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%.

Approximately 50–75% of aviation's climate impact is caused by non-CO₂ effects, like the production of ozone and the formation of contrail cirrus clouds, which can be effectively prevented by re-routing flights around highly climate-sensitive areas. Here, we discuss options how to incentivize re-routing approaches and apply multicriteria trajectory optimizations to demonstrate the feasibility of the concept of climate-charged airspaces (CCAs). We show that although climate-optimized re-routing results in slightly longer flight times, increased fuel consumption and higher operating costs, it is more climate-friendly compared to a cost-optimized routing. In accordance to other studies, we find that the averaged temperature response over 100 years (ATR (Formula presented.)) of a single flight can be reduced by up to 40%. However, if mitigation efforts are associated with a direct increase in costs, there is a need for climate policies. To address the lack of incentivizing airlines to internalize their climate costs, this study focuses on the CCA concept, which imposes a climate charge on airlines when operating in highly climate-sensitive areas. If CCAs are (partly) bypassed, both climate impact and operating costs of a flight can be reduced: a more climate-friendly routing becomes economically attractive. For an exemplary North-Atlantic network, CCAs create a financial incentive for climate mitigation, achieving on average more than 90% of the climate impact reduction potential of climate-optimized trajectories (theoretical maximum, benchmark). Key policy insights Existing climate policies for aviation do not address non- (Formula presented.) effects, which are very sensitive to the location and the timing of the emission. By imposing a temporary climate charge for airlines that operate in highly climate-sensitive regions, the trade-off between economic viability and environmental compatibility could be resolved: Climate impact mitigation of non- (Formula presented.) effects coincides with cutting costs. To ensure easy planning and verification, climate charges are calculated analogously to en-route and terminal charges. For climate mitigation it is therefore neither necessary to monitor emissions ((Formula presented.) (Formula presented.), etc.) nor to integrate complex non- (Formula presented.) effects into flight planning procedures of airlines. Its implementation is feasible and effective.

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.

Impacts of commercial aircraft operation upon the environment, which are caused primarily from emissions of CO2, NOx and the formation of contrails, are matter of growing concern, as aviation is one of the fastest developing industrial sectors worldwide and the awareness of its effects is expanding. Recent research has focused on the cost-benefit potential of different mitigation strategies, which optimize flight trajectories with respect to climate and economy, but most of these mitigation strategies cannot be implemented in the near future due to technical challenges.The objective of this paper is to present an interim mitigation strategy, which bridges this time period. In analogy to military exclusion zones, climate restricted airspaces (CRA) are defined based on 3-D climate change functions, characterizing the environmental impact caused by an aircraft emission at a certain location. Regions with climate costs greater than a threshold value are closed in the corresponding month; others are cleared for air traffic. To estimate the cost-benefit potential of this strategy, a preliminary analysis is conducted on the route from Helsinki (EFHK) to Miami (KMIA). Affected flight trajectories are re-routed optimally around resulting CRA with regard to monetary costs for varying threshold values. Therefore, flight simulation algorithms are developed, which solve a non-linear optimal control problem. For each optimized flight trajectory corresponding average temperature response (ATR) and cash operating costs (COC) are expressed relative to a reference great circle trajectory with constant Mach number and compared with the climate mitigation potential of climate optimized trajectories.

Climate optimized flight trajectories are considered to be a promising measure to mitigate non-CO₂ emissions’ environmental impact, which is highly sensitive to locus and time of emission. Within this study, optimal control techniques are applied in order to determine 2D (lateral) and 3D (lateral and vertical) cost-optimized flight trajectories while mitigating their climate impact by minimizing emissions and flight time in highly climate sensitive regions. Therefore, monetary and 4D-climate cost functions, describing the climate sensitivity in dependency of the emission location, altitude, time and weather situation, are integrated into the optimization algorithm. For both, 2D- and 3D-optimization, the cost-benefit potential (climate impact mitigation vs. rise in operating costs) is investigated for nine fictitious North Atlantic routes for eastbound and westbound directions in the presence of winds. The conducted study shows large potential for both measures as the reduction of climate sensitivities along the trajectory often predominates the additional emissions caused by headwinds, additional climb-and descent phases, additional flight distance, and off-design altitudes. Flight trajectories optimized within the horizontal plane can reduce the average temperature response (ATR) by approximately 15 % for a two percent increase in cash operating costs (COC). This mitigation potential is significantly improved by superposition of lateral and vertical optimization. 3D-optimized trajectories which are comparable in cost increase achieve a 20-35 % higher ATR reduction than their 2D-optimized counterparts. Further, they reduce global warming more efficiently (higher ATR reduction per unit cost increment) and to a higher extent. However, achieving maximum climate impact mitigation is linked with an disproportional rise of cash operating costs in both cases. Therefore, a careful consideration of the required climate impact savings as well as the accepted surcharges is necessary.

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 carbon dioxide, water vapour, nitrogen oxides and aerosols on radiative forcing agents is substantial. The projected duplication 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 arc 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 inter-mediate landings during a mission. Due to shorter flight distances the amount of fuel burnt over the mission can be reduced, as the amount of fuel necessary to transport a certain percentage of the fuel for a long distance can be omitted. Besides fuel cost saving implications, many previous studies anticipate a strong reduction of the environmental impact compared to direct flight operations. So far, none of them has actually quantified this impact in a realistic scenario. While for the amount of emitted species which arc produced proportional to fuel burn, a reduction is straightforward, this is not the case for other species. Moreover, the geographic location and altitude of the emissions have to be taken into account for a sound climate impact assessment. The paper presents results of the ecological analysis of the ISO concept for today's worldwide aircraft fleet, including its influence on global emissions distributions as well as the impact on climate change. A method is described that comprises of different models for 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.