Most of us realize we need to be on our best behavior while traveling by air. Gate crews and flight attendants are particularly concerned about the possibility of a petulant passenger going off the rails. But you might not realize that airlines are empowered to bump you from your flight or your seat even if you're not perceived as some sort of threat or hothead.

That reality made headlines recently when actor Andie MacDowell sounded off on social media after being ignominiously bumped from first class to coach while traveling with her dog on an American Airlines flight. It’s not clear why MacDowell couldn't be accommodated in first class but in a Twitter post, she wrote that the crewmember was in no mood to negotiate.

“Airline staff have been given a lot more power and have become policemen in the skies since 9-11,” says George Hobica, founder of Airfarewatchdog, a travel website. “I think many are abusing that authority."

Most of your rights and company policies—including when you are entitled to compensation for your inconvenience— are spelled out in the conditions of carriage which can be found on an airline’s website. The lengthy terms are filled with jargon and legalese, though they’re still worth eyeballing to understand the basics.

Here are key reasons why an airline can bump you or otherwise make travel difficult for you:

1. The airline priced tickets for this flight too low. If a carrier realizes through its electronic reservation system that a non-stop flight fills too fast, that could indicate the ticket fares are too cheap. In such a case, your flight could be switched from a non-stop to a connecting flight.

2. The air marshal needs your seat. Because air marshals protect the public, they are sometimes seated in first class without prior warning. If one of them shows up and needs your seat, you can be bumped, reassigned to another seat, or put on the next available flight. And you won’t even get an explanation; the government doesn’t want you to blab that there’s an air marshal on board.

3. The carrier abandons the route. Consolidation within the industry has prompted some airlines to cut back on the number of available flights. Some, such as Allegiant Air and Frontier, have also abandoned routes that are no longer profitable. "An airline should be required to put you on another carrier for the price you paid," says Hobica. "But that’s not the case.” While you’ll get your money back, you may be forced to buy a ticket at the last minute from another airline at top dollar.

4. You have poor fashion sense. The decision rests with the cabin crew, but if you wear a T-shirt with vulgar or semi-vulgar words or pictures, you could be refused boarding. The same holds true if you’re showing excessive skin.

5. You’re too large for one seat. Every airline has a rule on passenger size, but it’s generally not enforced—unless someone complains. If a passenger is spilling over the armrest, he may be required to buy two seats or wait for another flight in which two adjacent seats are available.

6. You can’t control your kids. Parents and their offspring have been booted from flights as a result of ill-mannered children who throw a temper tantrum, refuse to stay in their seats, wear a safety belt, or follow crew instructions.

7. The flight is overbooked. Despite sophisticated computer systems, airlines still overbook flights and, occasionally, double book seats. The latter usually happens with last-minute check-ins. Who gets priority? It depends on the airline, but many will give preference to upper tier frequent-flyer members, according to Hobica. It’s also based, to some extent, on when you checked in. If you’re bumped, you are entitled to receive up to $1,350 in cash, depending on the reason for the bump, the length of any delay, and the price of your ticket. Some airlines may prod you to take a travel voucher instead. Don’t take it. "Vouchers can be difficult to apply and they expire in a year," says Hobica. “Leave with the cash.”

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