## 3 Results
### Eco-Efficient Operations  
Our analysis shows that an implementation of the ISO concept can achieve significant reductions in ATR. However, results in Table 1 indicate that this has a major impact on operations and the stakeholders of the air transport system. On the one hand, longer flight times and higher fuel consumption lead to higher direct operating cost and thus limit an implementation of this concept from an operator's perspective in the climate-optimised set-up. On the other hand, the fuel-optimised concept reduces fuel consumptions and limits detours as well as additional flight times implicitly. Subsequently, a combination of both effects is expected to combine benefits both ecologically and economically. For this purpose, we investigate the maximum climate mitigation potential for defined limits of additional fuel consumption (_fuel penalty_) per individual flight. Results are displayed in Table 2.  
If additional fuel burn is limited to 0.5% per flight relative to the non-stop reference mission, ATR100 can still be reduced by more than 6%. By doing so, total fuel consumption is still reduced (\(-0.4\%\)), and flight time increase is limited to 3.1%. An analysis of this eco-efficient set-up shows that longer missions are preferred for an implementation of ISO, as shorter distances might not include fuel efficiency gains. The same holds true for detour and eccentricity caused by the selection of refuelling airports. A majority of the flights is associated with detours below 2.5% (97.3% of the ISO missions), and the maximum detour is approximately 5.2% (1.8% in the fuel-optimised set-up). Regarding the flight level, we find that a majority of the flights include step climbs (54.5%) or a cruise altitude of 35,000 ft (19.7%) so that the trade-off of reduced cruise altitudes with higher fuel consumption is mitigated.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Allowed Fuel** & \multirow{2}{*}{\begin{tabular}{c} **Total Change in** \\ **ATR100** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Total Change in** \\ **Fuel Burn** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Total Change in** \\ **Flight Time** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Share of ISO** \\ **Missions** \\ \end{tabular} } \\ \hline
0 & \(-5.66\) & \(-0.53\) & +2.75 & 37.16 \\
0.5 & \(-6.03\) & \(-0.40\) & +3.09 & 41.73 \\
1.0 & \(-6.73\) & \(-0.21\) & +3.38 & 45.43 \\
2.0 & \(-8.87\) & +0.28 & +4.04 & 54.19 \\
5.0 & \(-15.46\) & +2.57 & +6.03 & 82.10 \\
10 & \(-27.27\) & +7.21 & +7.01 & 96.84 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Climate impact mitigation when limiting additional fuel burn per flight (all numbers in % compared to the reference case).  
Figure 10: Change in ATR100 per emission species for fuel and climate-optimised ISO compared to the non-stop reference case at constant flight level of 35,000 ft.  
For more than one third of all flights (37.2%), fuel consumption and ATR100 can be reduced at the same time with an implementation of the ISO concept. If the climate-optimised solution is selected for all these win-win constellations, ATR100 could be reduced by approximately 5.7%, while fuel burn is reduced by 0.5%. Conversely, if the most fuel-efficient set-up is selected, as long as it is not associated with a higher climate impact, fuel consumption can be reduced by 1.3% and ATR100 by 2.1% in comparison to the reference case. Furthermore, we find that even accepting an individual fuel penalty of 1.5%, an implementation of ISO does not increase overall fuel consumption and still reduces the climate impact (\(-7.9\%\) in ATR100).