Flexibility within Flight Operations as an Evaluation Criterion for Preliminary Aircraft Design Michael Husemann1 and Eike Stumpf2 Institute of Aerospace Systems, RWTH Aachen University, D-52064 Aachen, Germany Katharina Schaefer3 DLR German Aerospace Center, D-51147 Cologne, Germany The evaluation of airline’s operation data reveals the deviation of design parameters such as mission payload or mission range from specifications defined throughout the design process. Operating an aircraft outside its planned design space may lead to increasing specific fuel consumption which again results in comparatively high operating costs and emissions. However, airlines justify the purchase of oversized aircraft by being able to maintain a certain level of flexibility within flight operations. In this case, such aircraft can be operated on different types of routes covering varying numbers of passengers, various distances and variable amounts of cargo depending on current demand. In this paper, one possibility to assess flexibility as an evaluation criterion during the aircraft design process is presented. Suitable attributes that affect flexibility are found and evaluated. In this context, a survey was conducted, sent to operating airlines worldwide and analyzed to explore the airline’s motivation for achieving preferably high flexibility. Participating airlines were asked for current and future decisions referring to desired aircraft characteristics, the fleet structure as well as the importance of flexibility parameters. Also, participants were subdivided into groups concerning their market orientation to gain insight in possible differences between operational aspects. Nomenclature CO2 DOC IFE ILR LCC MTOW PAX RITA RWTH TLAR TOFL = = = = = = = = = = = Carbon Dioxide Direct Operating Costs, US-$ Inflight-Entertainment-System Institut fuer Luft- und Raumfahrtsysteme (Institute of Aerospace Systems) Low Cost Carrier Maximum Take-Off Weight Passengers (Persons approximately) American Research and Innovative Technology Administration Rheinisch-Westfälisch Technische Hochschule Top-Level Aircraft Requirement Take-Off Field Length I. Introduction The aircraft design process is based on a set of top-level aircraft requirements (TLAR) that define the design mission after such the specific aircraft is sized accordingly. Range, payload, cruise altitude, cruise mach number as well as the required take-off and landing distance belong to the most important requirements specified by particular customers (see Figure 1). The actual and final transport capacity of the specific aircraft which is derived from the design process usually is represented by a payload-range-diagram. Demonstrating each range-payload-combination 1 Research Assistant and PHD Student at Institute of Aerospace Systems (ILR), RWTH Aachen University, husemann@ilr.rwth-aachen.de 2 Head of Institute of Aerospace Systems (ILR), RWTH Aachen University, stumpf@ilr.rwth-aachen.de 3 Assistant to the Member of the Executive Board of German Aerospace Center (DLR), katharina.schaefer@dlr.de 1 payload cruise Mach number payload it envelopes all possible missions that can be flown by the aircraft (see Figure 2). Apart from technical requirements economic factors like direct operating costs (DOC) are considered as absolutely essential for developing new aircraft designs so as to each design is evaluated by its economic performance. After all, today’s air traffic is characterized by a strong market competition due to customers demanding a broad variety of services and an increasing pressure from rising costs and ecological regulations [20]. Airlines and other operators therefore pay much attention to economic aspects in particular when purchasing new aircraft. To meet customers’ expectations and market demands operators usually aim for a heterogeneous fleet composed of different types of aircraft such as short and long distance aircraft developed by different manufacturers [8]. maximum payload design payload cruise altitude design range range take-off distance landing distance range Figure 2: Payload-range-diagram derived from design mission Figure 1: Aircraft design mission and definition of top-level aircraft requirements However, the evaluation of flight data statistics indicates that many airlines operate a large number of aircraft outside the determined design space which might cause higher weights, operating costs and emissions of greenhouse gases. Higher expenditures due to operations of larger aircraft on short distances and fewer payloads still are accepted by operators as long as the overall profitability is guaranteed or even increased. Both the actual mission range and mission payload frequently deviate from the values fixed during the design process as a result of fluctuating passenger respectively cargo volume and changing flight plans. The American Research and Innovative Technology Administration (RITA) publishes data amongst others on flown distances, loaded payload and operated aircraft type of daily flight operations of the American air traffic4. The corresponding payload-range-diagrams of two common single-aisle aircraft are illustrated in Figure 3 (Airbus A320-200) respectively Figure 4 (Boeing 737-800)5. For instance, it has to be mentioned that a design range of 2,000 NM respectively 2,600 NM and a design payload of 16,500 kg respectively 14,600 kg has been defined in the design process for the aircraft A320-200. Furthermore, the payload and range distribution is plotted for both aircraft types to illustrate daily operations patterns [29]. Figure 4: Payload-range-diagram of Boeing 737-800 Figure 3: Payload-range-diagram of Airbus A320-200 4 5 Data relating to the year 2010 Airbus A320-200 has been operated on 560,000 flights; Boeing 737-800 has been operated on 460,000 flights 2 It clearly can be observed that roughly any flight has been operated with maximum payload or rather maximum number of passengers since the majority of all flights carried about 12,000 kg of paid weight. In addition, most flights have been flown on relatively short distances of about 1,000 NM which, in fact, cover many inner-European, inner-American and inner-Asian connections but deviate massively from the design value. The findings of both illustrations demonstrate the initial presumption in terms of operating oversized aircraft. Thus, it has to be assumed that airlines expect a certain level of operational flexibility that allows covering a broad market segment. More specifically, one certain aircraft can be operated on different sorts of connections serving variable numbers of passengers and cargo-units. Furthermore, flexibility is expected to generate fewer expenses than operating two single aircraft designed for different missions. This paper deals with flexibility as an evaluation criterion by finding parameters that influence an aircraft’s operational performance. The motivation for achieving high flexibility is supposed to be detected by evaluating a survey that was sent to worldwide operating airlines. Particular attention is paid to different airline groups such as national, low cost and regional carriers to identify possible differences. II. Flexibility within Flight Operations In comparison to other public transportations air traffic is marked by an inflexible transport capacity since demand fluctuation due to unpredictable events such as storms or strikes hardly can be compensated within a short period of time [39]. Moreover, aircraft’s capacity is limited to a certain amount of payload (referring to passengers and cargo) and, for instance, it is not possible to increase needed space of both the cabin and cargo compartment by docking two aircraft with each other. Upcoming maintenance stops, unexpected failures due to malfunction of systems or changing commitments by authorities are further issues that make fleet planning more difficult. To maximize potential profitability – the actual principal objective of each business enterprise – operators have to focus on current market demands so as to choose the right aircraft type on appropriate routes in the right time [3]. In case of meeting current demand inadequately an exchange of the specific aircraft has to be made or, even worse, a second flight has to be offered which probably ends in degradation of the seat load factor and therefore to higher expenses. As a consequence, operators require aircraft that are characterized by a high flexibility, thus, such aircraft that can be adapted to changing market conditions without much effort [22]. A. Definition of Flexibility Flexibility generally is defined as the system’s ability to meet exogenous requirements as best as possible while ensuring retention of former characteristics [28]. Flexibility deals with the potential of adapting an aircraft’s characteristics to changing requirements over time. To get a better understanding when defining flexibility it has to be distinguished between predictable as well as unpredictable events an aircraft is confronted with [7]:  Predictable events such as the aggravation of valid technical and environmental regulations are quite easily taken into account since there usually is sufficient time to plan ahead. Aircraft that are capable of being flexible towards those occurrences are typified as versatile.  Unpredictable events complicate daily operations massively since any reactions responding to those events must be simple to deal with. Aircraft that are capable of being flexible towards those occurrences are typified as robust. Flexibility in terms of aviation therefore implies the scope of missions that can be flown efficiently over a broad range of flight speed, flight altitude and flight distances while carrying various amount of payload. Depending on current demand on a specific route the load factor can be increased easily while related operating costs are decreased. Consequently, operators attach more and more importance to operational flexibility when buying new aircraft [22] [9] [14]. B. Flexibility Parameters Before evaluating operational flexibility several specifications must be identified that have significant influence on an aircraft’s flexibility. However, in contrast to many other evaluation criteria, for example compared to direct operating costs (DOC), there is no adequate criterion to measure or compare an aircraft’s flexible constitution. Finding the actual level of flexibility therefore is quite difficult. Nevertheless, several characteristics are mentioned in literature that are summed up to so called flexibility parameters. A different relevance dependent on the particular type of business organization is attached to each parameter. It has to be mentioned that most aircraft characteristics are fixed during the design process so as to flexibility characteristics are limited to a certain extent. The most important parameters are listed as followed in Table 1: 3 Flexibility Parameters BoernerKleindenst [19] Clark [3] Patterson[22] Vasigh [33] ✔ ✔ ✔ ✔ Performance of Aircraft Speed Range Altitude ✔ ✔ ✔ ✔ Communality Engines Cockpit Design Fuselage ✔ ✔ ✔ ✔ ✔ ✔ Reconfiguration Cabin PAX ↔ Cargo Payload ✔ ✔ ✔ ✔ Ground Processes Ramp Services Loading ✔ ✔ Requirements Regulations Airport Issues ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ Table 1: Overview of flexibility parameters derived from different literature 1. Performance of Aircraft Parameters such as flight speed, flight altitude, and range belong to the group of TLARs and therefore to the most important, quantifiable criteria that draw a significant distinction between several aircraft types. Having explicit effects on profit positions of flight operations these parameters are of certain importance for operators. An aircraft is considered as flexible if no negative effects on profitability are caused due to changes of those mentioned parameters. For each aircraft design a specific initial cruise altitude and initial cruise mach number for minimizing specific fuel consumption due to reduced drag are derived from the design process (see Section I.) [19]. However, if the flight control allocates an altitude that is different from the optimal design value an aircraft’s flexible qualities help to maintain equal flight conditions, above all fuel consumption. Plus, daily operations require various connections so that different flight distances have to be flown. Therefore, flexible aircraft are supposed to be flying economically on both short as well as long routes. 2. Communality Defined as the similarity of at least two different components, communality has a huge effect on yielding both operational as well as technical advantages by reducing the system’s complexity. In the field of aviation communality mainly refers to the design of engines, cockpits and fuselage [10] [12]. To be permitted to fly one specific aircraft type a so called type-rating has to be acquired first. Thus, the more one aircraft resembles another one in terms of cockpit design and flight characteristics the less effort has to be made in terms of obtaining the particular license. Since a specific pilot is qualified to fly various aircraft types the effort of an airline’s fleet management including training expenditures can be reduced significantly. Moreover, finding a replacement for eventual failures is facilitated and, hence, the amount of replacement aircraft can be reduced. Dependent on the operator’s marketing strategy it has to be distinguished between the standardization and specialization of an aircraft type. National carriers particularly cover a broad market segment and, therefore, operate various aircraft types by different manufacturers. This strategy falls back on aircraft specialization in first place so as to a heterogeneous fleet is operated. In contrast to national carriers low cost carriers (LCC) mainly concentrate on specific market segments that are characterized by point-to-point-connections plus less passenger services. In order to decrease operating costs – the main objective of low cost carriers – one specific aircraft type is operated as homogeneous fleet and advantages of the so called ‘family concept’ are guaranteed. However, it might be difficult to react to short-term demand fluctuation because no other aircraft type that fits to the current demand situation would be available. 4 3. Reconfiguration Not only the growing overall passenger volume has to be taken into consideration by installing more and more seats into the cabin but also the current cabin layout needs to be flexible and adaptable in case of daily demand fluctuation [5]. Possible modifications have to be cost-efficient and implemented as quick as possible to minimize the ground process measured by the turn-around-time. Furthermore, logistics companies often charter available aircraft to transport postal items and packages by night when no regular flights are offered. In this case, the cabin must be convertible without much effort to save time and expenses [31] [11] [15]. 4. Ground Processes An aircraft’s productive period of time depends on ground processes plus the actual turn-around-time. Because operators try to avoid as much inefficient steps as possible to increase the productivity, alternative loading options such as rear cargo doors and the possibility of carrying different containers facilitate the dispatch process. Some special aircraft types are designed particularly for quick changes so as to available cabin space can be used for additional cargo pieces [23] [24] [25]. 5. Requirements The objective of regulations established by national and international authorities is to protect the civilization as well as the entire environment from harmful effects. In order to achieve these goals current laws and regulations have been sharpened year by year which makes it more difficult for operators to involve certain aircraft in the fleet management process [1] [5] [27]. An aircraft type that allows easy and affordable modifications, for example in terms of noise reduction, will be permitted to land on more airports worldwide and, thus, is considered as being more flexible than those that do not meet the requirements [6] [33]. Apart from complying with current international regulations, every aircraft also must meet an airport’s individual requirements to obtain the permission to take-off or land on each particular runway. Referring to the runway length, smaller aircraft usually require shorter runways and, thus, can land on more airports worldwide which makes it more flexible in terms of operations [19] [2]. III. Flexibility Survey The Institute of Aerospace Systems (ILR) created a survey titled “Flexibility within Flight Operations” that has been sent to operating airlines worldwide. The objective was to verify the definition of flexibility derived from literature and to find characteristics of particular airline groups. A. Characteristics of Airlines The particular market sighted by an airline influences the fleet concept. Therefore, it has to be assumed that fleet composition, route characteristics, flight scheduling and financial aspects differ between those groups depending on the individual strategic orientation. Not only high demanded connections have to be operated to maximize profitability but also customers’ demands need to be taken into account to be able to survive against potential competitors. For instance, business travelers make other requests in terms of seat comfort, seat spacing, possible cancellations of bookings and menu services than private travelers such as tourists [29]. The most important characteristics are summarized in Table 2. 1. National Carriers States often hold shares in native airlines not only to support the enterprise financially but also to guarantee a minimum of influence in local mobility [29]. Furthermore, by sharing interests in national carriers – also known as flag carriers – national independence and technical performance are demonstrated towards other countries [43]. Nevertheless, privatizations have been going on for years due to intensified efforts in strengthening economic intentions [11]. National carriers serve primary and secondary airports as well as international destinations to ensure an adequate mobility abroad. The provided route network therefore is planned after the hub-and-spoke-principle: While (international) long-haul flights are operated between centrally located airports, smaller regional airports are connected through coordinated feeder flights [37]. As a consequence, different types of aircraft have to be operated on short-, medium- and long-haul routes to meet customers’ requirements. This is, especially heterogeneous fleets provide benefits. 2. Regional Carriers The objective of regional carriers is to provide feeder service between smaller, mostly regional and bigger international airports [32]. For this purpose, smaller aircraft with a passenger capacity up to 100 seats are operated. 5 Additional point-to-point connections are offered on routes between regional airports when current passenger volume is not sufficient for operating larger aircraft [23]. It has to be mentioned that regional airports in particular provide comparatively short runways so that only smaller aircraft are technically permitted to land and take-off [12]. Due to short flight duration, regional carriers only offer one single passenger class, in rare cases even a class providing more services meant for business travelers [9]. The diversity of routes is limited to a certain extent [15]. 3. Low Cost Carriers Low cost carriers pursue a strategy that especially focusses on cost-conscious passengers: Avoiding costs in terms of giving up inclusive services such as meals and beverages on board or access to lounges at airports, passengers can be attracted by reasonable ticket prices [19]. After all, most passengers are tourists who are willing to swap comfort and legroom with opportunities to save costs. Besides, almost only point-to-point connections characterized by a high passenger volume and cost-effective airports are taken into account when planning flight operations. Turn-around-processes are supposed to happen as fast as possible to reduce ground operation expenses. Most low cost carriers operate homogeneous fleets to benefit from communality advantages [17]. Characteristics National Carrier Regional Carrier Low Cost Carrier ✔ ✔ ✔ ✔ ✔ ✔ (✔) ✔ (✔) ✔ ✔ (✔) ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ Primary Airports Secondary Airports International Destinations Long-Haul Flights Medium-Haul Flights Short-Haul Flights Homogeneous Fleet Heterogeneous Fleet Homogeneous Routes Heterogeneous Routes Hub & Spoke Point-to-Point Single Seat Layout Multiple Seat Layout ✔ ✔ ✔ ✔ ✔ Table 2: Comparison of operator’s characteristics B. Introduction to Survey The survey was sent in August 2014 to operating airlines worldwide and comprised questions referring to both current as well as future fleet strategies. Thus, the relevance of operational flexibility was supposed to find out from the airlines’ point of view. The focus on subjects and the number of all questions are summarized in Table 3. First of all, general information about the particular firm and its route network was requested to group all participants on the basis of its principle market orientation. Further questions were related to current fleet structures. By also giving information about desired aircraft features, a comparison with future and present fleets is possible and relevant aircraft characteristics that have to be considered in the design process can be identified. Finally, participating operators were asked after their opinion on the importance of listed flexibility parameters and operational flexibility as decision criterion in general. Focus on Subjects Number of Questions Classification of Airlines Location of Headquarters and Operations Current Fleet Composition Desired Characteristics of Future Aircrafts Assessment of Relevance of Flexibility Parameters Evaluation of Decision Criteria during Purchase Table 3: Overview of topic and number of questions 6 2 2 4 7 2 1 Until the closing date 30 airlines from all 100% over the world participated in completing the survey and sent their answers back to the 80% 67% author. Expressed in passenger kilometers that have been flown worldwide in 2012, more than 60% 10% have been operated by these participating airlines. The majority (67%) belong to the group 40% of national carriers followed by regional (20%) 20% and low cost carriers (13%). At least 10% of the 20% 13% 10% 10% 7% answers were submitted by both charter and network carriers. Other operators such as 0% logistics service provider or leasing companies National Regional Low Cost Charter Network Others Carrier Carrier Carrier Carrier Carrier come to 7% and represent the smallest group of all participants. In order to give an overview of Figure 5: Overview of participating airlines the distribution the results are plotted in Figure 5. As summarized in Table 4 almost two-thirds (62%) of participating airlines come from Europe and are represented the most. Whereas 10% of headquarters are located in both South America as well as in Africa, 7% of all participants come from each Asia and South East Pacific (Oceania). Only 4% of North American airlines sent back their completed surveys on time so that their answers were taken into account and contributed to the overall results. However, referring to the actual flight operations some deviations were noticed. Whereas more than fourfifths of participating airlines operate on European continent, only 24% of them consider destination in Oceania. Almost half of them (48%) operate in North America respectively Asia and one third (34%) in South America. At least 41% said that operations in Africa would take place. Europe 62% 83% Home Base Route Network North America 4% 48% South America 10% 34% Asia Africa Oceania 7% 48% 10% 41% 7% 24% Table 4: Overview of home base and route network of participating airlines C. Evaluation of Survey The most important responses have been analyzed to figure out similarities and typical pattern depending on particular market orientation. Since there have not been enough answers given by all participating groups, the focus is put on the three biggest airline groups (national carriers, regional carriers and low cost carriers) that also show significant differences in terms of chosen marketing strategies. 1. National Carriers Nine out of ten participants operate both short range as well as medium range aircraft. Furthermore, almost two thirds (62%) mentioned that long range aircraft are also part of the particular fleet6. These observations generally match with typical characteristics covering a broad market segment that have been found before (see Section III.A). In accordance with heterogeneous fleets, the maximum range spreads between 1,550 km and 15,700 km whereas the average range reaches from 4,440 km up to 10,490 km (see Figure 6 and Figure 7).  min 1 1,15 2 3 4 max 5 4,44 6 7 8 9  min 1 10 11 12 13 14 15 16 17 10,50 2 3 4 5 6 7 8 9 3,70 max 10 11 12 13 14 15 16 17 10,49 15,70 Figure 6: Ranges of current fleets of national carriers showing the shortest range (left) and longest range (right) in 1,000 km 6 Classification of aircraft types: short range (< 1,500 km); medium range (1,500 km – 3,500 km); long range (> 3,500 km) 7 However, when asking after possible mission ranges of future aircraft, most participants focus on shorter ranges for short-haul, medium-haul as well as long-haul flights and, hence, the corresponding distribution reveals smaller values. It has to be assumed that airlines therefore focus more on shorter connections since the fuel efficiency on long-haul flights generally gets worse [35]. Nonetheless, customers usually prefer nonstop flights to destinations worldwide and for this reason offering long distance flights still has priority for many national carriers. Referring to the passenger capacity of each aircraft type, no consistency can be detected. However, the actual results for medium range aircraft differ significantly from expected values and nearly correspond with the ones from long range aircraft types. Due to high passenger volume especially in Asia local airlines often operate large aircraft – even on short routes – to provide sufficient capacity. According to the answers national carriers operate aircraft with seating capacity between 40 and 471 passenger seats. An overview of the capacity distribution for all three aircraft types is illustrated in Figure 7. min 0  100 48 200 300 400 500 0 100 70 285 92 (a) short range (smallest aircraft type)  min 0 100 127 40 400 500 0 220  0 100 200 140 228 100 88 max 300 400 500 285  max 200 300 188 min 400 300 400 500 300 (d) medium range (largest aircraft type) (c) medium range (smallest aircraft type) min 200 144 min 300 max (b) short range (largest aircraft type) max 200  min max 500 0 300 100 200 190  300 max 400 289 500 471 (e) long range (smallest aircraft type) (f) long range (largest aircraft type) Figure 7: Passenger capacity of current fleet of national carriers In Table 5 the average number of passenger seats for each aircraft type is calculated from the results before. While the seating capacity for both short range as well as medium range aircraft matches expected dimensions, long range aircraft only provide a comparatively small number of seats. Since two or even three passenger classes7 are installed into most twin-aisle aircraft types certain space gets lost for additional partition walls and galleys and, thus, cannot be used for passenger seats. The respective average passenger capacity for each aircraft type generally matches the actual number of seats. Nevertheless, national carriers focus on those aircraft types that fit fewer seats such as Airbus A350 and Boeing 787 instead of Airbus A340 or Boeing 747. More than one third of all participants (35 %) expect an increasing passenger and cargo volume within the next few years. Route Type Ø Passenger Capacity (PAX) Planned Passenger Capacity (PAX) 118 157 258 100-150 150-200 200-300 Short Range Medium Range Long Range Table 5: Comparison of current and planned passenger capacity of national carriers National carriers give the most significance to direct operating costs (100 %) when evaluating individual decision criteria while purchasing new aircraft. Since the aviation industry is characterized by a generally small return on investment due to strong competition this result is not quite unexpected. Operational flexibility therefore is rated as 7 First Class, Business Class, Economy Class 8 the second most important feature of an aircraft because 65 % of all participants awarded two points to this attribute. Finally, environmental aspects such as emissions and noise exposure seem to be less relevant on account of high expenses that are related to technical modifications (see Table 6). Moreover, specifications such as payload capacity (passengers and cargo) and range are considered as the most essential flexibility parameters followed by communality and meeting an airport’s requirements due to operating heterogeneous fleets. n 5 4 3 2 1 Operating Costs 20 100% Flexibility 20 5% 30% 35% 25% CO2 -Emissions 20 45% 30% 20% 5% Noise 20 15% 25% 45% 10% 5% Air Pollution (NOx ) 20 20% 10% 65% 5% 5% Table 6: Evaluation of decision criteria during purchase process from national carriers’ point of view 2. Regional Carriers In accordance with typical characteristics, providing short-haul as well as medium-haul flights in particular, all participating regional carriers (100 %) state that short range aircraft types are part of their fleets. Apart from this, more than four fifths (83 %) operate medium range and at least one third (33 %) also long range aircraft. This is certainly the case when Asian airlines need to fall back on large aircraft to be able to serve current passenger volume. In addition to that, several participants categorize themselves as national and regional carriers so as to twin-aisle aircraft are operated as well. The average range extends from 3,600 km up to 7,100 km. Only a small distribution is noticeable which indicates a compliant business alignment. Due to the operation of large aircraft by certain airlines comparatively long distances also can be flown (11,000 km) even though these values are more an exception. The overall distribution for current ranges is illustrated in Figure 8. Future design range is supposed to be limited to a certain extent according to the results when potential fuel consumption can be reduced in return. min  1 2 3 4 max 5 2,60 3,60 6 7  min 8 9 1 10 11 12 13 14 15 16 17 2 3 4 3,70 6,85 5 6 7 7,10 max 8 9 10 11 12 13 14 15 16 17 11,00 Figure 8: Ranges of current fleets of regional carriers showing the shortest range (left) and longest range (right) in 1,000 km The distribution of passenger capacity matches the previous results as primarily short-haul aircraft are operated by regional carriers. Hence, less than 100 seats are installed on average into those aircraft types. However, large aircraft that are meant for long-haul flights only fit 185 seats on average which even present medium range aircraft types usually can cope with8. When asking after future passenger capacity respectively number of seats for each aircraft type, all participants agree on keeping up the current design so as to no additional seats are required. Thus, up to 100 seats are considered for short range and up to 200 seats for long range aircraft types. Medium range Route Type Short Range Medium Range Long Range Ø Passenger Capacity (PAX) Planned Passenger Capacity (PAX) 97 129 185 50-100 100-150 150-200 Table 7: Comparison of current and planned passenger capacity of regional carriers 8 For example, both Airbus A321 as well as Boeing 737-800 fit up to 180 passengers [17] [30]. 9 aircraft types are supposed to provide a number between those numbers. An overview of the average values is given in Table 7. The distribution of passenger capacity for each aircraft type generally spreads slightly around the prevailing average value as can be recognized in Figure 9. min  0 25 50 75 0 100 125 150 175 200 225 250 275 300 50 68 25 50  0 25 50 75 100 125 150 175 200 225 250 275 300 125 140 (b) short range (largest aircraft type) max  min 100 125 150 175 200 225 250 275 300 106 50 75 50 132 (a) short range (smallest aircraft type) min  max min max 0 25 50 75 140 100 125 150 175 200 225 250 275 300 (c) medium range (largest aircraft type)  min max 25 50 75 200 152 88 (d) medium range (smallest aircraft type) 0 max min 0 100 125 150 175 200 225 250 275 300 25 50 75 max 100 125 150 175 200 225 250 275 300 140 160 150 190 (f) long range (smallest aircraft type)  220 280 (e) long range (largest aircraft type) Figure 9: Passenger capacity of current fleet of regional carriers Regional carriers assess direct operation costs as the most relevant criterion in terms of purchasing new aircraft and, hence, agree with national carriers. Furthermore, two thirds of participating regional carriers hold the opinion that operational flexibility provides economic advantages by means of cost savings. Factors such as payload and passenger capacity, range as well as benefits from communality are considered as essential aircraft requirements followed by meeting an airport’s requirements. Operational flexibility consequently is rated as the second most significant criterion. However, environmental aspects – especially air pollution – are less relevant when considering new aircrafts. n Operating Costs 6 Flexibility 6 CO2 -Emissions 6 Noise 6 Air Pollution (NOx ) 6 5 4 3 2 1 100% 17% 67% 17% 67% 17% 17% 33% 17% 17% 50% 33% 17% 17% 17% Table 8: Evaluation of decision criteria during purchase process from regional carriers’ point of view 3. Low Cost Carriers As mentioned before, low cost carriers mainly concentrate on connections with high passenger volume and price-conscious passengers. Cost advantages achieved by renunciation of additional services such as cabin equipment9, included meals or seat reservations can be passed on in the form of cheap flight tickets. The main focus 9 e.g. Inflight-Entertainment-Systems (IFE) 10 therefore is put on the actual transport capacity. All participants (100%) operate both short range as well as medium range aircraft types which correspond with characteristics that have been derived before. However, since more and more low cost carriers even include long-distance destinations in their flight plan, appropriate aircraft models have to be considered [4]. For this reason half of participating low cost carriers (50%) also operate long range aircraft. Concerning the range distribution no significant spread can be identified since average values from 4,860 km up to 6,290 km match expected dimensions (see Figure 10). Hence, it has to be assumed that their respective business concepts resemble a lot trying to gain more share of the market.  min 1 2 3 4 3,00 max 5 6 7 8 9 10 11 12 13 14 15 16 17 1 2 8,15 4,86  max min 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 4,63 6,29 7,40 Figure 10: Ranges of current fleets of low cost carriers showing the shortest range (left) and longest range (right) in 1,000 km The current available passenger capacity of aircraft operated by low cost carriers shows a remarkable low deviance. Moreover, the results of all three aircraft types have a tendency to quite a consistent capacity that roughly is equal to the overall capacity of common medium range aircraft. The calculated average number of seats for short range and long range aircraft types only differ by 10 seats which confirm previous findings. Therefore, the operator’s intention of diversification towards competitors can be denied. When asking after future seating and cargo capacities low cost carriers mostly stick to the current design so as to the prevailing average number of seats exactly matches the given results that are summarized in Table 9.  min 0 20 40 60 80 100 120 140 130 90  min max max 160 180 0 200 20 0 20 40 60 80 120 100 120 140 140 160 min 160 180 0 200 20 40 60 80 100 120 140 160 189 40 60 80 100 120 140 160  182  min max 180 200 0 20 40 60 80 100 120 140 160 180 200 180 189 185 (f) long range (smallest aircraft type) (e) long range (largest aircraft type) Figure 9: Passenger capacity of current fleet of low cost carriers Short Range Medium Range Long Range 200 189 (c) medium range (largest aircraft type) 144 156 150 Route Type 200 189 max 180 170  min max 20 180 max (d) medium range (smallest aircraft type) 0 180 (b) short range (largest aircraft type) 145 90 80 156  100 60 170 (a) short range (smallest aircraft type) min 40 Ø Passenger Capacity (PAX) Planned Passenger Capacity (PAX) 155 164 165 100-150 150-200 150-200 Table 9: Comparison of current and planned passenger capacity of low cost carriers 11 Finally, participating low cost carriers were asked to evaluate specified decision criteria that are crucial when purchasing new aircraft. Since the main emphasis is put on avoiding expenditures of all kinds, direct operating costs are rated as the most significant factor followed by operational flexibility. Corresponding to other airline groups low cost carriers also graded passenger respectively cargo capacity as the most important aspect. After all, the overall load factor of each flight has to be optimized in order to maximize an airline’s profitability. Communality in terms of operating homogenous fleets generally also counts to relevant flexibility parameters. However, configurational features of an aircraft’s cabin are considered as less important since low cost carriers rarely modify cabins dependent on current passenger and cargo volume. From the participants’ point of view further decision criteria such as possibilities to minimize the climate impact are appraised as the least decisive characteristic features. Low carbon dioxide emissions (CO2) are considered as more significant as other air pollution which probably can be affiliated with higher operating costs. The complete distribution is plotted in Table 10. n 5 4 Operating Costs 4 Flexibility 4 CO2-Emissions 4 50% 50% Noise 4 50% 25% Air Pollution (NOx) 4 3 2 1 100% 25% 75% 25% 100% Table 10: Evaluation of decision criteria during purchase process from low cost carriers’ point of view IV. Studies on Flexibility As shown before in Section III. operational flexibility generally is considered as one absolutely significant criterion when it comes to evaluating aircraft models. Depending on flexibility characteristics an aircraft can contribute to reducing operating costs and, hence, is more profitable from an airline’s point of view. However, flexible properties are hardly to measure compared to other parameters such as direct operating costs which makes it more difficult to compare two different aircraft types. A reliable model consequently has to be established that allows the assessment of an aircraft’s flexibility during the aircraft design process by running parameter studies depending on an airline’s characteristics. A. Flexibility Model Such a model has been created by the Institute of Aerospace Systems in order to facilitate the quantification of an aircraft’s inherent flexibility by assigning a so called flexibility indicator to each aircraft. For this purpose, several characteristic values for each flexibility parameter, separated by each airline group, need to be extracted from real flight operations and additional technical information. Suitable data has been taken from RITA database that provides satisfactory data for specifications such as flown aircraft types, passenger load factor, payload and information about used airports for each origin-destination combination on the US-American market [18]. It has to be noted that data validity is assumed for other air traffic markets like Europe and Asia as well. However, finding sufficient flight operations data was not possible for all flexibility parameters presented in Section II. Therefore, only following flexibility parameters form the basis of the model to calculate the overall flexibility indicator:  Aircraft type rating and standardization  Suitability of airports  Cruising speed  Payload capacity  Passenger capacity  Range By analyzing flight operations data certain patterns and individual characteristics are supposed to be identified for each airline group. The flexibility model thus is predicated upon statistics and related correlation. 12 Number of Runways 60 National Carrier 50 Regional Carrier 40 Low Cost Carrier 30 20 10 0 1000 1500 2000 2500 3000 3500 Length of Runway [m] 4000 4500 5000 Figure 10: Frequency distribution of runway length for all airlines groups 100% Accumulated Frequency At first, a frequency distribution has to be derived for each characteristic value on the basis of flight operations data. An exemplary distribution for all three airline groups is illustrated in Figure 10 showing the different lengths of runways as part of the flexibility parameter “suitability of airports”. Hence, the number of events can be identified quite easily and a first impression of the main emphasis of the distribution is facilitated. Accumulating the particular frequencies (see Figure 11) helps to find out the individual gradation which serves as the prerequisite for deriving the actual flexibility curve expressed through a mathematically function. For this purpose, the accumulation must be reversed and, if wanted, smoothed out slightly which is plotted in Figure 12. The curve, illustrated for each airline group allocates a flexibility value between 0 (no flexibility) and 10 (total flexibility) to every related aircraft property. In order to determine the overall 80% 60% 40% National Carrier 20% Regional Carrier 0% 1000 Low Cost Carrier 1500 2000 2500 3000 3500 Length of Runway [m] 4000 4500 5000 Figure 11: Accumulated frequency distribution of runway length Flexibility Vaue flexibility indicator, all 10 flexibility values that have been National Carrier calculated before, must be Regional Carrier 8 summed up considering Low Cost Carrier weighting factors. Those can be 6 chosen individually by the model’s user for each airline 4 group. For further investigations the weighting factors already 2 have been preset by the author by means of previous findings 0 (see Section III.A). For instance, 500 1000 1500 2000 2500 3000 3500 4000 4500 from a regional airline’s point of Length of Runway [m] view, an Airbus A320-200 gets approximately 8.5 points since Figure 12: Flexibility curve for characteristic value “runway length” the typical take-off field length is about 1,960 m. In contrast to that, national carriers award 9.9 points, low cost carriers about 9.2 points. Once all flexibility values are calculated for every aircraft characteristic, all six flexibility parameters need to be determined and finally the overall flexibility indicator can be found. Furthermore, the model has been implemented into the design methodology MICADO to run parametric studies (see Section IV.B). 13 B. Design Environment MICADO Further parameter studies have been carried out using MICADO (Multidisciplinary Integrated Conceptual Aircraft Design Optimization), a design methodology for multi-disciplinary preliminary aircraft design that has been developed by the Institute of Aerospace Systems. Providing a small set of TLARs and further design specifications an optimized aircraft design can be generated while only limited processing power is required. The integrated sizing approach grants the consideration of design changes or innovative technologies which might lead to a full resizing of the whole aircraft design. Thus, the modular concept allows running parametric studies and assessing particular aircraft designs [16][21]. Based on specified aircraft requirements an initial sizing of the aircraft is derived comprising the prediction of aerodynamic characteristics, estimation of aircraft components and system masses followed by a detailed mission analysis. Afterwards, aircraft mass, derived flight conditions as well as drag polar and engine performance serve as the foundation for calculating the specific fuel consumption depending on the defined design mission. All sizing tools are iteratively re-executed until convergence in significant parameters, e.g. maximum take-off weight and block fuel, is achieved and further design specifications such as wing loading and thrust-to-weight ratio remain consistent. Finally, each aircraft design will be assessed and, if necessary, optimized towards economic and ecological factors, especially concerning operating Figure 13: MICADO process chain costs, emissions, aircraft noise and block fuel. The whole design synthesis is illustrated in Figure 13 [16] [26]. In order to run parametric studies a reference aircraft has been designed that is supposed to operate on short-haul and medium-haul routes. The aircraft is a common single-aisle model with two V2527-A5 engines that mainly resembles an Airbus A320-200 comparing technical and design specifications (see Figure 14). The most essential design specifications are listed in Table 11. As daily flight operations frequently differ from the specified design mission an additional study mission has been defined: Parameters such as payload capacity (passengers and cargo) and flight distance in particular deviate the most referred to findings that have been initially presented in Section I. Therefore, both values have been reduced drastically to simulate realistic flight operation conditions (see Table 11). Characteristics Maximum Take-Off Weight Operating Empty Weight Payload Wing Loading Thrust-to-Weight Ratio Take-Off Field Length Landing Distance Passenger Capacity Payload (Design Mission) Range (Design Mission) Payload (Study Mission) Range (Study Mission) Value Unit 76,988 42,098 20,000 629.085 0.3122 2,200 1,850 150 17,000 2,500 13,600 500 kg kg kg kg/m² m m kg NM kg NM Table 11: Design specification of reference aircraft Figure 14: Illustration of reference aircraft 14 C. Studies on Operational Flexibility Sensitivities of the aircraft design are investigated by running several parametric studies in which particular focus is put on the aircraft’s flexibility. Therefore, following requirements were successively altered individually for each airline group while further TLARs were maintained at a constant level:  Design Payload  Design Range The value of each flexibility parameter is varied gradually within an appropriate design space beginning with the current reference value that has been specified throughout the aircraft design process. However, for each variation a re-design has to be made while both ratios wing-loading as well as thrust-to-weight remain unchanged in order to ensure comparable aircraft designs. Besides the actual study results that will be plotted for each airline group further results of following parameters are investigated additionally:  Maximum Take-Off Weight  Wing Area  Block Fuel  Direct Operating Costs  CO2-Emissions The outcome of each parameter variation is expressed as a percentage of the reference case so that possible changes can be spotted and evaluated. Monetary values are given in 2010 US-Dollars per flight hour. 1. Payload Capacity The design payload capacity (17,000 kg) of the reference aircraft is increased respectively decreased by at most 2,000 kg (+/-) in order to investigate the influence on an aircraft’s flexibility. For this purpose, a step size of 100 kg has been chosen which results in 20 steps each up and down. It has to be noticed that the number of passengers (150 PAX) remains constant to focus only on the alteration of cargo weights. Thus, an alteration of the payload capacity influences the overall flexibility indicator for all three airline groups since the flexibility values of both parameters range as well as length of runway are changing significantly. The corresponding findings are plotted in Figure 15. In case of increasing the payload capacity by 2.5 % an overall flexibility growth of approximately 15 %, which is the largest of all, can be identified for low cost carriers. Contrary to this, for national carriers only 7.5 % and for regional carriers 11.5 % increased flexibility is noticeable. In fact, these results mainly depend on weighting factors chosen by the author when calculating the particular flexibility indicator for each airline group. The results therefore vary in case of weighting differently. The rise in flexibility regarding the range of the reference aircraft is influenced by the distribution of flexibility values. Compared to regional and national carriers, low cost carriers even assess shorter ranges as more important as many offered routes also cover shorter distances. This is, since most of the altered payload capacity affects longer ranges no real deviation is perceptible. Furthermore, the flexibility distribution of regional carriers flattens more than the corresponding distribution of national carriers. Increasing the payload capacity by 11.25 % consequently leads to a larger wing area (4.2 %) and therefore to a bigger wing tank (7.38 %). When more fuel can be filled up into the aircraft the range again increases by 5.022 %. The reason for this effect is the constant ratio of aircraft mass and wing area which requires an increasing wing area when the payload capacity is increased. On the other hand, larger wing area implies a bigger wing tank which finally increases the particular range and, hence, the flexibility value. Apart from influencing the wing area the variation of the payload capacity also have significant effects on maximum take-off weight, specific fuel consumption as well as emissions of carbon dioxide and direct operating costs. Due to higher aircraft masses the required thrust needs to be increased which ends in higher fuel consumption and emissions. For instance, increasing the payload capacity by 11.25 % higher specific fuel consumption (1.8 %) is induced which causes higher fuel CO2-emissions (1.4 %) as well as direct operating costs (2.1 %) [13]. Payload Capacity 15,400 kg 16,400 kg 17,000 kg 17,400 kg 18,400 kg Wing Tank Wing Area MTOW Block Fuel DOC/Flight 9,607 kg 9,989 kg 10,223 kg 10,387 kg 10,778 kg 117,46 m² 120,12 m² 121,85 m² 122,99 m² 125,72 m² 73,905 kg 75,617 kg 76,652 kg 77,358 kg 79,082 kg 3,629.74 kg 3,665.78 kg 3,686.80 kg 3,701.22 kg 3,738.20 kg 10,601.20 $/flight 10,711.82 $/flight 10,778.03 $/flight 10,823.05 $/flight 10,936.60 $/flight Table 12: Influences on aircraft specifications by altering design payload capacity 15 ∆ Flexibility, % ∆ Flexibility, % For both regional as well as low cost carriers a slight rise in flexibility in terms of the take-off field length can be identified. However, since both alterations are quite low an inaccuracy of the model is assumed. ∆Payload, % ∆Payload, % (b) Regional Carrier ∆ Results, % ∆ Flexibility, % (a) National Carrier ∆Payload, % ∆Payload, % (d) Aircraft Parameter (c) Low Cost Carrier Figure 15: Influence on overall flexibility by altering payload capacity of the reference aircraft 2. Range As daily flight operations in respect of payload and range frequently differ from the defined design mission, influences on the overall flexibility indicator by altering the range are investigated. Starting from the reference value of 2,500 NM the range gradually is increased (+2,500 NM) respectively decreased (-2,000 NM) providing a step size of 100 NM. Further specifications and requirements again are kept unchanged. In order to achieve longer ranges bigger airfoils with larger tanks are required amongst other requirements. For example, in case of increasing the range by 15 % the wing area enlarges by 4.25 %. The larger and therefore heavier airfoils as well as the additional fuel require enhanced wing boxes. Because of the resulting snowball effect, further reinforcements have to be taken into account and the maximum take-off weight increases by 4.25 %. The higher fuel consumption (1.7 %) has negative effects on CO2-emissions since these rises by 1.25 % and, after all, the direct operating costs increase by 2.08 %. Furthermore, the variation of the design range influences the flexibility value of the reference aircraft referring to its range and the required take-off field length. Low cost carriers in particular assess short ranges as comparatively important. The alteration of the design range therefore does not affect the calculation of the flexibility value since the maximum number of points is already allocated accordingly. In contrast, national carriers include long-haul flights and assign higher flexibility values to longer ranges. For this reason, in case of increasing the range by 15 % the flexibility rises by 6.04 %; if the range is decreased by also 15 % the flexibility decreases by even 10.42 %. For regional carriers both the gains as well as the loss of flexibility seem to be quite similar to the results of national carriers even though particular values differ slightly. Nonetheless, it has to be assumed that the design range as one of the specified TLAR generally seems to be an important attribute for airlines. 16 ∆ Flexibility, % ∆ Flexibility, % The overall flexibility indicator differs for each airline group due to different weighting factors. For instance, rising the design range by 15 % the overall flexibility increases by 2.08 % for national carriers and only by 0.41 % for regional carriers. The results of the parametric study are illustrated in Figure 16. ∆ Range, % ∆ Range, % (b) Regional Carrier ∆ Results, % ∆ Flexibility, % (a) National Carrier ∆ Range, % ∆ Range, % (d) Aircraft Parameter (c) Low Cost Carrier Figure 16: Influence on overall flexibility by altering range of the reference aircraft V. Conclusion and Outlook Comparing actual flight data with design specifications it can be stated that a large part of all operations differ from the defined design mission in terms of flown ranges and carried payload. Thus, it has to be assumed that most aircraft are sort of oversized for daily operations. However, when operating an aircraft that deviates from its design point – the defined payload-range-combination – higher specific fuel consumption, emissions of carbon-dioxide and after all increased operating costs have to be expected. The scope of this paper therefore was to investigate particular characteristics of several airline groups as well as their individual behavior referring to aircraft operations. Influences of certain design parameters on operational flexibility and further consequences such as operating costs were supposed to be found. Flexibility is defined as the aircraft’s adaptability towards current and changing requirements. In contrast to established and well-known evaluation criteria such as direct operating costs (DOC) there is no adequate method to measure flexibility appropriately which makes quantifying flexible features of an aircraft quite difficult. For this reason, certain design parameters that have a significant influence on an aircraft’s flexibility were defined. Besides essential top-level aircraft requirements such as range, payload, cruising altitude as well as cruising speed further characteristics as communality of components and possible reconfiguration of cabin and cargo departments influence the overall flexibility. A flexibility survey has been created and was sent to airlines worldwide in order to discover certain operational characteristics. National carriers mainly cover broad market segments so as to different aircraft types have to be 17 operated in heterogeneous fleets. Regional and low cost carriers focus more on specific markets characterized by shorter distances and one single type of passengers. Thus, usually one certain aircraft type, primary short- and medium-haul aircraft, is operated in homogeneous fleets. On basis of these findings a flexibility model has been established that allows the evaluation of an aircraft’s inherent flexibility. A mathematical function that assigns a value between 0 (no flexibility) and 10 (total flexibility) to each flexibility parameter has been derived from the RITA research database. The overall flexibility indicator finally can be determined by aggregating calculated values. In order to analyze the interaction of design parameters in terms of operational flexibility the model has been implemented into the multi-disciplinary design environment MICADO. The variation of fundamental aircraft design parameters has a significant influence on an aircraft’s flexibility. Possible effects on flexibility, fuel consumption, CO2-emissions and operating costs have been investigated by altering individual parameters such as payload capacity and range. Therefore, a reference aircraft has been designed that allows the performance of appropriate parametric studies. In fact, the increase of both the design payload capacity as well as design range results in higher flexibility which is, however, accompanied by higher fuel consumption, CO 2-emissions and operating costs. On the other hand, costs savings can be achieved through reducing both parameters. More specifically, both effects interact with each other and, thus, a compromise has to be found when designing aircraft. For future research, further improvements of the flexibility model have to be made by involving more operators in the design process. In particular, specified parameters that influence the overall flexibility must be identified as early as possible and a quantified value must be well-considered towards increasing operating costs. VI. Acknowledgements In loving memory of my mother who recently lost her fight and who I would like to express my special thanks to. Thanks for always taking care of me and for making me the person who I am today. You will never be forgotten. 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