After the United incident, passengers should know their rights. If bumped, ask for cash.

Most likely, you are one of the millions who have watched the video: A United Airlines passenger is sitting in his seat on a flight from Chicago O'Hare to Louisville when along come three Chicago airport police. One of the officers drags the man down the aisle as if he were a slab of beef.

An uproar ensues over his forced removal and continues days after the Sunday event. United's stock drops by 4 per cent. On Twitter, travellers threaten to boycott the carrier. A lawsuit is a near-certainty.

"This is a black eye on the industry," said George Hobica, president of Airfarewatchdog.com.

To a point, United was within its rights to bounce the man from the flight, thereby freeing up his seat for another passenger (in this case an employee). Read Rule 25 in the airline's contract of carriage. According to the document, if an airline overbooks, it must ask for volunteers to relinquish their seats. The agents usually sweeten the deal with a voucher, plus, depending on the wait until the next flight, a hotel room, transportation and meal voucher. However, if no one raises their hand, the airline must deny a passenger boarding. In this scenario, the passenger is entitled to a maximum of US$1,350 ($1940).

"United could've made this go away by offering more money," Hobica said.

Airlines can remove a passenger from the plane if the traveller exhibits aggressive or harmful behaviour. But the contract does not mention roughing up a passenger because he declines to give up his seat.

"It's denied boarding," Hobica said of the rule. "It's not ejecting you from your seat."

Several authorities are investigating the debacle, including the Transportation Department and the Chicago Department of Aviation. Travellers can also learn from this unfortunate occurrence.

First, airlines bump people. All carriers, with the exception of JetBlue, oversell flights. From October to December 2016, the DOT documented nearly 9,000 denied boardings, including 891 by United.

"Maybe it's time to rewrite the contract of carriage," Hobica said. He also urges the industry to resuscitate Rule 240, which requires airlines to provide a displaced traveller with a seat on a different carrier.

When an airline needs a seat for another passenger or an employee, it prioritises its customers. At the top of the don't-bump list are passengers with disabilities and unaccompanied minors under the age of 18 as well as members of the military. Also in the protective bubble, according to Hobica: passengers who fly first- or business-class, pay a higher fare and demonstrate their loyalty to the airline as a member of a frequent-flier program. He adds that pleading your case - that you have to attend a funeral or relieve your dogsitter - is useless.

Passengers should prepare for the reality of an overbooked plane. Take a morning flight, so you have more options if you are left behind at the gate. (The Louisville flight was the last one of the day.) If you agree to relinquish your seat, accept only cash, not vouchers, which expire after a year. (If the airline rebooks you within an hour of your original flight, compensation is not required, though Hobica received US$300 for losing his seat to an air marshal on a flight from New York to Los Angeles.) If the airline randomly ousts you, don't argue. Ask for your cash and try to enjoy an extra night of vacation.