If I might offer a suggestion here, and hopefully not step too far over into the bounds of 'helping' you do the assignment or giving away how I did it... consider how an airline would handle routes between cities on a timeschedule, as things change from day to day.
Basically, what I'd consider is the airline's ability to be flexible.
Remember, there are no 'right' or 'wrong' answers if you can defend and explain why you made one choice over another.