The Implications of Intermediate Stop Operations on Aviation Emissions and Climate

Florian Linke1∗, Volker Grewe2,3 and Volker Gollnick1

1Deutsches Zentrum für Luft- und Raumfahrt, Einrichtung Lufttransportsysteme, Hamburg, Germany 2Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany 3also at: Delft University of Technology, Faculty of Aerospace Engineering, Section Aircraft Noise & Climate Effects, The Netherlands

(Manuscript received December 15, 2015; in revised form August 5, 2016; accepted November 15, 2016)

Abstract Among the various transport modes aviation’s impact on climate change deserves special attention. Due to typical flight altitudes in the upper troposphere and above, the effect of aircraft engine emissions like e.g. water vapour, nitrogen oxides and aerosols on radiative forcing agents is substantial. The projected doubling of aircraft movements in the next 15 years will lead to an increase of aviation’s impact on climate and requires immediate mitigation options. Besides technological measures also new operational strategies are widely discussed; one of these concepts which has been subject of several studies in the past is Intermediate Stop Operations (ISO). It is based on the idea to reduce the stage length of flights by performing one or more intermediate landings during a mission. Here, we analyse the ISO concept by combining different models, which include a realistic traffic simulation taking into account operational constraints and ambient conditions, like e.g. wind, the calculation of engine emissions and the integration of a climate response model. We analyse the ISO concept for today’s worldwide aircraft fleet, including its influence on global emissions distributions as well as the impact on climate change by taking into account CO2 and non-CO2 effects, arising from contrail-cirrus, water vapour and nitrogen oxide emissions. We show in agreement with earlier findings that due to shorter flight distances the amount of fuel burnt over the mission can be reduced by roughly 5 % on average globally. For the first time, we quantify the climate impact of ISO, where the flight trajectory is optimised for fuel use and the aircraft is not redesigned for the ISO procedure. We find an increased warming effect, which arises from nitrogen oxide and water vapour emissions, which are released at higher cruise altitudes and which over-compensate reduced warming effects from CO2 and contrail-cirrus. However, we expect a climate impact reduction for ISO even with existing aircraft, avoiding the higher flight altitude in the first flight segment and hence reducing the fuel savings. Thus, climate impact benefits could be achieved if lower fuel savings were acceptable. Moreover, this negative climate impact is found for the particular case of introducing ISO using the current wide-body fleet. It does not necessarily apply to the adoption of ISO using aircraft redesigned for a shorter range. Keywords: Intermediate Stop Operations, Staging, emission inventory, climate assessment, operational concept, mitigation strategies, system-wide analysis

1 Introduction ronment, saturated with respect to water and they per- sist when the air is ice-supersaturated (e.g. Schumann, As aircraft most of the time cruise at high altitudes in 1996). The number of aircraft movements is expected the upper troposphere and lower stratosphere, the effect to double in the next 15 years causing aviation’s im- of gaseous emissions from aviation on radiative forc- pact on climate to increase further (Airbus, 2014). To ing agents is substantial (Lee et al., 2010; Brasseur limit these effects and to enable a sustainable develop- et al., 2016). While the effects of CO2 emission on the ment of aviation, immediate mitigation options are re- climate is generally independent of the emission locus, quired. Such mitigation options include technological the effects of non-CO2 emissions are depending on the measures like e.g. new combustion technologies, reg- weather situation (Grewe et al., 2014)aswellason ulatory measures, but also new operational strategies, the cruise altitude (e.g. Gauss et al., 2006; Frömming which change the way aircraft are operated (e.g. May- et al., 2012). Emitted nitrogen oxides produce ozone; the nard et al., 2015). Among the operational measures that higher aircraft emit NOx, the larger is its atmospheric have been discussed recently are general cruise altitude residence time and the more ozone develops (Grewe changes, i.e. flying at lower cruise altitudes (Frömming et al., 2002; Søvde et al., 2014). Contrails form when et al., 2012; Koch, 2013), selective closure of airspaces the exhaust air gets, during the mixing with the envi- (Niklass et al., 2015), changing horizontal flight tracks or optimizing the entire trajectory with respect to the ex- ∗Corresponding author: Florian Linke, DLR Lufttransportsysteme, Hamburg, pected climate impact (e.g. Grewe et al., 2014; Lührs Germany, fl[email protected] et al., 2016). These studies have shown that the options

© 2017 The authors DOI 10.1127/metz/2017/0763 Gebrüder Borntraeger Science Publishers, Stuttgart, www.borntraeger-cramer.com 2 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate Meteorol. Z., PrePub Article, 2017

can reduce the climate impact of flight operations sig- the concept’s non-CO2 emissions on the climate, how- nificantly for only comparably small cost penalties. An- ever, a detailed analysis of the changes of quantities and other such concept, which is often referred to as Interme- distribution of individual pollutant species is necessary. diate Stop Operations (ISO), suggests that aircraft oper- Creemers and Slingerland (2007) have estimated a ators conduct intermediate landings during a mission to global warming potential reduction of 13 % by ISO with reduce the stage length of flights. By refueling the air- optimized aircraft using a simplified method. For these craft at a stopover location the amount of fuel burnt over findings it was assumed that flight altitudes of the re- the entire mission can be reduced, as some fuel neces- designed aircraft will slightly increase and that the im- sary to transport the remaining fuel over a longer dis- pact of CO2,NOx and H2O emissions of one kilogram tance can be omitted. fuel can be modeled as a function of altitude. As stated The ISO concept has been subject of some studies in above, system-wide studies taking real-world air traffic the past, ranging from generic analyses of single mis- and route networks into account have been performed sions based on aircraft design methods to investigations with a focus on the global fuel saving potential only. A on fleet as well as global level. The focus of these stud- comprehensive study of the global impact of ISO on the ies was mainly to evaluate the potential fuel savings environment, i.e. emissions and climate, has not been that can be gained by the concept, but partly the au- done so far. That is the focus of this research. thors also looked into the effects on flight times, costs This paper presents a system-wide analysis of the (both single flight operating costs and lifecycle costs) short-term environmental impact of Intermediate Stop and safety. In some studies it was found that fuel sav- Operations. Due to the global character of the study ings are in the order of 13–23 % (the longer the mission, small-scale effects like changes of the local air quality the more fuel could be saved) for missions with a single (LAQ) at airports are not considered. The environmen- stopover if aircraft are used that are optimized for shorter tal impact is quantified by the amount and the distri- ranges (Martinez-Val et al., 2011; Lammering et al., bution of gaseous engine emissions as well as their ef- 2011; Langhans et al., 2010; Creemers and Slinger- fect on climate given as Average Temperature Response land, 2007). These findings were mainly obtained from (ATR). It is assumed that ISO are carried out with the payload-range efficiency considerations that have been current world-wide aircraft fleet in a real flight and air- derived from aircraft design relationships as provided by port network. All aircraft types and missions that poten- text books. Similar analyses have been done with current tially benefit from ISO are considered and realistic op- aircraft; here it was found that 5–15 % fuel can be saved erational influences, including wind, are taken into ac- depending on the aircraft type and mission length (Poll, count. By analysing previous emission inventories it can 2011; Lammering et al., 2011; Langhans et al., 2010; be shown that the selected set of flights account for ap- Creemers and Slingerland, 2007). For these analyses proximately 28 % of the fuel consumption of the global it was assumed that stopover airports were ideally lo- scheduled air traffic and a similar share of the relevant cated in the middle of the route, so no real flight and air- gaseous emissions like CO2,H2OandNOx. Introducing port networks were considered in these studies (generic ISO on these flights thus may have a significant effect on mission level). However, some authors also considered aviation’s fuel consumption and emissions. The applied real-world conditions; these so-called fleet and global models are described in detail in Section 2 and the simu- level assessments have been conducted by Poll (2011); lation set-up is given in Section 3. Results are presented Langhans et al. (2010); Green (2005); Linke et al. in Section 4 before they are discussed with respect to the (2011). E.g., assuming a real geographical distribution model assumptions in Section 5. of possible intermediate airports for flights operated by Boeing 777 or Airbus A330 Langhans et al. (2010) and Linke et al. (2011) found 10–11 % fuel savings glob- 2 Methodology ally if the aircraft is redesigned for 3000 NM (roughly 5600 km). The aircraft redesign was done using NASA’s A modeling system was developed that allows for the software for preliminary aircraft design called FLOPS assessment of operational concepts, like e.g. ISO, with (Flight Optimization System). The design range was respect to their impact on global emissions and climate. varied while other design parameters (like e.g. passen- As depicted in Figure 1 this system consists of dif- ger capacity) were kept constant. By analysing the ISO ferent models. Flight movements are simulated using opportunities of the different redesigns in the real flight the Trajectory Calculation Module (TCM, Linke, 2008; network it was found that the optimum design range for Lührs, 2013), which computes aircraft trajectories from a new mid-range aircraft optimized for ISO is approxi- lift-off to touch-down applying a kinetic mass-point mately 3000 NM. model that provides simplified equations of motion In addition to the positive implications of the ISO known as Total Energy Model. One of the key features concept on fuel consumption and operating costs many implemented in the TCM for the purpose of this re- authors infer that the concept may consequently reduce search is the use of the advanced aircraft performance the environmental impact of aviation. With regard to model (APM) BADA (Base of Aircraft Data) version 4, the CO2 footprint this conclusion is valid without fur- which allows for modeling typical flight operations re- ther ado, for a sound understanding of the impact of alistically. The BADA 4 models cover the whole flight Meteorol. Z., PrePub Article, 2017 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate 3

Generation of reduced emission profiles Profile reduction Reduced (Pre-processing) (Downsampling) emission profiles

Trajectory Environment

Emissions Inventory BADA 4 BADA APM Equations generation Inventory NOx , HC, CO

Flight profile CO2 , H 2O, SOx optimization Climate AirClim metrics

TCM Emission indices: ICAO EED

Missions/scenario Atmosphere

METEO, e.g. Determination of air distances Flight Planning ADI ECMWF

Orthodrome Route optimization IATA factor) Data processing and analysis Wind distributions constrained: unconstrained:

statistics Mission (O/D, A/C type, load Dijkstra/A* Optimal control Take-off distance Wind Climate

TOFL Air traffic charts infrastructure, e.g. EAD

Figure 1: Schematic diagram of the developed modeling system (abbreviations: A/C – aircraft; O/D – origin/destination; see text for further acronyms). envelope, capture the flight physics more accurately than it resulting in a data set of local wind distributions. This previous model versions and thus can be used to deter- database contains discrete wind cases (combinations of mine e.g. optimized vertical profiles, i.e. optimum alti- wind speed and direction) and their respective frequency tudes and speeds (Mouillet, 2013). Using this capa- of occurrence for every point in the grid. This data is bility airline-preferred cruise profiles can be estimated used to determine characteristic mean still air distances including the location of step climbs depending on the for any given flight route as a basis for system-wide selected step climb strategy and the heading-dependent analyses valid for longer periods of time, e.g. one year available flight levels. Regarding meteorological data (Swaid, 2013; Linke, 2016). These air distances are the TCM can either be used with International Standard eventually used in the emission distribution calculation Atmosphere conditions or with real atmospheric data in explained below to account for wind. NetCDF or GRIB format that can e.g. be obtained from Moreover, a flight planning functionality is included the European Centre for Medium-Range Weather Fore- that provides route optimization capabilities with respect casts (ECMWF). to different criteria. This is useful whenever realistic air- The consideration of wind in system-wide analyses line operations should be modeled. Today, many air- of new operational concepts is one of the main contri- craft operators already follow so-called wind-optimal butions of the work with respect to the methodology. routes that minimize flight time and fuel consumption From an aircraft performance point of view wind affects for a given mission in the presence of wind. Such wind- the mission and thus flight time, fuel burn and emis- optimal routes can be determined either without any sions by changing the actual distance the aircraft has to constraints using an optimal control approach (Lührs, cover, also known as (still) air distance. Whereas tail- 2013) or applying a constraining air traffic services wind shortens the air distance of a flight for a given (ATS) route network solving a shortest-path problem ground distance, headwind and crosswind increase it. (combined Dijkstra/A* method, Swaid, 2014). For this As the aircraft flies, its (true) airspeed overlays with the purpose the model accesses the European Aeronautical wind speed in a vector form; in a crosswind situation Information Services Database (EAD), a comprehensive some of the aircraft’s energy is needed to compensate air traffic infrastructure database containing geographi- for the drift by applying a wind correction angle in or- cal data on airports, waypoints and complete ATS routes. der to maintain a desired course. The required take-off field length (TOFL) at a given air- For considering the described wind effect a new and port for predominant temperature and pressure condi- highly efficient method has been developed which is tions can be determined with a TOFL model that is made able to process daily wind data and statistically analyse up by charts taken from airport compatibility manuals. 4 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate Meteorol. Z., PrePub Article, 2017

For the environmental analysis the modeling system appropriate cruise speed. This mach number generally includes an emission model that determines the gaseous represents a good compromise between fuel consump- emissions along resulting trajectories from TCM. Here, tion and flight time. The resulting standardized profiles both emission species that are produced proportionally are then down-sampled and only the relevant aircraft to fuel burn, i.e. CO2,H2O, as well as species that de- state parameters (flown air distance, flight time, altitude, velop in a non-proportional way, i.e. NOx, HC and CO, fuel flow, emission flows of all species) at the profile are determined. For the latter the state-of-the-art fuel vertices are stored into a database. Assuming a linear flow correlation method Boeing Fuel Flow Method 2 parameter gradient between each two flight phase ver- (DuBois and Paynter, 2006) is applied in combination tices, from those few points an entire profile can be with Emission Indices for sea level conditions obtained recreated. An analysis of the errors of all profiles in the from the Engine Emission Databank (EED) by the In- database resulting from this linearization has revealed ternational Civil Aviation Organization (ICAO). After- that it is generally in the order of ±0.1 percent and thus wards, these emission distributions can be mapped into can be neglected. With given mean air distances deter- a geographical grid, which allows for the generation of mined by the approach described above to consider the emission inventories by superposing the emissions of wind effect, for each flight the appropriate profile is ob- a large number of flights. These inventories are then tained from the database. Finally, the emission profile is used by the climate-chemistry response model AirClim mapped into the geographical grid by scaling it accord- (Grewe and Stenke, 2008; Dahlmann et al., 2016)to ing to the segment-wise air distance values to the respec- determine the climate impact resulting from the emis- tive ground distance and aligning it to the flight path. sions. The basis of this method constitute atmospheric Thereby, this method accounts for the effect of wind and concentration changes of radiative forcing agents as a can also be used to consider potential horizontal flight function of latitude and altitude caused by unit emis- inefficiencies. Through the above mentioned combina- sions, which were pre-calculated using the complex tion of trajectory computation, flight planning and en- climate-chemistry model ECHAM4.L39(DLR)/CHEM vironmental analysis capabilities, the modeling system (Hein et al., 2001) as well as the corresponding radiative can be used to evaluate the environmental effects result- forcing (RF). The model has been evaluated with respect ing from changes of flight and fleet operations (Linke, to concentration changes of water vapour and ozone, and 2016). especially RF values for changes in the flight altitude by comparing AirClim results with results from detailed atmosphere-chemistry models (Grewe and Stenke, 3 Study set-up 2008; Grewe and Dahlmann, 2012; Dahlmann et al., 2016). In addition, a comparison of the vertical sen- In this study we apply the modeling system to ana- sitivity of aircraft emissions on the RF between Air- lyse the implications Intermediate Stop Operations have Clim, LEEA (Köhler et al., 2008; Rädel and Shine, on global aviation emissions and climate. For this pur- 2008) and E39CA has been performed in Grewe and pose, we generate emission inventories for two scenar- Dahlmann (2012). The results clearly show a good ios and compare them to each other: the reference case is representation of the RF response caused by altitude made up of a large set of flight missions (approximately changes agreeing within a range of ±10 % for ozone, 1.023 million annual flights) in which every mission is contrails and ±15 % for water vapour. The model has conventionally performed as direct flight, whereas the been previously applied to assess the climate impact of ISO case contains for each mission two flight segments aircraft designs and trajectory options in a variety of connecting the origin to the destination airport via a studies (Grewe et al., 2010; Koch et al., 2012; Grewe stopover at the refueling airport. As the short-term ef- et al., 2016; Dahlmann et al., 2016). fects of ISO are of interest in this study, it is assumed In order to reduce the necessary number of trajec- that each ISO mission is performed by the same aircraft tory simulations in the course of a global analysis with type as used for the direct flight (self-substitution)and a large number of flights the developed modeling sys- no further changes of the aircraft fleet need to be consid- tem makes use of a method that is commonly applied ered. Flight movement data is obtained from Sabre ADI to reduce the complexity during the generation of emis- (Airport Data Intelligence, now: Sabre AirVision Market sion inventories. So-called reduced emission profiles are Intelligence) flight schedule database (http://www.airdi. used that were derived from pre-calculated trajectories net) for the first quarter of 2010 and flight frequencies and emission distributions. For each considered aircraft are scaled up to the period of one year. As previous stud- type (19 wide-body aircraft types were used that cover ies have revealed that only wide-body aircraft actually the entire Airbus and Boeing wide-body aircraft fleet), show a fuel saving potential in self-substitution on mis- missions of different air distances and load factors have sion lengths above 2500 NM (Linke et al., 2012), this been simulated with TCM and the corresponding emis- study is limited to the global wide-body aircraft fleet. In sion distributions along these trajectories were deter- order to estimate aircraft masses region-dependent pas- mined. For all missions it was assumed that the pilot senger load factors are calculated based on economics flies as close as possible to the optimum altitude and se- statistics published regularly by the International Air lects the so-called Long-Range Cruise mach number as Transport Association (http://www.iata.org/economics). Meteorol. Z., PrePub Article, 2017 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate 5

Flight profile Emission profile 40000

35000

30000

25000

20000 Altitude [ft] 15000

10000

Fuel,Fuel CO 22 , H O, SO 2 NOX 5000 Direct flight CO ISO segments HC 0 ! 0 1000 2000 3000 4000 5000 6000 −100 0 100 200 Range [NM] Rel. change of emissions [%]

Figure 2: Change of ﬂight altitudes and vertical emission distribution due to ISO shown on an exemplary 6000 NM mission ﬂown with an Airbus A340-600 aircraft assuming an ideal intermediate landing (in the middle).

In preparation for the inventory calculation, the generic mission the results of a system-wide analysis on stopover locations have to be defined; using an exhaus- a global level considering real flight networks, airport tive search algorithm for each ISO mission the respec- locations and meteorology are given. tive airport is determined by optimization, assuming that location is selected for the stopover which leads to the 4.1 Generic mission profile maximum fuel savings for the specific mission. The airports’ geographical coordinates are obtained from the Figure 2 shows the vertical flight profile of a standard EAD database which is filtered for only major airports 6000 NM mission simulated with an Airbus A340-600 with at least one asphalt-surfaced runway and an instru- aircraft. A distinct stepped climb cruise from an initial ment landing system assuming that certain equipment cruise flight level of 33000 ft (FL 330) up to FL 390 can needs to be installed such that commercial wide-body be observed. As the aircraft weight decreases over time airplanes are able to perform an intermediate stop there. due to the continuous fuel burn the optimum altitude Moreover, a minimum runway length needs to be avail- of the aircraft increases (Airbus Customer Services, able which is defined by the required TOFL of the air- 2002). The optimum altitude is defined as the altitude at craft for the given take-off weight (TOW) and the am- which the aircraft’s specific range becomes maximum. bient conditions at the field. The meteorological data is In today’s flight operations step climbs are conducted taken from ECMWF for a grid of 0.75° × 0.75° for the to follow the optimum altitude as good as possible while period of one year (2012). The statistical wind distribu- ensuring compliance with air traffic management (ATM) tions mentioned above are used to account for the effect constraints. Besides the profile of the direct flight mis- of wind by considering annual mean air distances be- sion, also the profiles of the two resulting flight segments tween every airport pair. These air distances are used to for a flight performing a stopover at the ideal location af- obtain the respective reduced emission profiles from the ter 3000 NM are shown. It can be seen that especially the database. By projecting these emissions into the ground- first segment has a higher initial cruise flight level than based grid additional emissions caused by headwind are the direct flight. The reason for this is the reduced TOW attributed to the grid cells; on the contrary, in case of tail- in the ISO case corresponding to a higher optimum alti- wind by stretching the profiles to match the ground dis- tude. Assuming that pilots try to fly as close as possible tances, a reduced amount of emissions is assigned to the to the optimum altitude for fuel economy reasons, this grid cells. For the sake of simplicity only orthodromic fact would lead to a shift of cruise emissions by approx- (great circle) routes are assumed. imately 4000 ft upwards (in this example). The relative changes of the amounts of the different emission species per altitude layer are also de- 4Results picted in the emission profile in Figure 2. In addition, Table 1 shows the emission split between cruise and The introduction of ISO affects the amount and the climb/descent both for the relative emission amounts distribution of engine exhaust emissions and thus leads and for the emission changes caused by ISO. It can be to a change of the climate impact. In the following seen that below cruise there is an increase of CO emis- the results of the study are presented. After a principal sions by about 96 % and an increase of HC emissions by investigation of the emission distribution changes on a approximately 83 % due to ISO. This can be attributed 6 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate Meteorol. Z., PrePub Article, 2017

Table 1: Relative amounts of emissions and emission changes below (< FL 310) and at (≥ FL 310) cruise altitude.

Species Direct ﬂight Changes due to ISO ≥ FL 310 < FL 310 ≥ FL 310 < FL 310

Fuel, CO2,H2O, SO2 93.50 % 6.50 % −8.94 % −13.33 % +54.11 % NOx 89.29 % 10.71 % −12.28 % −19.65 % +49.19 % CO 60.35 % 39.65 % +42.27 % +6.76 % +96.30 % HC 92.02 % 7.98 % +6.81 % +0.25 % +82.55 %

Figure 3: Geographical distribution of the top 20 most frequented ISO airports in the world.

to the doubling of flight phases with low thrust settings 4.2 System-wide analysis (e.g. descents and landings) in which the mixing process in the combustor is rather inefficient. Products de- In a first step for each mission the optimum stopover air- veloping proportionally to fuel burn, i.e. CO2,H2Oand port was determined. For 86.8 % of all simulated long- SO2,aswellasNOx increase by 49–54 % during climb haul flights appropriate airports were found that lead to in the ISO case, because the lower TOW allows for a positive fuel savings compared to the direct flight. It is steeper climb and less time is spent in grid cells be- assumed that as soon as positive savings can be achieved low cruise flight level. The increased cruise altitudes on by a stopover the flight is operated in ISO mode. The re- the first mission segment cause a reduction of emissions maining flights are conducted in direct mode. Table 2 by nearly 100 % on lower cruise flight levels (FL330 shows the 20 most affected airports together with the and FL350 in this example), whereas an emission in- number of additional landings and take-offs due to ISO. crease is caused on the upper flight levels (FL370 and It is not surprising that these airports are mainly located FL390), as can be seen in Figure 2. There is also a rel- in regions which are crossed by long-haul flights, includ- ative emission increase on the intermediate flight lev- ing Newfoundland, Greenland, Siberia as well as some els FL340 and FL360 (for ATM reasons there has to be islands in the Atlantic and Pacific Oceans. A map of a minimum separation between the available flight lev- these airports is depicted in Figure 3. Overall 440 in- els), which the aircraft only briefly flies through, due dividual airports were identified that serve as stopover to the doubling of climb and descent segments; how- locations for ISO missions. Approximately 40 % of all ever, the absolute amount of emissions on these levels is intermediate stops can be accommodated by the 20 most very small. This example helps understanding the gen- frequented airports in Table 2. These findings are consis- eral phenomena connected to profile adjustments due to tent with results from previous studies, including Linke ISO. In reality, suitable airports for intermediate stops et al. (2011) and Langhans et al. (2013),however,here are not ideally located and mission lengths differ con- we consider the effect of wind for the first time. It should siderably. Therefore, a system-wide study is needed to be noticed, that in reality, most of the listed airports quantify the effects that can be expected in a real opera- would not have the required capacity to accommodate tional environment. the additional landings and take-offs right now. How- Meteorol. Z., PrePub Article, 2017 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate 7

Table 2: Top 20 most frequented ISO airports with number of additional landings and take-offs.

Rank ICAO Airport name Location Flights 1 CYQX Gander International Airport Gander, Newfoundland, Canada 56090 6.31 % 2 CYYT St. John’s International Airport St. John’s, Newfoundland, Canada 46387 5.22 % 3 LPLA Lajes Airport Lajes, Azores, Portugal 27680 3.12 % 4 CYYR Goose Bay Airport Goose Bay, Labrador, Canada 26432 2.97 % 5 BGBW Narsarsuaq Airport Narsarsuaq, Greenland 23685 2.67 % 6 PADK Adak Island Airport Adak (Island), Alaska, USA 19239 2.17 % 7 YBRM Broome International Airport Broome, Western Australia, Australia 14586 1.64 % 8 CYFB Iqaluit Airport Iqaluit, Nunavut, Canada 12401 1.40 % 9 GVSV Cesária Évora Airport São Vicente, Capeverde 11878 1.34 % 10 CYVP Kuujjuaq Airport Kuujjuaq, Québec, Canada 11856 1.33 % 11 LTFH Carsamba Airport Samsun, Turkey 11644 1.31 % 12 PLCH Cassidy International Airport Banana, Kiritimati (Island), Kiribati 10959 1.23 % 13 WAPP Pattimura Airport Ambon, Indonesia 10946 1.23 % 14 GVFM Nelson Mandela International Airport Praia, Santiago (Island), Capeverde 10923 1.23 % 15 USNR Raduzhny Airport Raduzhny, Russia 9950 1.12 % 16 CYDF Deer Lake Airport Deer Lake, Newfoundland, Canada 9604 1.08 % 17 PABR Wiley Post-Will Rogers Memorial Airport Barrow, Alaska, USA 8874 1.00 % 18 PACD Cold Bay Airport Cold Bay, Alaska, USA 8797 0.99 % 19 UHMA Ugolny Airport Anadyr, Russia 8658 0.97 % 20 UOOO Norilsk Alykel Airport Norilsk, Russia 7495 0.84 % 348086 39.17 %

ever, it is assumed that as soon as the introduction of reduction in NOx, an increase by 33.3 % of CO and an the ISO concepts starts, a demand is created gradually increase of HC by 43.4 %. The latter findings indicate at these airports that would lead to the necessary infras- a potential LAQ issue, especially at highly frequented tructural expansions. ISO airports, and should be subject to further investi- Based on the identified ISO airports, emission in- gation. Figure 6 depicts the H2O inventory projections ventories were calculated both for the direct flight sce- into the longitude-altitude plane both for the direct flight nario and for the ISO mode scenario. The difference of and the ISO scenario. Obviously most cruise emissions these inventories with regard to pure fuel consumption are moved to approximately 12 km altitude due to the isshowninFigure4. Positive peaks (large differences shift of initial cruise flight levels to higher altitudes by to reference case) are marked in red. They can be found 4000–6000 ft. Additionally, the number of step climbs in those regions that were identified before, as additional in ISO mode is reduced due to shorter segment lengths emissions are produced near airports accommodating in- and therefore the altitude band is narrowed from 7000 ft termediate stops. From the latitude and altitude profiles to approximately 3000 ft. a shift of emissions towards higher latitudes and alti- Finally, a climate impact assessment is performed. tudes can be observed. The latter effect had already been We are evaluating the long-term future climate change discussed on a generic level in the previous section. of aviation when introducing ISO and compare this sce- Overall, the introduction of Intermediate Stop Oper- nario to a reference case without ISO. Hence, we are ations could save approximately 3 million tons of fuel evaluating the changes in the long-term climate impact per year representing 4.8 % of the entire fuel consump- for the mitigation strategy “Flying ISO”. A suitable cli- tion in the reference scenario, i.e. the respective wide- mate metric is the Average Temperature Response on body flights. Given an approximate portion of the se- a 100 year time horizon (e.g. Grewe and Dahlmann, lected flights of 28 % from the global fuel consumption 2015; Koch, 2013). The ATR is calculated as the time (scheduled air traffic), these savings amount to 1.3 % integral of the temporal course of the near-surface tem- globally. Realizing ISO on a global scale would only perature change divided by the time horizon and thus, require 0.4 % extension of the flown ground distance. as opposed to other metrics like RF or Global Warm- In general this implies that suitably located airports can ing Potential, allows for the quantification of a result- be found without requiring large detours. ISO can even ing temperature change that takes into account the dy- achieve a reduction of the flown air distance by 0.15 %, namics of the earth-climate system. As base scenario as in many cases airports can be found that are located the Fa1 growth scenario defined by the ICAO Fore- along the wind-optimal route shortening on average the casting and Economic Analysis Support Group was se- actual flight distance in the presence of winds. Figure 5 lected, which includes assumptions regarding the tem- shows the emission inventories also for NOx and CO poral development of global emissions. This emission species. While CO2,H2OandSO2 are reduced by 4.8 % scenario is hence based on conventional techniques and with respect to the direct flight scenario, there is a 4.6 % represents a business-as-usual-sccenario. The emission 8 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate Meteorol. Z., PrePub Article, 2017

Fuel consumption difference [kg/km ] [kg/km ] −20 0 20 40 2000 90

1500 60

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0 0

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] 10 400 8 12

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−2 Altitude [km] 4 Flight level [100 ft] −4 100 2 −6

Fuel consumption difference [kg/km Fuel consumption difference −8 0 0 −180 −150 −120 −90 −60 −30 0 30 60 90 120 150 180 −4 −2 0 2 4 Fuel consumption difference [kg/km ]

Figure 4: Redistribution of global fuel consumption due to ISO on long-haul missions (differences from direct flight scenario) in the first quarter of 2010; top left: geographical distribution of changes in the fuel sums per grid column; top right: latitude profile (zonal); bottom left: longitude profile (meridional); bottom right: altitude profile. effects caused by the ISO concept were scaled by the by a slight shift of emissions to higher latitudes as the evolution factors from the scenario to simulate traffic tropopause altitude falls towards the poles. The increase growth as well as technological advances and the ramp- of cruise altitude also leads to an increase in ozone up of the ISO concept introduction was assumed to last and to an increase in methane lifetime, which both are 10 years starting in 2015. The results can be seen in contributing to an increased warming. These results are Figure 7. Although the absolute amount of emissions consistent with previous findings, e.g. Frömming et al. of the species CO2,H2O, SO2 and NOx can be re- (2012); Søvde et al. (2014); Dahlmann et al. (2016). duced through ISO the Average Temperature Response On the other hand contrail formation is avoided, increases by 2.3 %, which is one of the major findings since many flights are above the main contrail forma- of the study. This temperature increase of 2.3 % results tion area. The radiative forcing by contrails reaches from reduced warming effects of CO2 (−0.72 %) and its maximum just below the tropopause and is depen- contrails (−0.35 %), which are over-compensated by an dent on contrail coverage and optical properties. Above increase in warming from NOx emissions (+2.12 %) and the (climatological) tropopause the radiative forcing by H2O emissions (+1.26 %). contrails then rapidly decreases with increasing altitude The most important effect is the increase in cruise al- (Lee et al., 2009). The shift to higher altitudes and lat- titude at mid latitudes for ISO compared to the reference itudes therefore helps avoiding contrails and reduces situation. This results in a shift of emissions to slightly the warming from contrails. The results are largely in higher altitudes, where mixing processes are slower and agreement with Frömming et al. (2012). In combina- hence result in a larger accumulation of nitrogen oxides tion, these findings show that in contrast to speculations and water vapor. The relative change in climate impact from previous studies a systematic introduction of ISO is largest for water vapor (almost 25 %, Fig. 7). The ra- on a global level would not necessarily have positive im- diative forcing of water vapor generally increases with plications for the climate, at least not for the current air- the altitude at which it is released (Grewe and Stenke, craft fleet. Our results show that the increase in warming 2008; Lee et al., 2010). The upwards shift of flight lev- effects from NOx and H2O emissions cannot be com- els into the lower stratosphere therefore intensifies the pensated by a reduction in the warming from less CO2 greenhouse effect of H2O. This effect is even increased emissions and from less contrails. Meteorol. Z., PrePub Article, 2017 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate 9

Figure 5: Aggregated geographical emission distribution for the ﬁrst quarter 2010; left: direct ﬂights, right: ISO mode.

5 Discussion the effects are estimated with large uncertainty ranges (Lee et al., 2009). However, Dahlmann et al. (2016) We have investigated the climate impact of an immedi- showed that these uncertainties are not limiting the as- ate introduction of intermediate stop operations. Natu- sessment of technology options, which include varia- rally, the results depend on assumptions we made. This tions in cruise altitude. Additionally, some potential im- includes the choice of the climate metric, regarded at- pacts, such as the effect of aerosol emissions on clouds mospheric processes, impacts from ground emissions or were not taken into account here, since the scientific possibilities of re-designing the aircraft, which we dis- understanding of these potential impacts is not mature cuss here in more detail. enough to be included here. In general, the choice of the climate metric plays a Further uncertainties result from inaccuracies in the crucial role in any assessment of aviation technologies emission quantification which are caused by model ef- (Fuglestvedt et al., 2010). However here, we ask the fects in the trajectory calculation and the fuel flow cor- question “What would be the long-term impact to the relation method. Comparisons between BADA 4 and climate if we started introducing ISO now?”. And this highly detailed aircraft performance data from Airbus limits the choice of climate metrics suitable to answer have shown, that for the respective aircraft types the fuel this question (Grewe and Dahlmann, 2015)andthe flow modelling error is 2.3 % on average (Nuic, 2013). impact of the choice of a suitable climate metric on the Furthermore, for a fuel flow that is precisely known results is low (Grewe et al., 2014). The time horizon Schulte et al. (1997) found by comparison to in-situ would have played a very crucial role, if we had chosen measurements that the Boeing Fuel Flow Method 2 a pulse emission, however then we would have asked a tends to systematically underestimate real NOx emis- different question. For an increasing emission scenario, sions by approximately 11 %. Other aspects that influ- as taken here into account, short-term and long-term ence the predicted fuel flow are the aircraft mass as well climate effects are more balanced (Grewe and Stenke, as the assumed altitude and speed profile. Here, we gen- 2008, their appendix). erally simulate optimum profiles such that the reduced The results presented above strongly depend on the drag might slightly underestimate the fuel flow as well. balance of the contributions from the various radiative However, in combination the emission quantification er- forcing agents to the overall climate response. Many of rors still seem to be acceptable, especially given the fact, 10 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate Meteorol. Z., PrePub Article, 2017

[Gg] 42000 16 40000

38000 14 36000 34000 12 32000 10 2 30000 26000 Direct flights 8

Altitude [ft] 22000 18000 6 14000 4 10000 Aggregated amount of H O emissions 6000 2 2000

−180! −150 −120 −90 −60 −30 0 30 60 90 120 150 180

! [Gg] 42000 18 40000 38000 16 36000 14 34000 32000 12 2 30000 10 26000 ISO 8 Altitude [ft] 22000 18000 6 14000 10000 4 Aggregated amount of H O emissions 6000 2 2000

−180! −150 −120 −90 −60 −30 0 30 60 90 120 150 180

Figure 6: Distributions of H2O emissions (absolute values meridionally aggregated) in longitude-altitude plane for the direct ﬂight mode (top) and the ISO mode (bottom); red line depicts a break in the vertical scale.

SP 2 &+ &2 +2 2 Contrails 100% $75UHI -4.7% -14.1% 14.7% 5.2% 54.8% 44.3%

$75,62 -4% -4% -4.9% +24.2% +2.5% -0.8% +2.3% ෥෥

Figure 7: Climate impact changes (metrics: ATR100) due to ISO separated by contributions of different radiative forcing agents (reference case: percentage of overall ATR; ISO case: relative changes with respect to reference case). that for the analysis the focus is laid on a relative com- as considered in this work the percentage of time, fuel parison rather than providing absolute numbers. and emissions spent during flight by far dominates the Emissions from aircraft ground operations were not ground operations portion. For a more detailed study on taken into account. However, as this study focuses on the implications of the ISO concept on local air quality the global emission distribution and the climate impact aspects at the affected airports, emissions from ground resulting primarily from emissions in the upper tropo- operations need to be considered. sphere and lower stratosphere this simplification is con- Furthermore, it should be noticed that the results re- sidered to be appropriate. Moreover, on long-haul flights fer to a scenario in which a short-term introduction of Meteorol. Z., PrePub Article, 2017 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate 11

ISO without any adaptations to the existing aircraft fleet 6 Conclusions and self-substitution are assumed. Given the current low kerosene price one can argue that this scenario is at present rather unrealistic. As mentioned in the introduc- A method has been presented that allows for the assess- tion there have been studies also on the cost implications ment of new operational concepts with respect to their of the ISO concept revealing that in spite of the possi- impact on global emissions and climate. The modeling ble fuel savings there are only few long-haul routes on system comprises a trajectory simulation module, flight which airlines actually are able to reduce the Direct Op- planning functionalities, atmospheric models including erating Costs on their flights (Linke et al., 2012). The an efficient wind distribution method as well as mod- reason for this is that fuel costs are only one portion els for generating global emission inventories and cal- (up to 50 % on long-haul flights) of the overall DOC culating their climate impact. The trajectory calculation bill and ISO induces additional costs that offset the fuel makes use of the most advanced aircraft performance cost savings. These additional costs are partly caused by models, namely BADA 4, provided by EUROCON- the increased flight times (e.g. higher crew costs and re- TROL, and for the first time provides means to model duced aircraft utilization) and include additional landing flight operations including cruise profiles from an airline and en-route navigation fees as well as increased mainte- point-of-view more realistically. A mechanism for com- nance costs for engine and airframe due to the doubling plexity reduction was applied by using a database of pre- of flight cycles. Furthermore, in a society for which calculated reduced emission profiles. The method was comfort is of high significance and an omnipresent part applied to analyse the environmental implications of the of the life-style the passengers’ willingness to accept ISO concept for today’s worldwide aircraft fleet. A large longer flight times and intermediate stops is limited un- air traffic scenario containing all world-wide long-haul less flight tickets are significantly cheaper than for di- flights in 2010 was considered and the effect of wind rect flights. This would further reduce the profit margin was accounted for. of the operator. In combination with optimistic assump- Overall, 4.8 % of fuel can be saved through ISO glob- tions regarding the available capacity at the ISO airports, ally, assuming a full coverage of the operational con- a global short-term implementation of the ISO concept cept. Airports serving as suitable stopover airports are is therefore rather unlikely. The results should be under- located mainly in Newfoundland, Greenland, Siberia, stood as indications of maximum possible savings and Azores and Capeverdes. While most emission species the corresponding climate impact in a “what-if” man- can be reduced by ISO, there would be an increase of ner. 33–43 % of HC and CO emissions due to the doubling A more realistic scenario may consider ISO with air- of descent and landing phases causing a potential LAQ craft types that are redesigned and optimized for shorter issue. Due to a lower TOW on ISO missions, the initial ranges. According to e.g. Langhans et al. (2013),in cruise flight levels are shifted up and the altitude band this case higher cost savings can be expected that would is narrowed as flight segments are shorter and less step make an adoption for airlines easier. Such an aircraft climbs are required. This emission relocation causes a would have a smaller wing and a lower initial cruise alti- warming climate impact compared to the direct oper- tude than the original long-haul aircraft fuelled for only ations by 2.3 % in the Average Temperature Response one ISO leg. It is therefore expected, that the utilization over 100 years as the increased warming effects, caused of redesigned medium-range aircraft would rather shift by the emitted NOx and H2O, dominates over the re- cruise emissions to lower altitudes which consequently duced warming effects from CO2 and contrails. could turn the negative climate impact of water vapour As discussed above, a more realistic adoption of and nitrogen oxides into a positive one while saving even medium-range aircraft for flying ISO could on the other more fuel. hand have a positive climate impact due to the expected Based on these findings, there is the need for a fur- lower cruise altitudes. A more detailed analysis and ther system-level study taking into account redesigned verification of this should be subject of future research. medium-range aircraft. Such a study should include a realistic DOC model and analyse various design options with different range and cruise altitude requirements. Acknowledgments For the existing aircraft fleet it could be investigated to what extent decreasing cruise altitudes during ISO mode would reduce the negative effects of the concept. As this Parts of this work were carried out within the DLR in- also reduces the potential fuel savings of the concept a ternal project WeCare. The authors would like to thank trade-off needs to be done and an optimization could be the project team for fruitful discussions leading to the conducted to determine the optimum altitude for the ISO presented results. Furthermore, the authors appreciate missions in order to have a combined environmental and the contributions by some colleagues at DLR, particu- fuel saving benefit. However, we expect a climate impact larly Majed Swaid and Benjamin Lührs who were in- reduction for ISO even with existing aircraft, avoiding volved in the development of the modeling system and the higher flight altitude in the first flight segment and contributed with their valuable knowledge on trajectory hence reducing the fuel savings. calculation and wind modeling. 12 F. Linke et al.: The Implications of ISO on Aviation Emissions and Climate Meteorol. Z., PrePub Article, 2017

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