# Airline Network Planning Considering Climate Impact: Assessing New Operational Improvements
## 3 Results
### Scenarios  
Four scenarios were investigated using the integrated modelling approach in airline network planning in conjunction with the network-related OIs. The scenarios are summarised in Table 1. The first scenario, namely the business-as-usual (BAU) scenario, was considered the reference scenario, representing what is assumed to be the common airline network planning strategy. The objective is to maximise profit while following the great circle trajectories at fuel-optimal altitude. No OIs are considered in this reference scenario. Three other scenarios represent multi-objective network planning and two ISO-related OIs, respectively. These scenarios were designed to capture the framework's capabilities and the synergy gained by incorporating OIs into the network planning problem. The second scenario (ATR) follows similar conditions to the BAU scenario, but in this case, the ATR20 associated with each route was incorporated using the transformed objective function (Equation (1)). The new objective function ensures that the resulting network has the highest profit-to-ATR20 ratio. This scenario is called ATR after its altered objective function. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scenario** & **Objective** & **Flight Level** & **Case Study Airlines** \\ \hline BAU & Profit & Fuel optimal & KLM, TAP, Easylet \\ ATR & Profit/ATR20 & Fuel optimal & KLM, TAP, Easylet \\ ATRISO & Profit/ATR20 & Fuel optimal & KLM, TAP \\ ATRLISO & Profit/ATR20 & 31,000 ft. & KLM, TAP \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Scenario descriptions. The last two scenarios (ATRISO and ATRLISO) also optimise the network using the same objective function but consider the option of ISO in some of the long-distance routes for which a feasible stop-over airport is available. A minimum range threshold of 2500 nautical miles was used when defining which routes could be considered for ISO. The ISO option was considered an alternative to a direct flight in all the ISO routes. Therefore, the solutions for these scenarios may include both direct and ISO flights for the same route, depending on the aircraft type allocated to the flight, flight time, associated costs, and ATR impact. The difference between these two last scenarios is that the maximum flight altitude for the ISO flights in the last scenario (ATRLISO) was considered to be 31,000 ft, lower than what is usually the fuel-optimal flight altitude. This represents a trade-off between the flight cost efficiency and climate impact. These scenarios were analysed for three European airlines representing different operation types: KLM, one main-hub-and-spoke; TAP, one smaller multi-hub airline; and Easylet, a point-to-point low-cost carrier. Since Easylet had no flights exceeding the ISO's minimum range threshold, it was not considered in the last two scenarios. The overall modelling framework is the same for all three airline types in this study. The only difference is the network structure and routing constraints. Flights on hub-and-spoke airlines are only to or from the hub. Passengers on connecting flights are transported to the hub, where they wait for the next flight to their destination. In comparison, multi-hub airlines have a variety of operational strategies. TAP operates two main hubs with frequent flights connecting the hubs throughout the day. Almost the same policy for connecting passengers applies here. The difference is that some of them must catch an additional connecting flight between hubs because their final flight leg departs from the other hub. Lastly, low-cost carriers mainly operate a point-to-point network where connecting passengers is not the primary concern. When analysing the results for the scenarios, we considered climate and non-climate KPIs. For the climate KPIs, we considered ATR20, ATR100, CO\({}_{2}\), H\({}_{2}\)0, NO\({}_{x}\), HC, SO\({}_{2}\), CO, and Soot.