## 3 Results
### Comparison of Climate- and Fuel-Optimised Configurations
#### 3.2.2 Impact of Reduced Cruise Altitudes  
Selected cruise altitudes of both scenarios differ significantly: in the fuel-optimised case, step climbs are performed in a way that the absolute fuel consumption along the trajectories is minimised. By contrast, the constant climate-optimised flight level is selected  
Figure 8: Distribution of additional take-offs and landings per grid cell with introduction of fuel-optimised ISO concept (**left**) and climate-optimised ISO concept (**right**).  
Figure 7: Latitude (**a**) and longitude (**b**) along resulting routes: difference between climate-optimised and fuel-optimised ISO set-up. Positives values indicate a higher share in the climate-optimised case, negative ones, a higher frequency in the fuel-optimised case.  
from the defined set so that in most cases, the lowest possible flight level of 29,000 ft is chosen. On average, we identify a shift to higher flight altitudes in the fuel-optimised ISO case (+0.2%) compared to the non-stop reference missions and to lower altitudes in the climate optimised case (\(-\)23.8%). This is also influenced by additional on-ground time through the refuelling stop.  
Figure 9 shows the relative frequency change per flight level compared to the reference case. One can derive that emission altitudes in the fuel-optimised case are generally higher. More fuel is consumed above 38,000 ft compared to the reference case and less below. The climate optimised case conversely is significantly more frequent at the lowest considered cruise flight level of 29,000 ft.  
However, the significant mitigation potential of climate-optimised ISO and the preference of lower flight altitudes is not only due to the ISO concept itself. We observe an overlay of ATR reduction potentials through optimised refuelling location and reduced aircraft weights by the ISO concepts with a reduction in flight level in general. Consequently, we aim to differentiate both effects caused by a sole reduction of the cruise altitude on the one hand and by the impact of intermediate stops on the other hand. The effect of different cruise altitudes can be eliminated from the ISO effect by assuming constant cruise altitudes so that the impact from changing flight levels is excluded: higher cruise flight altitudes are not considered for the fuel-optimised case, as step-climbs are avoided and effects through a reduction of cruise flight levels are also excluded from the selection in the climate-optimised scenario.  
Figure 10 shows the climate mitigation potential of fuel-optimised and climate-optimised ISO for a constant flight level of 35,000 ft in comparison and normalised to the non-stop reference case at the same cruise altitude. In the fuel-optimised case, fuel efficiency enhances by approximately 2% and ATR100 decreases by 5%. By contrast, fuel consumption increases in the climate-optimised case (+2.5%). Mainly, the additional reduction in contrail-cirrus-induced ATR leads to the higher mitigation potential of that scenario. We also see that mitigation potentials in the climate-optimised case are significantly below the combined case of ISO with reduced flight levels. Furthermore, it can be confirmed that a reduction in climate impact caused by non-CO\({}_{2}\) effects overcompensates the rise in fuel consumption and resulting CO\({}_{2}\) emissions in this case. Additionally, this shows that climate mitigation potentials can be increased by a climate-optimised selection of intermediate stop airports. In accordance with the results in Table 1, climate-optimised ISO is beneficial for nearly all  
Figure 9: Distribution of fuel consumption over flight levels in the different scenarios compared to the reference case.  
selected long-range flights while (99.4%), whereas it is only beneficial for 44.0% of all flights to improve fuel efficiency.