Airline passengers have surprisingly few rights when their flights are “overbooked.”

Federal regulations do not prevent carriers from selling more seats than a flight can accommodate, a practice the airline industry says allows carriers to try to fill planes despite the number of no-shows that they can expect on any given flight.

Typically, airlines will ask for volunteers on oversold flights, promising some sort of compensation in return for taking a different flight. If few volunteer, carriers typically will keep increasing their offers until enough fliers agree to take another flight.

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If not enough volunteers are found, the airline has the power to decide who gets “bumped” off the flight. Carriers spell this out in the “contract of carriage” that customers are bound to when they buy their tickets. But few fliers ever read that fine print, and such contracts don't always specify a clear order for such situations.

The Department of Transportation does have clear guidance about compensation due to fliers "involuntarily denied boarding," and that can pay off in the case of a significant inconvenience. For example, an involuntarily bumped traveler who arrives to his final destination more than four hours late is entitled to an amount worth 400% of his one-way fare (capped at a maximum of $1,350).

Otherwise, however, the government offers little regulation in how carriers go about deciding who gets bumped.

Some airlines select passengers who were the last to check in. Others look for fliers who has paid the lowest fare.  Many decide on a case-by-case basis, reluctant to boot fliers who may be traveling for a real emergency.

As for United, it’s contract of carriage states that on oversold flights, “no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA.”