## 1 _Applied_  
### _Sciences_  
_Article_  
_Operational Improvements to Reduce the Climate Impact of Aviation--A Comparative Study from EU Project ClimOP_  
_[PERSON]_ **1,**~  
-frequent extreme weather events [1; 2]. In this context, aviation plays an important role in emitting carbon dioxide (CO\({}_{2}\)) and non-CO\({}_{2}\) species such as nitrogen oxides (NO\({}_{x}\)) and water vapor (H\({}_{2}\)O) into the atmosphere. The effects resulting from non-CO\({}_{2}\) emissions such as NOx-induced ozone (O\({}_{3}\)) and methane (CH\({}_{4}\)) changes or contrail-induced cirrus (CiC) are estimated to contribute to the majority of the total RF of aviation emissions [3; 4]. Hence, ambitious goals have been set for the aviation industry to increase fuel efficiency and to reduce the climate impact from both CO\({}_{2}\) and non-CO\({}_{2}\) emissions [5; 6; 7]. Nevertheless, previous analyses have shown that current regulations and measures from the aviation sector do not suffice to comply with the goals that have been defined by the Paris agreement [8]. Thus, the realization of an extensive set of additional measures from the technical, operational, and regulatory sides will be required. Technical climate mitigation measures typically aim to enhance fuel efficiency, e.g., through lower weights, higher lift-to-drag ratios, or improvements of thrust-specific fuel consumption [9; 10]. More radical approaches target innovative engine concepts, e.g., hydrogen or battery-electric propulsion, as well as sustainable alternative fuels [9; 11]. However, radically innovative technical measures are not expected to enter into service on a larger scale in the next decade. In addition, technical approaches mostly aim for a reduction in fuel burn as well as associated CO\({}_{2}\) emissions and might neglect significant non-CO\({}_{2}\) effects [12; 13], as these non-CO\({}_{2}\) effects are not necessarily proportional to fuel consumption but also influenced by emission location and time [3; 4]. In contrast to technical approaches, operational measures can be implemented without major adjustments to the current air transport fleet and thus benefit from short realization times. Also, non-CO\({}_{2}\) effects can directly be addressed by such operational measures. Hence, operational climate mitigation measures are expected to play an important role in the short term to reach the defined climate goals before technical innovations can be implemented on a large scale [5; 7]. The current state of literature comprises various studies on individual operational measures to reduce the climate impact of aviation, e.g., [14; 15; 16; 17; 18; 19; 20], which can be categorized into three types of measures:  
* **Trajectory optimization** including in-flight measures that reduce the climate effect along trajectories by re-routing or improving procedures without changing the investigated flight network itself. The climate impact can directly be related to one individual mission. Exemplary measures are weather-optimized flight planning [21; 22], climate-optimized trajectories [17; 23; 24], flying at reduced altitudes and speeds [14; 25; 26], direct or wind-optimal routing [21; 27], continuous climb and descent operations [21; 28], and routing optimization for contrail avoidance [16; 29]. * **Climate-optimized operation of airline networks** comprising in-flight measures that affect the network for example by changes in the network flight schedule, where reduction in the climate impact is not directly related to one individual mission. This includes approaches such as Formation flight [19], Intermediate Stop Operations (ISO) [15; 30] and climate-aware network design and planning [31; 32; 33], e.g., by merging or separating flights, reassigning aircraft, frequencies, and creating optimal hub-and-spokes or point-to-point operations.