The United Airlines incident shows how few rights you have on a flight

United Airlines has come under fire for a viral video that shows a passenger being forcibly removed from a plane after refusing to give up his seat.

The whole affair, in which the 69-year old man was dragged through the plane by airport police, serves to highlight how few rights passengers have even after they're already seated.

The airline, which says the flight from Chicago to Louisville was overbooked, asked for volunteers to give up their seats for crew members in exchange for $800. When no one volunteered, several passengers were told they had to leave.

The passenger in the video refused to give up his seat and was ultimately dragged off the plane by police officers working for the Chicago Aviation Department.

The entire incident was handled improperly: the passenger suffered injuries to his face and was treated at a local hospital, and a police officer was put on leave for acting outside of "standard operating procedure." The Department of Transportation is now looking into what occurred.

But the truth is, that it stemmed from something that any airline claims the right to do. Buried in United's "Contract of Carriage" is a line stating that the airline can deny boarding to passengers if the flight is overbooked:

"If a flight is Oversold, 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. If there are not enough volunteers, other Passengers may be denied boarding involuntarily in accordance with UA's boarding priority."

United does not define "boarding" in its contract, but a spokesperson told Business Insider that it refers to any period before the plane physically takes off. So you can be told to leave the plane at any point before it actually lifts off - even after you've boarded.

When no one volunteered to leave the United Airlines plane, a United manager said they would select random passengers to give up their seats, Audra Bridges, the passenger who posted the viral video, told the Associated Press.

But United's policy shows there's more to the selection process in these kinds of circumstances.

"The priority of all other confirmed passengers may be determined based on a passenger's fare class, itinerary, status of frequent flyer program membership, and the time in which the passenger presents him/herself for check-in without advanced seat assignment," United's policy states.

The airline further states that passengers with disabilities and minors under the age of 18 will be the last to be denied boarding. In 2016, the biggest US airlines bumped 475,054 passengers from flights, showing how the practice isn't limited to United.

Passengers do have some recourse.

The Department of Transportation writes in its "Fly Rights" that it requires all airlines to provide a written statement to passengers bumped from planes describing their rights and explaining how they decide who gets a seat on an oversold flight. Those who are denied boarding are eligible for some form of compensation.

It means that you can ask for proper compensation and alternative travel accommodations.