Special Issue "Selected Papers from 3rd ECATS Conference on Making Aviation Environmentally Sustainable"

A special issue of Aerospace (ISSN 2226-4310).

Deadline for manuscript submissions: closed (31 December 2020).

Special Issue Editors

Prof. Dr. David Raper
E-Mail Website
Guest Editor
Chair of Environmental Science, Centre for Aviation, Transport and the Environment, Faculty of Science and Engineering, Manchester Metropolitan University, Manchester, M1 5GD, UK
Interests: airport air quality; aircraft emissions; sustainable development of aviation
Prof. Dr. Volker Grewe
E-Mail Website
Guest Editor
1. Deutsches Zentrum für Luft- und Raumfahrt (DLR), German Aerospace Center, Institute of Atmospheric Physics (IPA), Oberpfaffenhofen, 82234 Wessling, Germany;
2. Delft University of Technology, Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlands
Interests: climate impact of aviation; atmospheric chemistry; technical and operational mitigation options
Special Issues and Collections in MDPI journals
Dr. Sigrun Matthes
E-Mail Website
Guest Editor
Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Erdsystem-Modellierung, Münchner Straße 20, 82234 Oberpfaffenhofen-Wessling, Germany
Interests: aviation climate impact and mitigation; environmentally-optimized aircraft trajectories; chemistry-climate modelling; green flight and interdependency modelling
Special Issues and Collections in MDPI journals
Dr. Simon Blakey
E-Mail Website
Guest Editor
Department of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
Interests: fuel thermal stability; alternative fuels; combustion system efficiency; experimental design
Prof. Dr. Ola Isaksson
E-Mail Website
Guest Editor
Division of Product Development, Department of Industrial and Materials Science, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
Interests: multidisciplinary design; value driven design; collaborative engineering; platform based development; design automation
Prof. Dr. Huadong Yao
E-Mail Website
Guest Editor
Division of Fluid Dynamics, Department of Mechanics and Maritime Sciences, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
Interests: computational acoustics; fluid-structure interaction
Prof. Dr. Tomas Grönstedt
E-Mail Website
Guest Editor
Division of Fluid Dynamics, Department of applied mechanics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
Interests: turbomachinery conceptual design; aeroengine conceptual design

Special Issue Information

Dear Colleagues,

This Special Issue is cooperating with the 3rd ECATS conference on Making Aviation Environmentally Sustainable, which will take place online 13-15 October 2020 as an open access event.

Building upon the success of the 1st ECATS Conference (Berlin 2013) and 2nd ECATS Conference (Athens 2016), this scientific conference ECATS 2020 will give the opportunity to present state-of-the art research, review recent achievements and at the same time will provide a strategic perspective on future directions in environmentally sustainable aviation. The conference programme will present research in eight key areas of multidisciplinary expertise, allowing in-depth discussions and poster presentations.

  • Alternative fuels for aviation
  • Propulsion integration
  • Airport air quality
  • Aviation climate impact and mitigation concepts
  • Green flight—climate optimal flight trajectory
  • Future materials for aircraft
  • Interdependency and aviation environmental modelling
  • General session for the Swedish National Aeronautics Programme (NFFP)

Authors of contributions relating to the above key themes are welcome to submit extended versions of their conference work to this Special Issue for publication in our journal Aerospace.

Prof. Dr. David Raper
Prof. Dr. Tomas Grönstedt
Prof. Dr. Volker Grewe
Dr. Simon Blakey
Dr. Sigrun Matthes
Prof. Dr. Ola Isaksson
Prof. Dr. Huadong Yao
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Aerospace is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

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Published Papers (17 papers)

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Article
Fuel Planning Strategies Considering Operational Uncertainties of Aerodynamic Formation Flight
Aerospace 2021, 8(3), 67; https://doi.org/10.3390/aerospace8030067 - 07 Mar 2021
Viewed by 597
Abstract
The operational concept of aerodynamic formation flight, also referred to as aircraft wake-surfing for efficiency (AWSE), has high potential in terms of fuel savings and climate impact mitigation. In order to implement this concept, many technological and operational challenges have to be coped [...] Read more.
The operational concept of aerodynamic formation flight, also referred to as aircraft wake-surfing for efficiency (AWSE), has high potential in terms of fuel savings and climate impact mitigation. In order to implement this concept, many technological and operational challenges have to be coped with. As the fuel consumption during a mission strongly depends on a successful execution of AWSE, the existing uncertainties regarding flight planning increase. While a conservative fuel planning ensures a follower to complete the mission even in the case of a formation failure, it might result in high amounts of excess fuel and, therefore, additional fuel consumption. In this study, this issue is addressed by the adaptation of flight planning procedures to the requirements of AWSE focusing on fuel planning in particular, considered from the perspective of a designated follower aircraft of a two-aircraft formation. This trade-off is modeled as an n-action two-event decision-making problem. Each of the possible actions is represented by a combination of mission routing and a corresponding diversion airport, taking atmospheric effects (e.g., wind) into account in order to determine the resulting amount of trip fuel. The two events under consideration are a total formation failure in contrast to a complete success. Based on a scenario with a set of double origin destination pairs characterizing the formations and representative weather patterns for the North Atlantic region, each action is analyzed with regard to the expected fuel consumption and expense. Based on a set of assumed formation success probabilities, we find that the proposed method holds a savings potential to reduce the follower’s fuel consumption by 4.8% and its monetary expenses by 1.2% compared with a conventional flight planning. In order to gain a monetary profit margin applying this method, the required formation success probability is shown to vary between 92% and 96%, depending on the assumed fuel price. Full article
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Article
Evaluation of the Climate Impact Reduction Potential of the Water-Enhanced Turbofan (WET) Concept
Aerospace 2021, 8(3), 59; https://doi.org/10.3390/aerospace8030059 - 25 Feb 2021
Viewed by 621
Abstract
Aviation faces increasing pressure not only to reduce fuel burn, and; therefore, CO2 emissions, but also to provide technical solutions for an overall climate impact minimization. To combine both, a concept for the enhancement of an aircraft engine by steam injection with [...] Read more.
Aviation faces increasing pressure not only to reduce fuel burn, and; therefore, CO2 emissions, but also to provide technical solutions for an overall climate impact minimization. To combine both, a concept for the enhancement of an aircraft engine by steam injection with inflight water recovery is being developed. The so-called Water-Enhanced Turbofan (WET) concept promises a significant reduction of CO2 emissions, NOx emissions, and contrail formation. Representative missions for an A320-type aircraft using the proposed new engine were calculated. Applying a first-order one-dimensional climate assessment prospects the reduction of more than half of the Global Warming Potential over one hundred years, compared to an evolutionarily improved aero-engine. If CO2-neutrally produced sustainable aviation fuels are used, climate impact could be reduced by 93% compared to today’s aircraft. The evaluation is a first estimate of effects based on preliminary design studies and should provide a starting point for discussion in the scientific community, implying the need for research, especially on the formation mechanisms and radiation properties of potential contrails from the comparatively cold exhaust gases of the WET engine. Full article
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Article
Harmonic Forcing from Distortion in a Boundary Layer Ingesting Fan
Aerospace 2021, 8(3), 58; https://doi.org/10.3390/aerospace8030058 - 24 Feb 2021
Viewed by 588
Abstract
Integrating a fan with a boundary layer ingestion (BLI) configuration into an aircraft fuselage can improve propulsion efficiency by utilizing the lower momentum airflow in the boundary layer developed due to the surface drag of the fuselage. As a consequence, velocity and total [...] Read more.
Integrating a fan with a boundary layer ingestion (BLI) configuration into an aircraft fuselage can improve propulsion efficiency by utilizing the lower momentum airflow in the boundary layer developed due to the surface drag of the fuselage. As a consequence, velocity and total pressure variations distort the flow field entering the fan in both the circumferential and radial directions. Such variations can negatively affect fan aerodynamics and give rise to vibration issues. A fan configuration to benefit from BLI needs to allow for distortion without large penalties. Full annulus unsteady computational fluid dynamics (CFD) with all blades and vanes is used to evaluate the effects on aerodynamic loading and forcing on a fan designed to be mounted on an adapted rear fuselage of a Fokker 100 aircraft, i.e., a tail cone thruster. The distortion pattern used as a boundary condition on the fan is taken from a CFD analysis of the whole aircraft with a simplified model of the installed fan. Detailed simulations of the fan are conducted to better understand the relation between ingested distortion and the harmonic forcing. The results suggest that the normalized harmonic forcing spectrum is primarily correlated to the circumferential variation of inlet total pressure. In this study, the evaluated harmonic forces correlate with the total pressure variation at the inlet for the first 12 engine orders, with some exceptions where the response is very low. At higher harmonics, the distortion content as well as the response become very low, with amplitudes in the order of magnitude lower than the principal disturbances. The change in harmonic forcing resulting from raising the working line, thus, increasing the incidence on the fan rotor, increases the forcing moderately. The distortion transfers through the fan resulting in a non-axisymmetric aerodynamic loading of the outlet guide vane (OGV) that has a clear effect on the aerodynamics. The time average aerodynamic load and also the harmonic forcing of the OGV vary strongly around the circumference. In particular, this is the case for some of the vanes at higher back pressure, most likely due to an interaction with separations starting to occur on vanes operating in unfavorable conditions. Full article
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Article
Climate Impact Mitigation Potential of European Air Traffic in a Weather Situation with Strong Contrail Formation
Aerospace 2021, 8(2), 50; https://doi.org/10.3390/aerospace8020050 - 12 Feb 2021
Cited by 1 | Viewed by 955
Abstract
Air traffic contributes to anthropogenic global warming by about 5% due to CO2 emissions and non-CO2 effects, which are primarily caused by the emission of NOx and water vapor as well as the formation of contrails. Since—in the long term—the [...] Read more.
Air traffic contributes to anthropogenic global warming by about 5% due to CO2 emissions and non-CO2 effects, which are primarily caused by the emission of NOx and water vapor as well as the formation of contrails. Since—in the long term—the aviation industry is expected to maintain its trend to grow, mitigation measures are required to counteract its negative effects upon the environment. One of the promising operational mitigation measures that has been a subject of the EU project ATM4E is climate-optimized flight planning by considering algorithmic climate change functions that allow for the quantification of aviation-induced climate impact based on the emission’s location and time. Here, we describe the methodology developed for the use of algorithmic climate change functions in trajectory optimization and present the results of its application to the planning of about 13,000 intra-European flights on one specific day with strong contrail formation over Europe. The optimization problem is formulated as bi-objective continuous optimal control problem with climate impact and fuel burn being the two objectives. Results on an individual flight basis indicate that there are three major classes of different routes that are characterized by different shapes of the corresponding Pareto fronts representing the relationship between climate impact reduction and fuel burn increase. On average, for the investigated weather situation and traffic scenario, a climate impact reduction in the order of 50% can be achieved by accepting 0.75% of additional fuel burn. Higher mitigation gains would only be available at much higher fuel penalties, e.g., a climate impact reduction of 76% associated with a fuel penalty of 12.8%. However, these solutions represent much less efficient climate impact mitigation options. Full article
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Article
Towards Determining the Contrail Cirrus Efficacy
Aerospace 2021, 8(2), 42; https://doi.org/10.3390/aerospace8020042 - 06 Feb 2021
Cited by 1 | Viewed by 694
Abstract
Contrail cirrus has been emphasized as the largest individual component of aircraft climate impact, yet respective assessments have been based mainly on conventional radiative forcing calculations. As demonstrated in previous research work, individual impact components can have different efficacies, i.e., their effectiveness to [...] Read more.
Contrail cirrus has been emphasized as the largest individual component of aircraft climate impact, yet respective assessments have been based mainly on conventional radiative forcing calculations. As demonstrated in previous research work, individual impact components can have different efficacies, i.e., their effectiveness to induce surface temperature changes may vary. Effective radiative forcing (ERF) has been proposed as a superior metric to compare individual impact contributions, as it may, to a considerable extent, include the effect of efficacy differences. Recent climate model simulations have provided a first estimate of contrail cirrus ERF, which turns out to be much smaller, by about 65%, than the conventional radiative forcing of contrail cirrus. The main reason for the reduction is that natural clouds exhibit a substantially lower radiative impact in the presence of contrail cirrus. Hence, the new result suggests a smaller role of contrail cirrus in the context of aviation climate impact (including proposed mitigation measures) than assumed so far. However, any conclusion in this respect should be drawn carefully as long as no direct simulations of the surface temperature response to contrail cirrus are available. Such simulations are needed in order to confirm the power of ERF for assessing contrail cirrus efficacy. Full article
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Article
Fuel Tankering: Economic Benefits and Environmental Impact for Flights Up to 1500 NM (Full Tankering) and 2500 NM (Partial Tankering)
Aerospace 2021, 8(2), 37; https://doi.org/10.3390/aerospace8020037 - 31 Jan 2021
Viewed by 823
Abstract
The majority of emissions from aviation come from the combustion of the fuel required to operate each flight. Keeping the fuel consumption required for a safe flight to the absolute minimum is therefore the simplest and most effective way to ensure that emissions [...] Read more.
The majority of emissions from aviation come from the combustion of the fuel required to operate each flight. Keeping the fuel consumption required for a safe flight to the absolute minimum is therefore the simplest and most effective way to ensure that emissions from that flight are kept to a minimum. In practice, however, the fuel load is determined by each aircraft operator on the basis of a number of criteria maximizing first cost efficiency, rather than fuel savings. In this context, tankering is the practice of carrying more fuel than is necessary for the safe execution of the flight to avoid or minimize refueling at the destination airport. It offers an economic advantage when there is a significant difference in fuel prices between the departure and arrival airports, but considerably increases the amount of emissions produced, because the more fuel an aircraft carries, the heavier it is, and carrying this extra weight increases its fuel consumption. This paper presents the steps followed by EUROCONTROL in conducting a first study to estimate the number of times this practice would offer an economic benefit and the amount of extra CO2 emissions that would result. This study, limited to flights up to 1500 and 2500 NM, corresponding mainly to short and medium-haul flights, estimates that, in 2018, 21% of ECAC (In this paper, ECAC refers to the geographical region defined by the 44 member states that signed the European Civil Aviation Conference) flights would perform fuel tankering beneficially. This would represent a net saving of 265 M€ per year for the airlines, but the burning of 286,000 tonnes of additional fuel (equivalent to 0.54% of ECAC jet fuel used), or 901,000 tonnes of CO2 per year. At a time when aviation is challenged for its contribution to climate change, the use of fuel tankering for economic reasons is therefore highly questionable. Full article
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Article
Mitigation of Non-CO2 Aviation’s Climate Impact by Changing Cruise Altitudes
Aerospace 2021, 8(2), 36; https://doi.org/10.3390/aerospace8020036 - 31 Jan 2021
Cited by 1 | Viewed by 899
Abstract
Aviation is seeking for ways to reduce its climate impact caused by CO2 emissions and non-CO2 effects. Operational measures which change overall flight altitude have the potential to reduce climate impact of individual effects, comprising CO2 but in particular non-CO [...] Read more.
Aviation is seeking for ways to reduce its climate impact caused by CO2 emissions and non-CO2 effects. Operational measures which change overall flight altitude have the potential to reduce climate impact of individual effects, comprising CO2 but in particular non-CO2 effects. We study the impact of changes of flight altitude, specifically aircraft flying 2000 feet higher and lower, with a set of global models comprising chemistry-transport, chemistry-climate and general circulation models integrating distinct aviation emission inventories representing such alternative flight altitudes, estimating changes in climate impact of aviation by quantifying radiative forcing and induced temperature change. We find in our sensitivity study that flying lower leads to a reduction of radiative forcing of non-CO2 effects together with slightly increased CO2 emissions and impacts, when cruise speed is not modified. Flying higher increases radiative forcing of non-CO2 effects by about 10%, together with a slight decrease of CO2 emissions and impacts. Overall, flying lower decreases aviation-induced temperature change by about 20%, as a decrease of non-CO2 impacts by about 30% dominates over slightly increasing CO2 impacts assuming a sustained emissions scenario. Those estimates are connected with a large but unquantified uncertainty. To improve the understanding of mechanisms controlling the aviation climate impact, we study the geographical distributions of aviation-induced modifications in the atmosphere, together with changes in global radiative forcing and suggest further efforts in order to reduce long standing uncertainties. Full article
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Article
Analysis of Aircraft Routing Strategies for North Atlantic Flights by Using AirTraf 2.0
Aerospace 2021, 8(2), 33; https://doi.org/10.3390/aerospace8020033 - 28 Jan 2021
Viewed by 917
Abstract
Climate-optimized routing is an operational measure to effectively reduce the climate impact of aviation with a slight increase in aircraft operating costs. This study examined variations in the flight characteristics among five aircraft routing strategies and discusses several characteristics of those routing strategies [...] Read more.
Climate-optimized routing is an operational measure to effectively reduce the climate impact of aviation with a slight increase in aircraft operating costs. This study examined variations in the flight characteristics among five aircraft routing strategies and discusses several characteristics of those routing strategies concerning typical weather conditions over the North Atlantic. The daily variability in the North Atlantic weather patterns was analyzed by using the European Center Hamburg general circulation model (ECHAM) and the Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model in the specified dynamics mode from December 2008 to August 2018. All days of the ten complete winters and summers in the simulations were classified into five weather types for winter and into three types for summer. The obtained frequency for each of the weather types was in good agreement with the literature data; and then representative days for each weather type were selected. Moreover, a total of 103 North Atlantic flights of an Airbus A330 aircraft were simulated with five aircraft routing strategies for each representative day by using the EMAC model with the air traffic simulation submodel AirTraf. For every weather type, climate-optimized routing shows the lowest climate impact, at which a trade-off exists between the operating costs and the climate impact. Cost-optimized routing lies between the time- and fuel-optimized routings and achieves the lowest operating costs by taking the best compromise between flight time and fuel use. The aircraft routing for contrail avoidance shows the second lowest climate impact; however, this routing causes extra operating costs. Our methodology could be extended to statistical analysis based on long-term simulations to clarify the relationship between the aircraft routing characteristics and weather conditions. Full article
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Article
Climate Impact Mitigation Potential of Formation Flight
Aerospace 2021, 8(1), 14; https://doi.org/10.3390/aerospace8010014 - 08 Jan 2021
Viewed by 763
Abstract
The aerodynamic formation flight, which is also known as aircraft wake-surfing for efficiency (AWSE), enables aircraft to harvest the energy inherent in another aircraft’s wake vortex. As the thrust of the trailing aircraft can be reduced during cruise flight, the resulting benefit can [...] Read more.
The aerodynamic formation flight, which is also known as aircraft wake-surfing for efficiency (AWSE), enables aircraft to harvest the energy inherent in another aircraft’s wake vortex. As the thrust of the trailing aircraft can be reduced during cruise flight, the resulting benefit can be traded for longer flight time, larger range, less fuel consumption, or cost savings accordingly. Furthermore, as the amount and location of the emissions caused by the formation are subject to change and saturation effects in the cumulated wake of the formation can occur, AWSE can favorably affect the climate impact of the corresponding flights. In order to quantify these effects, we present an interdisciplinary approach combining the fields of aerodynamics, aircraft operations and atmospheric physics. The approach comprises an integrated model chain to assess the climate impact for a given air traffic scenario based on flight plan data, aerodynamic interactions between the formation members, detailed trajectory calculations as well as on an adapted climate model accounting for the saturation effects resulting from the proximity of the emissions of the formation members. Based on this approach, we derived representative AWSE scenarios for the world’s major airports by analyzing and assessing flight plans. The resulting formations were recalculated by a trajectory calculation tool and emission inventories for the scenarios were created. Based on these inventories, we quantitatively estimated the climate impact using the average temperature response (ATR) as climate metric, calculated as an average global near surface temperature change over a time horizon of 50 years. It is shown, that AWSE as a new operational procedure has a significant mitigation potential on climate impact. For a global formation flight scenario, we estimated the average relative change of climate response to range between 22% and 24% while the relative fuel saving effects sum up to 5–6%. Full article
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Article
Assessing the Climate Impact of Formation Flights
Aerospace 2020, 7(12), 172; https://doi.org/10.3390/aerospace7120172 - 08 Dec 2020
Cited by 2 | Viewed by 982
Abstract
An operational measure that is inspired by migrant birds aiming toward the mitigation of aviation climate impact is to fly in aerodynamic formation. When this operational measure is adapted to commercial aircraft it saves fuel and is, therefore, expected to reduce the climate [...] Read more.
An operational measure that is inspired by migrant birds aiming toward the mitigation of aviation climate impact is to fly in aerodynamic formation. When this operational measure is adapted to commercial aircraft it saves fuel and is, therefore, expected to reduce the climate impact of aviation. Besides the total emission amount, this mitigation option also changes the location of emissions, impacting the non-CO2 climate effects arising from NOx and H2O emissions and contrails. Here, we assess these non-CO2 climate impacts with a climate response model to assure a benefit for climate not only due to CO2 emission reductions, but also due to reduced non-CO2 effects. Therefore, the climate response model AirClim is used, which includes CO2 effects and also the impact of water vapor and contrail induced cloudiness as well as the impact of nitrogen dioxide emissions on the ozone and methane concentration. For this purpose, AirClim has been adopted to account for saturation effects occurring for formation flight. The results of the case studies show that the implementation of formation flights in the 50 most popular airports for the year 2017 display an average decrease of fuel consumption by 5%. The climate impact, in terms of average near surface temperature change, is estimated to be reduced in average by 24%, with values of individual formations between 13% and 33%. Full article
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Article
The Contrail Mitigation Potential of Aircraft Formation Flight Derived from High-Resolution Simulations
Aerospace 2020, 7(12), 170; https://doi.org/10.3390/aerospace7120170 - 05 Dec 2020
Cited by 3 | Viewed by 1021
Abstract
Formation flight is one potential measure to increase the efficiency of aviation. Flying in the upwash region of an aircraft’s wake vortex field is aerodynamically advantageous. It saves fuel and concomitantly reduces the carbon foot print. However, CO2 emissions are only one contribution to the aviation climate impact among several others (contrails, emission of H2O and NOx). In this study, we employ an established large eddy simulation model with a fully coupled particle-based ice microphysics code and simulate the evolution of contrails that were produced behind formations of two aircraft. For a large set of atmospheric scenarios, these contrails are compared to contrails behind single aircraft. In general, contrails grow and spread by the uptake of atmospheric water vapour. When contrails are produced in close proximity (as in the formation scenario), they compete for the available water vapour and mutually inhibit their growth. The simulations demonstrate that the contrail ice mass and total extinction behind a two-aircraft formation are substantially smaller than for a corresponding case with two separate aircraft and contrails. Hence, this first study suggests that establishing formation flight may strongly reduce the contrail climate effect. Full article
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Article
How Well Can Persistent Contrails Be Predicted?
Aerospace 2020, 7(12), 169; https://doi.org/10.3390/aerospace7120169 - 02 Dec 2020
Cited by 2 | Viewed by 929
Abstract
Persistent contrails and contrail cirrus are responsible for a large part of aviation induced radiative forcing. A considerable fraction of their warming effect could be eliminated by diverting only a quite small fraction of flight paths, namely those that produce the highest individual [...] Read more.
Persistent contrails and contrail cirrus are responsible for a large part of aviation induced radiative forcing. A considerable fraction of their warming effect could be eliminated by diverting only a quite small fraction of flight paths, namely those that produce the highest individual radiative forcing (iRF). In order to make this a viable mitigation strategy it is necessary that aviation weather forecast is able to predict (i) when and where contrails are formed, (ii) which of these are persistent, and (iii) how large the iRF of those contrails would be. Here we study several data bases together with weather data in order to see whether such a forecast would currently be possible. It turns out that the formation of contrails can be predicted with some success, but there are problems to predict contrail persistence. The underlying reason for this is that while the temperature field is quite good in weather prediction and climate simulations with specified dynamics, this is not so for the relative humidity in general and for ice supersaturation in particular. However we find that the weather model shows the dynamical peculiarities that are expected for ice supersaturated regions where strong contrails are indeed found in satellite data. This justifies some hope that the prediction of strong contrails may be possible via general regression involving the dynamical state of the ambient atmosphere. Full article
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Article
Climate-Optimized Trajectories and Robust Mitigation Potential: Flying ATM4E
Aerospace 2020, 7(11), 156; https://doi.org/10.3390/aerospace7110156 - 30 Oct 2020
Cited by 5 | Viewed by 1259
Abstract
Aviation can reduce its climate impact by controlling its CO2-emission and non-CO2 effects, e.g., aviation-induced contrail-cirrus and ozone caused by nitrogen oxide emissions. One option is the implementation of operational measures that aim to avoid those atmospheric regions that are [...] Read more.
Aviation can reduce its climate impact by controlling its CO2-emission and non-CO2 effects, e.g., aviation-induced contrail-cirrus and ozone caused by nitrogen oxide emissions. One option is the implementation of operational measures that aim to avoid those atmospheric regions that are in particular sensitive to non-CO2 aviation effects, e.g., where persistent contrails form. The quantitative estimates of mitigation potentials of such climate-optimized aircraft trajectories are required, when working towards sustainable aviation. The results are presented from a comprehensive modelling approach when aiming to identify such climate-optimized aircraft trajectories. The overall concept relies on a multi-dimensional environmental change function concept, which is capable of providing climate impact information to air traffic management (ATM). Estimates on overall climate impact reduction from a one-day case study are presented that rely on the best estimate for climate impact information. Specific weather situation that day, containing regions with high contrail impact, results in a potential reduction of total climate impact, by more than 40%, when considering CO2 and non-CO2 effects, associated with an increase of fuel by about 0.5%. The climate impact reduction per individual alternative trajectory shows a strong variation and, hence, also the mitigation potential for an analyzed city pair, depending on atmospheric characteristics along the flight corridor as well as flight altitude. The robustness of proposed climate-optimized trajectories is assessed by using a range of different climate metrics. A more sustainable ATM needs to integrate comprehensive environmental impacts and associated forecast uncertainties into route optimization in order to identify robust eco-efficient trajectories. Full article
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Article
Quantifying the Environmental Design Trades for a State-of-the-Art Turbofan Engine
Aerospace 2020, 7(10), 148; https://doi.org/10.3390/aerospace7100148 - 13 Oct 2020
Cited by 1 | Viewed by 1077
Abstract
Aircraft and engine technology have continuously evolved since their introduction and significant improvement has been made in fuel efficiency, emissions, and noise reduction. One of the major issues that the aviation industry is facing today is pollution around the airports, which has an [...] Read more.
Aircraft and engine technology have continuously evolved since their introduction and significant improvement has been made in fuel efficiency, emissions, and noise reduction. One of the major issues that the aviation industry is facing today is pollution around the airports, which has an effect both on human health and on the climate. Although noise emissions do not have a direct impact on climate, variations in departure and arrival procedures influence both CO2 and non-CO2 emissions. In addition, design choices made to curb noise might increase CO2 and vice versa. Thus, multidisciplinary modeling is required for the assessment of these interdependencies for new aircraft and flight procedures. A particular aspect that has received little attention is the quantification of the extent to which early design choices influence the trades of CO2, NOx, and noise. In this study, a single aisle thrust class turbofan engine is optimized for minimum installed SFC (Specific Fuel Consumption). The installed SFC metric includes the effect of engine nacelle drag and engine weight. Close to optimal cycles are then studied to establish how variation in engine cycle parameters trade with noise certification and LTO (Landing and Take-Off) emissions. It is demonstrated that around the optimum a relatively large variation in cycle parameters is allowed with only a modest effect on the installed SFC metric. This freedom in choosing cycle parameters allows the designer to trade noise and emissions. Around the optimal point of a state-of-the-art single aisle thrust class propulsion system, a 1.7 dB reduction in cumulative noise and a 12% reduction in EINOx could be accomplished with a 0.5% penalty in installed SFC. Full article
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Article
Impact of Hybrid-Electric Aircraft on Contrail Coverage
Aerospace 2020, 7(10), 147; https://doi.org/10.3390/aerospace7100147 - 12 Oct 2020
Cited by 1 | Viewed by 1588
Abstract
Aviation is responsible for approximately 5% of global warming and is expected to increase substantially in the future. Given the continuing expansion of air traffic, mitigation of aviation’s climate impact becomes challenging but imperative. Among various mitigation options, hybrid-electric aircraft (HEA) have drawn [...] Read more.
Aviation is responsible for approximately 5% of global warming and is expected to increase substantially in the future. Given the continuing expansion of air traffic, mitigation of aviation’s climate impact becomes challenging but imperative. Among various mitigation options, hybrid-electric aircraft (HEA) have drawn intensive attention due to their considerable potential in reducing greenhouse gas emissions (e.g., CO2). However, the non-CO2 effects (especially contrails) of HEA on climate change are more challenging to assess. As the first step to understanding the climate impact of HEA, this research investigates the effects on the formation of persistent contrails when flying with HEA. The simulation is performed using an Earth System Model (EMAC) coupled with a submodel (CONTRAIL), where the contrail formation criterion, the Schmidt–Appleman criterion (SAC), is adapted to globally estimate changes in the potential contrail coverage (PCC). We compared the HEA to conventional (reference) aircraft with the same characteristics, except for the propulsion system. The analysis showed that the temperature threshold of contrail formation for HEA is lower; therefore, conventional reference aircraft can form contrails at lower flight altitudes, whereas the HEA does not. For a given flight altitude, with a small fraction of electric power in use (less than 30%), the potential contrail coverage remained nearly unchanged. As the electric power fraction increased, the reduction in contrail formation was mainly observed in the mid-latitudes (30° N and 40° S) or tropical regions and was very much localized with a maximum value of about 40% locally. The analysis of seasonal effects showed that in non-summer, the reduction in contrail formation using electric power was more pronounced at lower flight altitudes, whereas in summer the changes in PCC were nearly constant with respect to altitude. Full article
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Article
Beyond Contrail Avoidance: Efficacy of Flight Altitude Changes to Minimise Contrail Climate Forcing
Aerospace 2020, 7(9), 121; https://doi.org/10.3390/aerospace7090121 - 21 Aug 2020
Cited by 3 | Viewed by 1507
Abstract
Contrail cirrus introduce a short-lived but significant climate forcing that could be mitigated by small changes in aircraft cruising altitudes. This paper extends a recent study to evaluate the efficacy of several vertical flight diversion strategies to mitigate contrail climate forcing, and estimates [...] Read more.
Contrail cirrus introduce a short-lived but significant climate forcing that could be mitigated by small changes in aircraft cruising altitudes. This paper extends a recent study to evaluate the efficacy of several vertical flight diversion strategies to mitigate contrail climate forcing, and estimates impacts to air traffic management (ATM). We use six one-week periods of flight track data in the airspace above Japan (between May 2012 and March 2013), and simulate contrails using the contrail cirrus prediction model (CoCiP). Previous studies have predominantly optimised a diversion of every contrail-forming flight to minimise its formation or radiative forcing. However, our results show that these strategies produce a suboptimal outcome because most contrails have a short lifetime, and some have a cooling effect. Instead, a strategy that reroutes 15.3% of flights to avoid long-lived warming contrails, while allowing for cooling contrails, reduces the contrail energy forcing (EFcontrail) by 105% [91.8, 125%] with a total fuel penalty of 0.70% [0.66, 0.73%]. A minimum EFtotal strategy (contrails + CO2), diverting 20.1% of flights, reduces the EFcontrail by the same magnitude but also reduces the total fuel consumption by 0.40% [0.31, 0.47%]. For the diversion strategies explored, between 9% and 14% of diversions lead to a loss of separation standards between flights, demonstrating a modest scale of ATM impacts. These results show that small changes in flight altitudes are an opportunity for aviation to significantly and rapidly reduce its effect on the climate. Full article
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Review

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Review
Energy Transition in Aviation: The Role of Cryogenic Fuels
Aerospace 2020, 7(12), 181; https://doi.org/10.3390/aerospace7120181 - 18 Dec 2020
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Abstract
Aviation is the backbone of our modern society. In 2019, around 4.5 billion passengers travelled through the air. However, at the same time, aviation was also responsible for around 5% of anthropogenic causes of global warming. The impact of the COVID-19 pandemic on [...] Read more.
Aviation is the backbone of our modern society. In 2019, around 4.5 billion passengers travelled through the air. However, at the same time, aviation was also responsible for around 5% of anthropogenic causes of global warming. The impact of the COVID-19 pandemic on the aviation sector in the short term is clearly very high, but the long-term effects are still unknown. However, with the increase in global GDP, the number of travelers is expected to increase between three- to four-fold by the middle of this century. While other sectors of transportation are making steady progress in decarbonizing, aviation is falling behind. This paper explores some of the various options for energy carriers in aviation and particularly highlights the possibilities and challenges of using cryogenic fuels/energy carriers such as liquid hydrogen (LH2) and liquefied natural gas (LNG). Full article
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