload of 4536 kg. Both the cruise speed and flight time are unknown due to the fact that the design is at an early stage. At the same time, it has a range that would allow it to operate routes of up to 483 km. As mentioned above, its power source is batteries[37]. 32 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe Figure 4.8: Unnamed eVTOL developed by Kelekona [37] Illini Air Shuttle Second eVTOL is the so-called Illini Air Shuttle developed by students at the University of Illinois (see Figure 4.9). Maturity of the design of this aircraft is at an early stage, as is the case with the aforementioned aircraft. It is considered to be a eVTOL passenger aircraft in which the means of piloting (‘pilot or autopilot mode’) has not been defined. Its seating capacity is 10 passengers and its cruising speed is 290 km/h[38]. Flying time is 1 hour and the estimated range of the design is 321 km. Regarding the relevant weight characteristics of the aircraft, the empty weight is 3632 kg, maximum payload is 1250 kg and its maximum take-off weight (MTOW) is 4882 kg, being propelled by 10 electric motors. [38] Figure 4.9: Illini Air Shuttle eVTOL from the University of Illinois [38] Genesys X-2 The next eVTOL aircraft according to developer Skynet Project SRL can be developed as a passenger aircraft with an all-electric or hybrid-electric source at customer’s convenience. The Genesys X-2 eVTOL aircraft (see Figure 4.10) accommodates a capacity of 8 passengers with Decision making process 39 taken into account. Thus, selecting the most suitable eVTOL aircraft capable of operating a route according to its characteristics and needs can be a complex decision. This being said, it is necessary that we rely on a structured system such as the AHP which allows us to assess different aircraft with specific quantitative parameters in a consistent framework. This procedure will then be used to normalise quantitative values of different scales and, according to the weights assigned in the several existing criteria, numerical scores will be obtained for each aircraft. One of the benefits of using the AHP process for complex decisions is that the criteria can be weighted in a way that meets the operational and technical needs of any given route. As an example, depending on the distance of a route, a criterion such as range will be weighted higher (over longer distances), whereas if the route needs are different, perhaps parameters such as cruising speed will be preferred. 5.1.3 AHP methodological framework for the eVTOLs selection process The analytical hierarchical process (AHP), as its name describes, is defined by a structured hierarchy permitting the formation of a network via which connections are made between the nodes. As shown in Figure 5.1 below, the hierarchy of the network is based on 3 clusters that make up 3 levels: the objective, the criteria to be assessed and weighted with respect to how decisive they are with respect to the goal, and the alternatives. Therefore, the hierarchy of the analytical hierarchical process used in the case studied is as follows: ● Goal: Select the best eVTOL for scenario X ● Criteria: Criterion 1 , Criterion 2 , Criterion 3 , Criterion 4 , Criterion 5 ● Alternatives: , , , , , … 𝐴1𝐴2𝐴3𝐴4𝐴5𝐴𝑛𝐴11 This means that the target node is the parent node of each of the criterion nodes (these being the child nodes of the target node). Meanwhile, the criterion nodes are the parent nodes of the alternatives nodes (and these will be the child nodes of the criterion nodes). Figure 5.1: AHP Process hierarchy levels and structure definition According to the model presented above, the objective cluster contains the node ‘Select the best eVTOL for scenario X’. Then, the criteria cluster contains the 5 nodes for each criterion. Lastly, the alternatives cluster contains 11 nodes corresponding to each candidate. 40 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe The hierarchical model allows all nodes at different levels to be fully connected to each other. For this reason, pairwise comparisons will be made between criterion nodes to determine the relative importance for each of them in relation to the objective of the problem. Following the same procedure, due to the connection of each criterion node with each alternative node, specific data (related to the criterion) from each candidate will be used to compare each other and to assign a weight of importance in relation to the criterion node. Scenarios that will be examined in this AHP model deal with routes that are constrained and discussed earlier in the project. As mentioned on numerous occasions, restrictions apply to a set of 5 ultra-short haul intra-European routes. In this sense, and for the sake of simplicity, this process will only be performed for 2 scenarios, i.e. for 2 routes. These routes to be analysed in the model are: Vienna-Salzburg (VIE-SZG) and Paris-Orly-Bordeaux (ORY-BOD). These two routes were selected for a specific reason. As for the analysis of the 3 banned routes from Paris-Orly, the most restrictive route has been chosen because of its length, this route being the longest of the 5 restricted routes. Similarly, the analysis of the Amsterdam - Brussels (AMS-BRU) has been discarded due to the fact that only the frequency of flights has been reduced from 5 to 4 per day, being the impact of the loss of traffic on this route less significant than the entire substitution of the VIE-SZG route. Scenario 1 of the study will be the Vienna-Salzburg route and, as explained above, both the flight distance and the previous operations as well as the past traffic is a relevant input to be taken into account in the AHP model. Flight distance is 268 km and was previously operated 2 to 3 times per day with a total of 909 annual flights. Secondly, the Paris-Orly-Bordeaux (ORY-BOD) route covers a distance of 493 km and was previously operated 6 to 7 times a day representing a higher demand. The AHP process architecture and hierarchy is shown in the figure below. Assisted by the SuperDecisions software, the analytic hierarchical process structure for Scenario 1 is as follows (Figure 5.2): Figure 5.2: AHP Process hierarchy on Vienna-Salzburg route with SuperDecisions tool Decision making process 41 After defining the criteria upon which the alternatives will be selected, pairwise comparison matrices are used to assign the weights of each criterion in relation to the purpose of the study. To create these matrices, each criterion will be compared against the other criteria using the Saaty scale. This scale, which comprises values from 1 to 9, assigns values based on the importance of one criterion over the other. That is, if criterion 1 is 9 times more important than criterion 2, this value will be assigned in the corresponding row and column of the matrix. Inversely, when C2 is compared to C1, the reciprocal value to the one mentioned above will be assigned, being 1/9. Those values are not arbitrary and they are ranked in the following table (Table 5.1) comprising the values of intensity of importance from 1 to 9. The Saaty scale is useful because it allows converting qualitative preferences into numerical values. For example, if the pair of criteria being compared contribute equally to the objective and are of equal importance, the value assigned is 1. If the judgement determines a subtle or moderate preference between two criteria, a 3 is assigned. Finally, if there is a demonstrated and extreme dominance in favour of one criterion, the values 7 and 9 should be used; alternatively, intermediate values are used when intermediate judgements are made between those defined above. Table 5.1: Saaty's fundamental scale for AHP process [48] For matrix completion, the AHP process assumes that the comparisons are reciprocal in order to obtain robust and consistent preferences. Thus, if C1 > C3 = 5, the inverse comparison implies that C3 > C1 = ⅕ in order to maintain a consistent matrix.. To get consistent results, it is crucial to ensure that the ratings entered in the matrix are consistent (e.g. if C2 > C3 > C4, then C2 > C4). Via the Saaty scale, the consistency of the matrix can be assessed through various useful parameters such as the Consistency Index (CI) and the Consistency Ratio (CR). The desired consistency ratio should be less than 10%. [48] Both indices will be calculated after defining the pairwise comparison matrices between criteria for both scenarios. 5.2 Criteria Definition and Pairwise Comparison Hierarchies within the analytical hierarchical process structure have been previously defined by levels, which are differentiated 3 times. The aim of the analytical hierarchical process has been defined, and is to select the most suitable eVTOL to operate within 2 different scenarios. All 11 42 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe alternatives are the candidates that, according to the results given in the form of a ranked score list, intend to operate the VIE-SZG and ORY-BOD routes. Hence, it remains ultimately to define the criteria that will be taken into account for each alternative in relation to the target node. In addition to the definition of these criteria, a pairwise comparison matrix will be created. This matrix, consisting of assigning weights using the Saaty scale, will allow comparing pairs of criterion based on their relative importance or contribution to the main objective. Values for each criterion in each alternative have been quantified in Table 4.1. In other words, in addition to obtaining the weights of each criterion in relation to the main objective through the criteria pairwise comparison matrix, quantitative data will be used together to obtain the ranked score for each eVTOL candidate. 5.2.1 Criteria for eVTOL selection The first criterion of the analytical hierarchical process (AHP) to be taken into account is the seating capacity. This criterion is related to the number of passengers the aircraft can accommodate on the flight and has a non-negligible impact on meeting the demand for the route. From an airline's point of view, an aircraft that has the capacity to operate the route with a higher seating capacity would satisfy the past traffic demand with less frequency resulting in a cost reduction that should be taken into account. The second criterion is the maximum take-off weight (MTOW) and represents the combined sum of the operating empty weight (OEW), maximum payload (passengers, cargo) and the trip fuel. In other words, it is the maximum weight at which the aircraft can take off including all the weights mentioned above. Third criterion is the maximum payload and refers to the total weight that the aircraft can carry including passengers, passenger cargo and other cargo. In this sense, a greater or lesser maximum payload depends on the traffic and demand generated on the specific route but also on the capacity of the aircraft to accommodate the load. The fourth criterion is the range. This means the maximum distance without battery recharging stops in our electric-powered aircraft case study. With this parameter it is possible to determine whether the aircraft can complete the mission without intermediate stops for recharging, a fact that would considerably increase the flight time. Lastly, the final criterion is the aircraft cruising speed. This characteristic is considered in the study due to its importance in competitiveness and operational efficiency, since an eVTOL aircraft capable of reaching higher average cruise speeds allows routes to be operated in less time while offering a greater frequency and offer of flights. To summarise, within the analytical hierarchical process we identify 11 candidates that will be rated according to the weight of each criterion in relation to the main objective, and the performance of each candidate in each criterion by means of quantified and normalised data. The selection criteria are 5 and are as follows: seating capacity, maximum take-off weight, maximum payload, range and cruise speed. 5.2.2 Pairwise comparison matrices As mentioned above, depending on the context of each route, characteristics of the same and variables such as traffic prior to the ban, relative importance has been assigned among the Decision making process 43 criteria for each scenario. These matrices should maintain a consistency ratio of less than 10%. In this sense, Table 5.2 depicts the pairwise comparison matrix between criteria for the restricted route between Vienna and Salzburg. Criteria C1: Seat Capacity C2: MTOW C3: Maximum Payload C4: Range C5: Cruising Speed C1: Seat Capacity 1 3 5 1/3 1/2 C2: MTOW 1/3 1 3 1/5 1/3 C3: Maximum Payload 1/5 1/3 1 1/7 1/5 C4: Range 3 5 7 1 3 C5: Cruising Speed 2 3 5 1/3 1 Table 5.2: Pairwise comparison matrix between criteria for scenario 1: Vienna - Salzburg Additionally, Table 5.3 shows the matrix for the second case study, which corresponds to the Paris-Orly-Bordeaux route. Criteria C1: Seat Capacity C2: MTOW C3: Maximum Payload C4: Range C5: Cruising Speed C1: Seat Capacity 1 4 5 1/3 5 C2: MTOW 1/4 1 3 1/5 2 C3: Maximum Payload 1/5 1/3 1 1/7 1/3 C4: Range 3 5 7 1 7 C5: Cruising Speed 1/5 1/2 3 1/7 1 Table 5.3: Pairwise comparison matrix between criteria for Scenario 2: Paris-OrlyBordeaux 5.2.3 Justifications for pairwise comparisons The two pairwise comparison matrices have been displayed with the assigned weights determined using the Saaty scale. Given knowledge of the domestic nature of both the Vienna-Salzburg and the Paris-Orly-Bordeaux routes, the estimated traffic, past frequency and flight distance information has been used to consider the values of the two matrices. 5.2.3.1 Scenario 1: Vienna-Salzburg As for Scenario 1, we first compare the first criterion with all other criteria. Seating capacity is moderately more important than maximum take-off weight because a demand has to be met where historically 74 passengers were flown on board. Although maximum take-off weight is of structural relevance, from the airlines' point of view, the criterion that has an impact on 44 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe operations is given preference. When comparing C1 vs. C3, seat capacity is strongly more important than the maximum payload value because variables such as the passenger load on such a short route are less relevant. Comparing seating capacity with range, the fourth criterion (range) is moderately more important. The rationale is simple, unless an aircraft has a range of more than 268 km (non-stop) it would not be possible to operate the route even if the aircraft offers a large number of passengers. In this sense, the range criterion is a crucial element in order to be able to resume this route. Finally, the first criterion is assigned a value of ½ compared to the cruise speed. The reason behind this is that high cruising speeds, for this ultra-short route scenario, would compensate for a low seating capacity while increasing the frequency. Since traffic and demand prior to the cancellation of this route was lower than scenario 2, it would be feasible to resume this route by increasing frequencies with a lower seat capacity since only 2 to 3 flights per day were operated. Maximum take-off weight is moderately more important when compared to the maximum payload value. Although payload must be taken into account, on this least demanding route it is intended to operate the flights with a lower seating capacity and, consequently, with a lower number of bags or cargo. If we compare C2 (MTOW) vs C4 (range), the latter is extremely more important since range is essential for the operational viability of the route. C2 vs C5 implies that cruise speed is moderately preferable, since MTOW is relevant in aircraft design as opposed to cruise speed which plays a dominant role in route viability. Maximum payload is 7 times lower than the range. Similar to previous arguments, if the aircraft cannot complete the route, the maximum allowable payload value is irrelevant. If we compare C3 with the cruise speed, the latter is strongly more important than the third criterion. Ultimately, range is moderately important with respect to cruising speed. Although the last criterion enhances competitiveness and helps in the implementation of the route, range is the most critical criterion. 5.2.3.2 Scenario 2: París-Orly-Bordeaux In contrast to the previous route, Scenario 2 is 493 km in distance and the passenger demand is higher with an average of 120 passengers per flight and 6 to 7 daily flights. These differences in characteristics are taken into account in the second pairwise comparison matrix for the ORY-BOD route. For this case, a justificatory comparison table setting out the differences in weights assigned between criteria for Scenario 1 and Scenario 2 is shown below (Table 5.4). Decision making process 45 Comparison Scenario 1 Scenario 2 Justifications C1 vs C2 3 4 Seating capacity is slightly more critical due to higher demand C1 vs C3 5 5 Maximum payload value remains secondary C1 vs C4 1/3 1/3 In both scenarios range remains moderately dominant over seating capacity C1 vs C5 1/2 5 Cruise speed loses importance due to the passenger seating needs which are crucial on long, high-traffic routes C2 vs C3 3 3 MTOW preserves relevance to maximum payload due to the higher weight of the battery packs for the route. C2 vs C4 1/5 1/5 Range is strongly dominant over MTOW C2 vs C5 1/3 2 MTOW is substantially more important relative to cruise speed in the second scenario, due to the more restrictive empty operational weights C3 vs C4 1/7 1/7 Range is very strongly dominant over maximum payload C3 vs C5 1/4 1/3 A reasonable speed is moderately more necessary to meet demand with sustainable frequency on a route of almost 500 km C4 vs C5 3 7 Range expands the preference towards this scenario due to the 468 km distance and the restrictions on battery life. Table 5.4: Comparison and justification of ratings of the pairwise comparison matrices in both study scenarios 5.3 AHP Calculations for Each Scenario In this section, weights , , , , will be calculated. Then, these weights will be 𝑊1𝑊2𝑊3𝑊4𝑊5 used in each scenario to, together with the normalised quantitative values of each candidate, generate the ranked list of scores in order of priority. For this purpose, the method used is the principal eigenvalue method. Through the pairwise comparison matrix, both the principal eigenvalue and the priority vector are calculated. This priority vector provides the weights of each criterion in relation to the target node according to quantitative considerations using the Saaty scale. Based on this, the consistency index (CI) and the consistency ratio (CR) will be computed. Results obtained will be compared and checked 46 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe with the SuperDecisions tool. This is a free downloadable software that is available online and allows to perform the analysis of this AHP process. 5.3.1 Scenario 1: Vienna–Salzburg First, the following characteristic the equation must be solved, where is the pairwise 𝐴 comparison matrix. At the same time, is the priority vector (eigenvector) and corresponds to 𝑤 eigenvalues (eigenvalues). To obtain the eigenvector, the largest eigenvalue is required . λ𝑚𝑎𝑥 (5.1) 𝐴·𝑤=λ𝑚𝑎𝑥·𝑤 being the prior pairwise comparison matrix for this scenario: 𝐴 The characteristic equation is solved, where I is the identity matrix and is subtracted on the λ diagonal of : 𝐴 (5.2) The characteristic polynomial calculated from the determinant allows us to calculate the eigenvalues of the matrix. −λ5+5λ4+2993λ2 630 +5168λ 4725 +1952 4725 =0 λ𝑚𝑎𝑥=5. 1851, λ2=0.0384,λ3=0. 0384, λ4= −0. 1309, λ5= −0. 1309 Since is the largest eigenvalue of the matrix, the following equation is solved to calculate λ𝑚𝑎𝑥 the corresponding eigenvector: (5.3) Resulting in the principal eigenvector: 𝑤= 0.75748, 0.36556 , 0.18137, 1.99999,1 [ ] In order to convert the principal eigenvector into the vector of priorities (weights), it is normalised. So, we normalise the eigenvector: 𝑤= 0.17598, 0.08493 , 0.04214, 0.46463,0. 23233 [ ] Recalling the criteria order, being C1: seating capacity, C2: MTOW, C3: Max Payload, C4: Range and C5: Cruise Speed, priorities and weights of each criterion in relation to the objective is: Decision making process 47 ● Weight of seating capacity criterion: 𝑊1= 0. 17598 ● Weight of MTOW criterion: 𝑊2= 0. 08493 ● Weight of Maximum Payload criterion: 𝑊3= 0. 04214 ● Weight of Range criterion: 𝑊4=0. 46463 ● Weight of Cruising speed criterion: 𝑊5=0. 23233 With the help of the SuperDecisions tool, the weights assigned for the Vienna-Salzburg route match with precision and are shown in Figure 5.3. Figure 5.3: Weights assigned to the criteria of Scenario 1 of the AHP process using the eigenvalue method. To validate the consistency of the matrix, the consistency index and ratio are checked. The former is calculated as follows, taking into account the largest eigenvalue and the number of criteria of the problem (being ). 𝑛 (5.4) 𝐶𝐼=λ𝑚𝑎𝑥− 𝑛 𝑛−1 =5.1851− 5 5−1 =0.046275 As mentioned before, for a matrix to be valid and for its weights to be coherent and consistent, we must analyse the consistency ratio, which must be less than 10%. To do so, a random index created by Saaty, for matrices of order , will be needed (see Table 5.4). 𝑛 Table 5.5: Random consistency index (RI) for matrices of order [48] 𝑛 (5.5) 𝐶𝑅=𝐶𝐼 𝑅𝐼 =0,046275 1,11 = 0. 0417 𝐶𝑅 (%)= 4.17%<10% The pairwise comparison matrix and the values assigned according to the Saaty scale are consistent and allow further study and analysis of eVTOL aircraft selection for the Vienna-Salzburg route. 5.3.2 Scenario 2: Paris Orly-Bordeaux A parallel approach is then followed for the second scenario. In this case, the matrix is shown in Table 5.3. Following the same procedures, the eigenvalues are obtained: λ𝑚𝑎𝑥=5. 2664, λ2=0.0225,λ3=0. 0225, λ4= −0. 1557, λ5= −0. 1557 With the largest eigenvalue and the pairwise comparison matrix, the principal λ𝑚𝑎𝑥=5,2664 eigenvector is : 𝑤= 3.76673, 0.60004 , 1.43764, 6.95874,1 [ ] 48 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe To convert the main eigenvector into the vector of priorities (weights), the vector is normalised. Therefore: 𝑤= 0.27368, 0.0436 , 0.10446, 0.50561,0. 07266 [ ] Just as earlier, priorities and weights of each criterion in relation to the objective are verified with the SuperDecisions programme to ensure that the weights are accurate. = 𝑊10.27368 , 𝑊2= 0.10446 , 𝑊3= 0.04360, 𝑊4=0.50561,𝑊5=0.07266 Figure 5.4: Weights assigned to the criteria of scenario 2 of the AHP process using the eigenvalue method Concerning the Paris-Orly-Bordeaux route, the pairwise comparison matrix is also consistent as it is clearly below the 10% threshold. These are the calculations: (5.6) 𝐶𝐼=λ𝑚𝑎𝑥− 𝑛 𝑛−1 =5.2664− 5 5−1 =0,0666 (5.7) 𝐶𝑅=𝐶𝐼 𝑅𝐼 =0,0666 1,11 = 0. 05946 𝐶𝑅 (%)= 5.946%<10% To sum up, the relative weights of the criteria in relation to the importance given to the target node have been obtained. Despite slight differences between the matrices due to the context of each route, the most fundamental aspect is that both matrices are consistent. Application of AHP Results to eVTOL Selection 55 selection process is rigorous and objective. For each candidate, the score is calculated as follows: (7.3) 𝑇𝑜𝑡𝑎𝑙 𝑆𝑐𝑜𝑟𝑒=∑(𝐴𝑑𝑗𝑢𝑠𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒·𝐶𝑟𝑖𝑡𝑒𝑟𝑖𝑜𝑛 𝑊𝑒𝑖𝑔ℎ𝑡) 𝑇𝑜𝑡𝑎𝑙 𝑆𝑐𝑜𝑟𝑒=(𝐶1·𝑊1)+(𝐶2·𝑊2)+(𝐶3·𝑊3)+(𝐶4·𝑊4)+(𝐶5·𝑊5) Where: ● C1, C2, C3, C4, C5 are the normalised quantitative values for each criterion. ● , , , and are the weights of each criterion. 𝑊1𝑊2𝑊3𝑊4𝑊5 7.2.1 Ranked score list for Scenario 1 with 9 and 11 alternatives The scores of each eVTOL aircraft were determined for the Vienna-Salzburg and Paris-Orly-Bordeaux routes following the entire analytical hierarchical process. Through a pairwise comparison of criteria, the weights ( , , , and ) could be defined and, 𝑊1𝑊2𝑊3𝑊4𝑊5 in combination with the normalised quantitative data for each criterion (C1, C2, C3, C4, C5) in each alternative, the decision making process has yielded the following results. These are shown in Table 7.4 and displayed graphically in Figure 7.1 with the SuperDecisions tool. The normal weighted score is the one computed using 7.3 described above. The ideal weighted rating, on the other hand, takes the highest normal weighted score as a reference (1 being ideal, or 100% analogously). For the other scores, each normal weighted score is divided by the highest normal weighted score in the data list. Thus, if A1 is the ideal alternative it will represent a 1 and if A3 is 0.55 of the ideal value, this means that this candidate's score is 55% from that of candidate A1. 56 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe VIE-SZG (11 candidates) VIE-SZG (9 candidates) Alternatives Ideal W Normal W Ranking Ideal W Normal W Ranking Unnamed Evtol (A1) 1 0.219234 1 - - - Illini Air Shuttle (A2) 0.546002 0.119702 3 1 0.221807 1 Genesys X-2 (A3) 0.502381 0.110139 5 0.853739 0.189366 2 Trinity (A4) 0.574552 0.125961 2 - - - eTransporte r(A5) 0.339498 0.074429 7 0.537165 0.119147 5 S4 (A6) 0.253341 0.055541 8 0.322289 0.071486 6 Mistral (A7) 0.464606 0.101858 6 0.625267 0.138689 4 Prosperity (A8) 0.151071 0.033120 10 0.192425 0.042681 8 ET9 001(A9) 0.204228 0.044774 9 0.267448 0.059322 7 Bartini (A10) 0.001267 0.000278 11 0.003801 0.000843 9 GJet Kóan (A11) 0.524391 0.114964 4 0.706282 0.156659 3 Table 7.5 Ranked score list for Scenario 1 with 9 and 11 alternatives. Figure 7.1: (a) Ideal score list for Scenario 1 with 11 alternatives (b) Ideal score list for Scenario 1 with 9 alternatives by means of SuperDecisions tool. Application of AHP Results to eVTOL Selection 57 7.2.2 Ranked score list for Scenario 2 with 3 alternatives Again, the list of weighted scores is obtained and presented in the form of a ranking. This time, for Scenario 2 (ORY-BOD), consideration is given only to the 3 candidates who are able to meet the operational requirements of the route. Table 7.5 below shows these results: París-Orly–Bordeaux Alternatives Ideal W Normal W Ranking Unnamed eVTOL (A1) 0.629786 0.317283 2 Mistral (A7) 0.355146 0.178921 3 GJet Kóan (A11) 1 0.503796 1 Table 7.6: Ranked score list for Scenario 2 with 3 alternatives. Assisted by the SuperDecisions tool, similar as in Figure 7.1, Figure 7.2 shows the ideal and normal score for each of the 3 candidates in graphical form. It is important to note that data has been inserted into and checked in the tool. Figure 7.2: Ranked score list for Scenario 2 with 3 alternatives by means of SuperDecisions tool. 7.3 Comparison of eVTOL Rankings Across Scenarios Once the rankings have been achieved for both scenarios, a comparison of the results for the Vienna-Salzburg and Paris-Orly-Bordeaux routes is performed, taking into account the 3, 9 and 11 candidates case study. 7.3.1 Comparison of eVTOL Rankings Across Scenario 1 Under the first scenario, Vienna-Salzburg, in the case of 11 alternatives (including A1 and A4), the best aircraft in the pool of choices was Kelekona's Unnamed eVTOL (1st place), followed by Trinity (2nd place). Coincidentally, both aircraft are the ones that were excluded in the study of 9 alternatives due to lack of information in some criteria. It is also worth noting the advantage of the first candidate over the second, as Ace VTOL's Trinity aircraft scored 57% relative to the first. And, although the Illini Air Shuttle (3rd place), Gjet Koan (4th place) and Genesys X-2 (5th place) scored close to A4, the second to fifth ranked aircraft are still far from the best candidate for this route. So, if we were to consider all 11 alternatives for the Austrian domestic route, the candidate chosen to resume operations 58 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe would be candidate A1. This is mainly due to the fact that A1 stands out on all 3 criteria: seat capacity (C1), maximum payload (C3) and range (C4). Bearing in mind that in this case, range occupied 46.46% of the weight and seating capacity 17.6%, its 483 km of range and 40 seats to accommodate passengers made alternative 1 a clear candidate to be chosen. On the other hand, weighted scores were extracted from the analytical hierarchical process for the 9 aircraft with all the defined criteria for the Vienna-Salzburg. The most suitable and highest scoring candidate for operating in scenario 1 is the Illini Air Shuttle (1st place), followed by Genesys-X-2 (2nd place) and Gjet Koan (3rd place). Overall, the Illini Air Shuttle is the most complete eVTOL aircraft across the different criteria assessed in the AHP process. With a 10-passenger capacity, 321 km range and 4882 kg maximum take-off weight (among others), this candidate is the chosen one to operate the route. 7.3.2 Comparison of eVTOL Rankings Across Scenario 2 Within the second scenario, the weightings of each criterion varied in proportion due to the contextual difference of the route. For the French domestic route, we proceed to discuss the results of the weighted scores obtained in the AHP process, for the case of 3 candidates. The highest scoring candidate for the Paris-Orly-Bordeaux route is the Gjet Koan, followed by the Unnamed eVTOL aircraft and the Mistral. The strength of this aircraft relative to its competitors is its overall balance, as it only excels in one criterion yet remains competitive in the others. Bearing in mind that the fourth criterion (range) makes up 50.56% of the weight amongst the criteria for this scenario and that the Gjet Koan is the most competitive aircraft with a range of 562 km, first place is defensible. However, being the seating capacity its biggest limitation (4 passengers), it should be noted that this feature will decrease to a large extent the chances of recovering past traffic due to the high passenger demand prior to the restriction. Viability of eVTOL implementation 59 8. Viability of eVTOL implementation Aiming to resume the Vienna-Salzburg and Paris-Orly-Bordeaux routes which were banned for sustainability reasons and regarding environmental emissions, an eVTOL aircraft selection process has been carried out through an analytical hierarchical process. The candidates under consideration are electrically powered aircraft, as opposed to conventional fossil-fuelled aircraft. Carrying out the procedure described during the course of the study has made it possible to determine the best candidates for operating the routes under analysis. This section, taking into account several aspects, will analyse the feasibility of implementing these aircraft in the air transport market as of today. 8.1. Summary of AHP results The eVTOL aircraft decision-making process, in connection with the routes substituted by a rail alternative, resulted in a ranking in the form of weighted scores where the best candidate received the highest score. The results for both scenarios were as follows: The Unnamed eVTOL has been chosen for the Vienna-Salzburg route by assessing all candidates (including A1 and A4) with the quantitative characteristics of each criterion as defined in Table 4.1. Likewise, considering only 9 candidates, the chosen candidate is the Illini Air Shuttle (A2). On the French domestic route scenario, the Gjet Koan from eJet Aerospace was selected while competing with Mistral and Unnamed eVTOL. These 3 were only considered for the AHP process due to the fact that they were the only ones which were able to meet the operational needs of the route. A more detailed discussion of the constraining factors to be considered in the feasibility of implementing eVTOL operations in the short term, will be provided. Restrictive factors to be taken into account in assessing how realistic it is to include this type of aircraft in day-to-day operations are: ● accommodating the previous demand to be recovered, ● the timeframe for regulation and certification of these aircraft, ● the current infrastructure, ● the financial challenge that would involve. 8.2. Accommodating Previous Traffic Demand ● Scenario 1: Vienna-Salzburg Based on 2019 data, the traffic to be recovered prior to the replacement of the route is of 909 flights and a total of 78467 seats [26]. Selecting the 40-seat Unnamed eVTOL would require a total of 1960 flights per year. These annual flights would correspond to 5-6 daily flights between the cities of Vienna and Salzburg, meaning a two-fold increase in frequency in order to meet the historical needs of the route. If the selected aircraft were the Illini Air Shuttle, since it offers 25% of the seats to serve passengers in comparison to the A1, then the daily frequency should be increased to 21 flights 60 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe per day, corresponding to 7840 flights per year. Over the course of a year, the daily frequency should be multiplied by a factor of 8 compared to that established with conventional aircraft. ● Scenario 2: Paris Orly-Bordeaux Challenges in meeting the operational needs of the route and the previous high passenger demand creates major constraints for resuming this route as of today with this type of non-conventional aircraft. According to the statistical bulletin of commercial air traffic in France for 2023, the Paris-Bordeaux route had a traffic of 649109 passengers [50]. Although this is the traffic coming from Charles de Gaulle (CDG), it is the one we will use for the frequency estimation. With the objective of bringing in the Gjet Koan, the 4-seat aircraft, it is neither feasible nor practical to operate it, as it would require 162277 flights per year and 445 flights per day in order to maintain the 2023 traffic. Due to the limited seating capacity of the aircraft to serve passengers on the Paris-Orly-Bordeaux route, enhancing the possibility to operate these 445 daily flights with the Gjet Koan could be achieved through a fleet expansion. In this regard, the airline concerned, Air France, would need to undertake a larger investment and dispose of multiple eVTOL aircraft of this model in order to cope with the expected traffic. So, all in all, recovering previous demand through the best-rated aircraft according to the AHP process is a complex and demanding task when compared to the Austrian route. 8.3. Certification challenges and infrastructure requirements Prior to considering the infrastructure requirements needed to accommodate any eVTOL aircraft, certification challenges for these aircraft are the priority in the hierarchy to be considered. Given their technological innovation, as well as the changes their implementation would entail in the industry, entities such as the FAA (Federal Aviation Administration in the USA) and the EASA (European Aviation Safety Agency) currently need to speed up the process of creating new regulatory frameworks in order to certify the deployment of these aircraft and allow them to operate in the short term. If not, delays in certification may postpone the implementation of eVTOL aircraft in the market and, indirectly, hinder the reintroduction of these banned routes. Airports involved in the Vienna-Salzburg and Paris-Orly-Bordeaux routes are not designed to accommodate and attract eVTOL aircraft operations, as they constitute conventional airports. Therefore, even if the lost traffic of 78467 seats for Scenario 1 and 649109 passengers is to be recovered via high frequencies and boosted by the increase in eVTOL fleet by Austrian Airlines and Air France, airports of Vienna, Salzburg, Paris-Orly and Bordeaux would have to be upgraded. Vertiports at near urban locations could be an option to accommodate these aircraft in order to reduce passenger access times and to enhance the mode of transport. City vertiports should therefore be created in the vicinity of large cities or, if possible, conventional airports should be adapted to include vertiports. In addition to this, battery charging infrastructure has to be considered when analysing the feasibility of implementing eVTOL in commercial air traffic operations. In order to reduce waiting times for battery charging and increase operational efficiency, the electric charging grid Viability of eVTOL implementation 61 capacity will have to be adapted at vertiports and conventional airports to meet the demand. Grid capacity can be configured with fast chargers or battery swapping. Fast chargers would reduce battery charging time and battery swaps would directly eliminate this waiting time. The downside of a grid that allows automatic battery swaps is that, in order to do so, battery size and capacity would require standardisation across all manufacturers. 8.4 Economic Feasibility Ultimately, it remains to assess the impact and economic viability of resuming this pair of routes by means of electric air transport. Certainly, most of these eVTOL aircraft are in the development or prototyping phase and, because of the technological innovation that these vehicles represent to date, their purchase costs will be more significant than those of conventional aircraft. This may cause a financial barrier of entry for airlines, since, even if fuel costs and emission taxes are cost-saving over the lifetime of the aircraft, the entry costs for acquiring more than just an aircraft are high. A measure to be considered by EU countries would be to facilitate low-interest financing to encourage investment from these airlines. Besides this, the infrastructure to be accommodated is also associated with significant costs to meet the operational requirements. Finally, successful resumption of these routes (partly for economic reasons) will be about convincing and attracting the common public to use this mode of transport instead of high-speed rail. Should airlines make a significant investment to operate these routes with electric aircraft, and passenger demand is not attracted, the solution would not be economically feasible. 62 Selection of appropriate eVTOLs as a tool to revive ultra-short haul routes in Europe 9 Sustainability and social commitment A project sustainability matrix including environmental, economic and social perspectives is required for 3 stages of the project (see Table 9.1). Point of view Phase Description Development of the work Carbon footprint limited to computer energy consumption for information research and remote meetings with work supervisor. Environmental Project execution Implementation of eVTOLs would significantly reduce the carbon footprint of air routes. Still, adapting infrastructure to existing needs would have an impact on urban centres. Risks and limitations The need for electricity generation may entail an environmental risk when it does not come from renewable sources. Development of the work The economic impact has been limited to academic resources on multi-criteria problems. Economic Project execution Economic viability will be determined, largely by support from governmental entities, as the acquisition costs of eVTOLs and vertiports are significant. Risks and limitations Early certification of aircraft and social acceptance by the community will be needed to recover the initial costs. Development of the work Acquiring experience in objective decision-making. Social Project execution eVTOLs could generate rejection in the initial phase due to their limitations compared to conventional aircraft. Risks and limitations Should the barrier of entry be costly for the general public, social inequalities could be generated. Table 9.1: Sustainability matrix Conclusions and future work 63 10 Conclusions and future work A few intra-European routes were indirectly highlighted as contributors to greenhouse gas emissions and subsequently affected by sustainability-focused policy measures. Given the European Union's objectives, as defined in the European Green Deal, of zero net carbon footprint by 2050, new legislations were put in place to replace and substitute five previously operated air routes. The affected routes consisted of: Vienna-Salzburg, Amsterdam-Brussels and Paris-Orly-Bordeaux with Nantes and Lyon, being mainly operated by KLM Royal Dutch Airlines, Air France and Austrian Airlines. However, as previously reported, only 6.1% of CO2 emissions correspond to flights with a radius of less than 500 km. As a result, the potential for emission reductions by targeting restricted ultra-short routes is low and focus should be put on more sustainable means towards medium and long-haul routes. Thereby, through the analytical hierarchical process in which eVTOL aircraft are the candidates to resume the Vienna-Salzburg and Paris-Orly Bordeaux routes, a multi-criteria analysis using quantitative data determines the best aircraft capable of operating in each scenario. To recover the 78467 seats offered on the Austrian route, Kelekona's Unnamed eVTOL would have to operate five to six daily routes. This would mean a doubling of the past frequency in 2019. Should the Illini Air Shuttle be selected, in order to meet historical demand, its daily frequency should be increased to 21 flights per day. Given the 493 km flight distance and the high demand of 649109 passengers, Paris-Orly-Bordeaux requires demanding conditions for the implementation of eVTOL aircraft. The annual number of movements with the implementation of the first alternative, Gjet Koan, would amount to 162277 flights and 445 daily flights. For the applicant Gjet Koan, the operation does not seem realistic. It is worth noting that, during this decision-making process, no consideration has been taken into account regarding the required development in terms of either batteries, infrastructure required to accommodate this innovative means of transport or the time horizon foreseen to obtain flight certifications. And, although the benefits of implementing eVTOL aircraft in commercial air transport have been highlighted, there are numerous difficulties in reintroducing air routes coupled with eVTOLs. Financial sustainability for airlines is uncertain, largely because the prices of these aircraft are unknown and therefore the ROI (Return On Investment) is uncertain. Elements such as the social acceptance of this new mode of transport remain an open question, as well as the integration of vertiports in major European cities that would facilitate passenger access times. Regulation is a challenge and entities such as the FAA and EASA must accelerate the certification process to promote the reintroduction of both routes. On top of this, Vienna, Salzburg, Paris-Orly and Bordeaux airports will need to be remodelled to include vertiports within the current design concept. There is a limited potential of emission reduction on these ultra-short haul routes and eVTOL implementation is challenging at this stage. Stakeholder collaboration will be key in order to promote commercial deployment of these aircraft in the near-term. At the end, short-haul routes will be the first ones to be fully decarbonised once the technology is fully in place. 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