Air transportation liberalization led to significant changes on aviation because of a substantial increase in air transportation demand. The airline industry has diversified its business models, adopting a hub-and-spoke network structure and creating low cost carriers. Consequently, airfares dropped and competition and flight frequency increased for the main markets. But it has also contributed for increasing congestion at airports and to the establishment of subsidy schemes to serve sparsely populated regions with implications for airlines and passengers. This thesis is concerned with the development of specific tools to deal with these two types of air transportation networks. The thesis extends the existing work in network design formulation to the situation of both congested and low demand networks.Aviation network design models determine the flight schedule and fleet assignment for a given network. Typically, they are developed by airlines as a core part of the airline planning process. In this thesis, these models are extended to encompass other perspectives. Three main perspectives are dealt with in this thesis: airline, aviation authority, and government. The optimization models proposed are linear and mixed-integer with the objective of maximizing profits or minimizing costs.Networks operated under congestion are becoming more frequent as airport capacity is incapable to cope with the increasing demand for air transportation. This phenomenon is responsible for very large costs for airline and passengers. Therefore, it is necessary to mitigate delays without compromising airline profitability, passengers’ connectivity and service frequency. Three optimization models were developed in the thesis to study delay impacts in the perspective of governments, airlines, and aviation authorities.After liberalization, both the US and the EU launched subsidy schemes to prevent sparsely populated and remote regions from becoming underserved with respect to air transportation. In this case, schedule services without subsidies would not be viable. Governments set a minimum level of service to be satisfied by the operating airline and, at the same, give public subsidies for its operation. In this thesis, two decision approaches are developed to help governments setting the level of service required to be satisfied by the airlines. These decision approaches are based on network design optimization models in which all relevant costs and revenues for the government analysis are involved, including airline and airport costs and revenues, and passenger time costs.Another objective of the thesis was to apply the models to case studies based on real world networks. Indeed, all models were applied to different networks, depending on their objectives, with significant improvements from the current networks. Formulations could cope with the size and complexity of real world networks, as all the applications reached optimal solutions. For congested networks were used the Portuguese air transportation system (Chapter 2), the main European network (Chapter 3), and the TAP network (Chapter 4). For subsidized networks were used the Azores network (Chapter 5) and the Norwegian regional network (Chapter 6). Results show the usefulness of the optimization models as capable tools to help airlines, aviation authorities, and governments in their network design decisions.
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