In my last blog post, I said that I was going to deviate a bit from my normal career advice in order to cover a few points in the actual business of running an airline. After all, if you’re going to work for one, it helps to understand the how’s and why’s of some of what airlines do. This time, I’d like to delve into another not-clearly-understood practice: overselling flights.

There is no question that it is one of the most frustrating business practices in the United States. Airlines routinely sell more tickets than they have seats (except for jetBlue, which makes a point of not doing this).

This doesn’t happen anywhere else, or at least not by design for a time-sensitive product. It’s one thing for Apple to run out of new phones or tablets because demand exceeds production capability. But those products are not as subject to meeting a need based on a time factor.

A seat on an airplane, however, is about as time-sensitive as it gets, especially if it’s important. And let’s face it, it’s always important to the person getting—or about to get—screwed. I can’t help you feel better when it happens to you, but I can at least give you a bit of insight.

Airlines oversell for one simple reason: because they can. Generally speaking, they know that a certain number of people are not going to show up. We’re not talking about missed connections; we’re talking about the traveler—usually a business traveler—who doesn’t show for one reason or another. Those travelers make up a percentage of the seats on a given airplane, and it is that percentage that is usually—but not always—oversold.

If an airline has data that says that 10 passengers on flight 123 from ABC to XYZ don’t show up on a consistent basis, they will oversell by (usually) no more than that number, and often by less than that number. On the days when everyone shows, they then figure out what to do.

So, how do they determine who is going to get stuck? Each carrier has its own formula to follow, but it usually consists of some mix of the following (not necessarily in this order, and not limited to this list): last ticket sold; cheapest ticket sold; connecting versus non-connecting passengers; vacation package bought from the airline; the last person checked in; does the person getting bumped live locally (thus saving the airline a hotel room)?

The airline may also take into account that the next person who should be losing a seat may be part of a group of passengers, yet they only need to lose one passenger total. Unfortunately, you’ll never know. One thing you can count on: Unaccompanied minors are almost never denied boarding.

How can you fight back if it happens to you? It helps if you can argue persuasively that your travel plans will be unduly disrupted. For instance, if you are trying to make a cruise, you may be able to avoid being pulled. Likewise if you are connecting to an international flight, especially one that doesn’t run every day. Otherwise, you’re at the airline’s mercy.

There is one exception to the above rule of overselling by the number of no-shows, and that is the issue of performance. Sometimes, the airplane isn’t necessarily oversold, but the flight can’t meet performance requirements (usually related to single-engine climb) because of hot temperatures/short runways or runway contamination (snow or standing water).

Other times it may be because the flight is carrying an abnormal amount of fuel for the alternate, or because it’s overweight because of excessive bags or mail. As sharp as the airline reservation computer programs are, they often can’t take such random events into account well enough to try to control sales and loads.

One last trick: If you want to know if a flight is oversold, go through the act of buying a ticket on that flight the day before or the morning of, and see if one is available on the airline’s website. And then just hope that if it is oversold, it isn’t your number that comes up.—Chip Wright