## 2 Materials and Methods  
### Aviation Fuels and Previous Sustainability Studies  
At the 2017 Conference on Aviation and Alternative Fuels, definitions in the area of turbine fuels and their production pathways were proposed. Fuels are classified as conventional aviation fuel (CAF), aviation alternative fuel (AAF), and sustainable aviation fuel (SAF) [22; 23; 24]. A schematic overview of the different types of aviation fuels is shown in Figure 1.  
CAFs are aviation fuels derived entirely from petroleum sources (e.g., crude oil, liquid natural gas condensates, heavy oil, or oil sands) produced by refinery processes. A typical CAF is the conventional Jet-A1 fuel. In contrast to CAFs, AAFs are derived from sources other than petroleum. These sources can be renewable or non-renewable (e.g., biomass, natural gas, electricity, or hydrogen). Fuels based on renewable sources within AAFs are referred to as SAFs [22; 23]. AAFs initially included all alternatives to fossil kerosene. At this point, however, a distinction can be made between drop-in fuels and non-drop-in fuels [25]. While drop-in fuels can be blended with fossil kerosene or replace it completely without changing the current powertrain structure, non-drop-in fuels (e.g., liquid hydrogen (LH\({}_{2}\))) require extensive modifications to the powertrain. These structural changes to the powertrain require longer development phases and are cost-intensive. Therefore, drop-in capable fuels in particular are considered promising AAFs to contribute to sustainable aviation in the short term. Drop-in AAFs are mostly produced by X-to-liquid pathways [25; 26; 27], where the X stands for the underlying energy source. Potential energy sources can be gas, coal, biomass, and hydrogen. Gas-to-liquid (GtL) and coal-to-liquid (CtL) are used as pathways for producing AAFs from non-renewable sources. In contrast, biomass-to-liquid (BL), power-to-liquid (PtL), and sun-to-liquid (StL) are potential pathways to produce alternative fuels from renewable sources, which are SAFs [22; 23; 25]. Different approved synthesis processes for the aforementioned pathways exist, such as Fisher-Tropsch synthesis (FTS), methanol synthesis, or methanation. FTS is the most widely used industrial process and the most commercially mature, resulting in low process costs. The first process step is the reverse water gas shift reaction, in which synthesis gas is produced using hydrogen, electricity, and CO\({}_{2}\). This synthesis gas is converted into syncrude during the FTS, which is further processed into alternative fuel with the help of hydrocracking and the addition of hydrogen [25; 28; 29; 30; 31]. The synthesis processes are often preceded by electrolysis to generate the hydrogen required to conduct the synthesis. In addition, with regard to biomass, there are other production processes for SAFs aside from the BtL pathway. These are hydrothermal liquefaction (HTL), hydroprocessed esters and fatty acids (HEFA), alcohol-to-jet (Atl), and direct sugar to hydrocarbons (DSHC). However, the production capacities of fuels using these processes are still small and are not considered in this study. Numerous studies exist in which the negative environmental impacts of the aviation sector are analyzed (e.g., [18; 24; 33; 34]). These studies primarily highlight that a large part of the negative environmental impacts of the aviation sector results from flight operations, or more precisely, the combustion of fossil kerosene. It is responsible for about 77-91% (depending on the type of aircraft and the mission profile) of air travel-related GHG emissions [25; 33; 34]. The remaining GHG emissions are generated in other phases and can be attributed to aircraft production, kerosene production, transport, and airport infrastructure. In this context, studies on alternative fuels, which replace fossil kerosene and are expected to reduce GHG emissions due to savings in production, have been increasingly conducted in recent years. [PERSON] et al. [18], [PERSON] et al. [35], and [PERSON] et al.