How do airlines decide which type of aircraft to operate on certain routes in cases when they have multiple types that are interchangeable? For example, Alaska Airlines operates 73G, 734 and MD80 on many of the same routes. I fly out of BUR frequently to either PDX or SEA. They keep changing the a/c on these flights. Another example, Southwest and 732, 733, 735, 73G -- is it arbitrary which planes get assigned to which routes or is there actually a system for determining this?