This research is aimed at developing a model that maximizes system profit when determining the aircraft routes and flight frequencies in a network. The model employs network flow techniques to effectively collect or deliver passenger flows from all origins to all destinations using non-stop and multi-stop flights in multi-fleet operations. The model was formulated as a multi-commodity network flow problem. A Lagrangian-based algorithm was developed to solve the problem. To test the model in practice, a case study is presented.