I would proceed as follows:
Step 1: Clarify the objective. Does the client want to increase revenues, decrease costs, increase profits, or other?
Step 2: Assuming the most complete scenario - profitability optimization: calculate the point where marginal revenues for the additional airplane are equal to marginal costs. There are several elements that could have an impact on that:
- Customers issues (eg expected growth in demand)
- Competitors issues (eg ability to increase their market share against us)
- Supplier issue (eg availability to provide in the required time the planes)
- Our expected actions on price (eg introduce price discrimination thanks to more routes)
- Our expected actions on volume (eg future marketing campaign, new distribution channels, new routes, etc)
- Customers issues (eg new requests in terms of functionalities on new routes which implies new costs)
- Competitors issues (eg lobby against us with regulator to increase costs of labour for new destinations)
- Supplier issue (eg increase in costs for new planes compared to budget)
- Elements influencing cost per unit (eg ability to negotiate with supplier so far that we increase size)
- Elements influencing number of units (eg economies of scale so far that we grow)
Step 3: Check capabilities. Do we have the money and other resources required to do the investment (or disinvestment)
Step 4: Consider risks. Anything that could go wrong with the previous mentioned elements.