Show up, pay, and ride? Like getting on the subway? Not workable, for two reasons: 1, from the passengers' perspective, you need to get to Indianapolis, and when you show up, there are no seats, or worse, no flights. 2, from the airlines' perspective, no one buys advance tickets, you have no idea how big of a plane (and thus also how much fuel and how many support staff) you'll need, nor when, nor even whether you'll need a plane at all. Planning. Airplanes aren't subways.