Updated 11am, April 12th

The US airline United has found itself under fire after a passenger was forcibly removed from an flight.

The incident happened after a number of people were randomly chosen to leave the plane, which was travelling from Chicago to Louisville, Kentucky.

The Chicago Department of Aviation is investigating after viral videos showed a passenger being dragged from the plane by airport security after refusing to give up his seat.

Other footage, reportedly taken after the man re-boarded the plane, showed him dazed and with a bloody face.

US media has obtained a letter from United CEO Oscar Munoz to staff, which suggests the incident happened after a number of crew members said they needed to board the already fully boarded flight.

INBOX: @united CEO sends letter to employees about United Express flight. pic.twitter.com/obVdl6G2E0

— Ryan Ruggiero (@RyanRuggiero) April 10, 2017

The situation has led to widespread outrage across social media, with many calling for a boycott of the airline.

The company initially claimed the flight was overbooked. 

The Washington Post later reported that a spokesperson for the airline backed off the claims, and instead suggested the passengers were asked to leave to accommodate off-duty crew.

Overbooking

For many, the fact that flights are regularly overbooked might come as a surprise - but it is indeed a relatively common occurrence, even if it very rarely leads to scenes as dramatic as the ones seen in Chicago.

Fintan Ryan, aviation expert and former Aer Lingus captain, spoke to Newstalk Breakfast about the incident in the US and also his own experience of overbooking.

"Overbooking happens all the time," he explained. "On a Boeing 747, for instance, which had around 400 seats... you could expect that around 50 people wouldn't show up.

"The statistics are such that you overbook by different amounts on different flights. For instance, in a [Dublin to London] flight you might overbook by quite a bit because there's another flight behind. Normally somebody would say 'OK, that's fine, give me €100 and I'll go on a flight an hour behind'."

However, he said that over the course of a 25-year-long career he has never come across anything like forcibly removing someone from an aircraft due to overbooking.

"You sort out these things before people get on the aircraft," Fintan said. "It's very difficult to sort something out when it's actually on the aircraft, because people are sitting down and ready to go."

@United overbook #flight3411 and decided to force random passengers off the plane. Here's how they did it: pic.twitter.com/QfefM8X2cW

— Jayse D. Anspach (@JayseDavid) April 10, 2017

Your rights

What are passengers' rights, and what would happen if you found yourself denied boarding on a flight from Ireland?

EU regulations govern what happens when a flight is overbooked when departing from or due to arrive in a member state.

'Denied boarding' takes place when a flight is overbooked - but the process happens before passengers get on the plane.

The airline must first ask for volunteers to give up their seats in return for an agreed benefit or compensation.

The volunteers are also entitled to either a refund, a re-routing to their destination on the next available flight, or another flight on a later date.

If the airline does not find enough volunteers, then they can deny boarding to passengers against their will.

However, in addition to the above entitlements, in such a situation the airline must compensate passengers when they are not allowed board - compensation ranges between €250 and €600 depending on the length of the flight.

Anyone who has their flight cancelled is also entitled to free meals and refreshments, as well as free hotel accommodation if an overnight delay takes place. Transport between the hotel and airport must also be provided.

Image: DAA

Fintan argued that it can make sense to disrupt a handful of passengers' journeys if crew members need to be make another flight at the destination airport.

He suggested: "It's more important in the overall scheme of things to actually get people down to move that plane out of Louisville than to have a situation where four or five passengers are bumped in Chicago O'Hare. Normally what happens in these cases, you do get enough people to forego their seats and to be compensated.

"You're talking about discommoding maybe 300-400 passengers, particularly if the employees that were going down were pilots or cabin crew for [another] flight."