The incident that is widely regarded as a public relations disaster for US carrier United Airlines took place on Sunday night at Chicago's O'Hare International Airport.

United was trying to make room for four employees of a partner airline, meaning four people had to get off the flight to Louisville.

At first, the airline asked for volunteers, offering $400 and then when that didn't work, $800 per passenger to relinquish a seat. When no one voluntarily came forward, United selected four passengers at random.

Three deplaned but the fourth, a man who said he was a doctor and needed to get home to treat patients on Monday, refused.

He was then forcibly thrown out of the plane by city aviation department security officers.

After it sparked widespread fury, United Airlines' CEO Oscar Munoz apologized, saying: "This is an upsetting event to all of us."

Overbooking and cancellation

Airlines are allowed to sell more tickets than seats on the plane. They routinely overbook flights because some people do not show up. Overbooking and cancellations are not a problem in the US alone.

In Europe, a 2004 EU regulation set common rules on compensation and assistance to passengers in the event of denied boarding and of cancellation or long delay of flights.

In these instances, airlines are obliged to offer their passengers alternative transport as well as make all arrangements for providing them with accommodation and all necessary information.

The regulation applies to passengers departing from an airport located in any EU member state as well as to those flying from a third country to any airport within the 28-nation bloc. 

Compensation

In case passengers are denied boarding due to an overbooked flight, they are entitled to a complete reimbursement of their tickets. In addition, the carrier will have to provide them with an alternative flight as well as financial compensation.

The amount of compensation paid to travelers ranges from 125 euros to 600 euros, depending on the length of their flight and the delay caused as a result of the cancellation. 

Airlines are also directed to offer free of charge meals, refreshments and accommodation to their passengers in a reasonable relation to the waiting time. In addition, passengers have to be offered free of charge two telephone calls, telex or fax messages, or emails.

Arranging accommodation to stranded passengers is financially burdensome for airlines, as they have to pay for hotel rooms. If passengers accept an alternative flight to another airport, the carrier must still bear the cost of transport to the original destination.

Special rules

There are a few special regulations governing compensation paid on account of canceled flights. 

If passengers are informed of the cancellation at least two weeks before the scheduled time of departure, the airline gets away scot-free as it need not pay any compensation.

It's also the case if they are informed of the cancellation between two weeks and seven days before the scheduled time of departure and are offered re-routing, allowing them to depart no more than two hours before the scheduled time of departure and to reach their final destination less than four hours after the scheduled time of arrival.

Furthermore, air carriers are exempt from paying compensation when the cancellation occurs in extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken.

A clause in the EU regulation states: "If an insufficient number of volunteers comes forward to allow the remaining passengers with reservations to board the flight, the operating air carrier may then deny boarding to passengers against their will."

It's probably not meant to suggest that an airline can forcibly and violently drag a passenger out of a plane, as the issue can be resolved in other ways such as by offering travelers higher compensation to voluntarily take a later flight.