## 4 Discussion and Conclusions  
This study shows the climate mitigation potential of ISO as well as how configurations differ from a fuel-optimised set-up. In general, our results are consistent with what has been presented in previous research: in accordance with [PERSON] et al. [8], we confirm that fuel-optimised ISO increases fuel efficiency and decreases emission quantities but in turn leads to higher ATR through a rise in emission altitudes. We find that this effect can be mitigated by a reduction in flight level, e.g., as shown by [PERSON] et al. [18]. If ISO airports are additionally selected based on the minimum climate impact, high mitigation potentials can be achieved. Nevertheless, the quality of results depends on the made assumptions in the course of the simulation process and limitations to the modelling workflow itself. This includes pre-selection of missions and candidate airports, trajectory and emissions modelling as well as simulation of the climate impact. The selection of considered routes is limited to flights to or from the ECAC area with a great circle distance of more than 2500 NM. By doing so, we ensure to cover a significant share of all global missions (18% in terms of ASK). When transferring the achieved results to the global scale, different regional distributions in terms of routes but also regarding atmospheric boundary conditions need to be considered. Furthermore, the preselection of candidate airports does not ensure that the overall optimal refuelling airport is selected. Due to this approach, the results in terms of fuel efficiency and climate mitigation potential deviate moderately from the absolute optimum, which can be achieved (see Appendix A). From an operational perspective, we did not take available infrastructure and capacities, such as runway lengths or equipment availability as well as the geopolitical situation at the selected intermediate refuelling airports, into account. Consideration of these effects can influence availability of suitable airport candidates and thus the mitigation potential. Nevertheless, this study is capable of reliably indicating the significant potentials of ISO as an operational improvement to reduce the climate impact. Further inaccuracies may result from the trajectory and emissions modelling: simplifying assumptions in terms of an average load factor and the resulting aircraft mass, great circle connections instead of actual routes, exclusion of wind effects and consideration of ISA as well as optimised flight altitudes and speeds influence fuel consumption and resulting emission quantities on the considered missions. On this basis, we hypothesise that fuel consumption and emission quantities are slightly underestimated. For instance, the impact of wind on a set of global long-haul routes is expected to increase fuel consumption by approximately 1-2% [26]. However, the overall effect is expected to be minor due to the fact that results are generally presented in relation to the reference case. Application of aircraft performance data as provided by BADA4 further influences fuel burn and thus emission quantities. However, BADA4 enhances reliability and improves realism compared to the previous version 3 [43], and a comparison with more detailed models and flight data showed mean errors in fuel consumption below 5% [25]. The applied simplified emission modelling techniques in terms of Boeing and DLR fuel flow correlation methods as well as the standardised ICAO landing take-off cycle further lead to uncertainties. Inaccuracies of NOx emission quantities during cruise flight for example are estimated at, on average, around 10-12% [28; 44]. With regards to the climate impact modelling, the choice of the climate metric as well as the assessment approach have an important impact on the study. ATR is the selected as the preferred climate metric. It is well-suitable for describing the long-term temperature reduction potential through an implementation of the ISO concept [40].