I was wondering how do an airline decide which is the optimal aircraft for its fleet based on operating costs?
For example, an airline operates a network of 24 routes with different frequancies for each route, and each route has different operating costs. These costs imply that for example, on 10 routes the lowest cost is a certain Boing type, and in the rest it is a certain Airbus type. Now, the airline can choose from a veriety of aircraft types (A321, B737-600, B737-900, etc.) to form its fleet, but it has to be a single-type fleet - an Airbus or a Boeing.
The question is, how can it decide which aircraft to choose, given the above constraints?