What you need to know about oversold flights and denied boarding While involuntary denied boarding is not common, it is legal.

 -- The disturbing scene that played out on board a United Airlines plane Sunday evening in Chicago has made headlines around the world, sparking questions as to how or why such a dramatic event could occur.

Many are now wondering what rights passengers have when flights are overbooked and if there are rules and regulations surrounding this circumstance.

Here are some answers to those questions:

United Airlines first reported early Monday morning that it had oversold the Chicago-Louisville, Kentucky, flight, a common practice for airlines to make up for customers who do not show. United later said it needed to get four additional crew members on board for a flight departing from Louisville. When no passengers volunteered to take the travel voucher offered by United for giving up their seats, the airline generated a list of four passenger names to be removed from the aircraft.

United said it generated the names in accordance with its Contract of Carriage. The parameters spelled out in United's version are typical for major air carriers. The rule says passengers that are involuntarily denied boarding "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."

The rule notes that those with disabilities and unaccompanied minors will be the last ones to be chosen.

While the practice of overselling flights is common, the chances of falling victim to involuntary denied boarding is very small. In 2016, 40,629 U.S. airline passengers -- or .0049 percent -- were denied boarding involuntarily, according to the Department of Transportation. The vast majority of these passengers were given some form of compensation.

According to Department of Transportation regulations, if a passenger is not able to be rebooked on another flight that arrives with two hours of the original flight's planned arrival, the passenger's "compensation shall be 400 percent of the fare to the passenger's destination or first stopover, with a maximum of $1,350."

United offered passengers on the Chicago-Louisville flight up to $1,000 to take another flight, according to an internal letter from United CEO Oscar Munoz.

Although the man was already on board the plane and apparently seated, a statement from the Department of Transportation indicates the event is treated as an "involuntary denied boarding." The department added that the incident is under review.

Munoz told United employees today in an internal email that the company will take "full responsibility and we will work to make it right."

"I have committed to our customers and our employees that we are going to fix what’s broken so this never happens again. This will include a thorough review of crew movement, our policies for incentivizing volunteers in these situations, how we handle oversold situations and an examination of how we partner with airport authorities and local law enforcement," he wrote in the email.

United will share the results of its review on April 30, Munoz said.

The tone of Tuesday's email was different from one sent by Munoz a day before. On Monday, Munoz told employees that “there are lessons we can learn from this experience,” but he said employees “followed established procedures.”

Following Sunday evening's incident in Chicago, one of the law enforcement officers involved was placed on leave Monday. The Chicago Department of Aviation said the officer's actions were not in accordance with standard operating procedure and are not condoned by the department.

In 2016, United denied boarding at a rate of 0.43 per 10,000 passengers, according to the Department of Transportation's Air Travel Consumer Report. American Airlines' rate was 0.64, and Delta Air Lines' was 0.10. These numbers include passengers who have confirmed reservations and are involuntarily denied boarding on a flight that is oversold.

The Department of Transportation suggests that the most effective way to reduce the risk of being bumped from a flight is to arrive at the airport early. Typically, for passengers in the same fare class, the last ones to check in are usually selected to get bumped, if necessary.