Selling more tickets than there are seats available on an aircraft strikes some people as an appalling business practice – bordering on fraud.

After all, theatres and sports venues don’t sell the same seat twice. So how can airlines justify trying to boost their profits at the travelling public’s expense?

In one celebrated case, Taylor Swift fan Megan Ridout was denied boarding a British Airways flight from London Heathrow to Lyon for the city’s gig in the Eras tour. BA turned her away from the plane and she made the concert only after an exhausting trek via Marseille.

Yet when handled correctly, overbooking makes good sense.

These are the key questions and answers.

200 seats on the plane? You sell 200 tickets. What’s wrong with that?

Airlines know from experience that on most flights, a proportion of passengers won’t turn up. So they place a bet that they can sell more tickets than there are seats, predicting that a certain percentage (typically 5 per cent) will not turn up because of a short-notice event such as an illness, a work demand or simply being stuck in traffic.

Mostly, they get away with it. You and I have probably been on plenty of oversold flights without realising it, because the number of “no shows” has been predicted correctly.

By selling the same seats twice, an airline can make much more money: on busy flights, often the last passengers to book are desperate to travel and will pay very high fares.

Britain’s biggest budget airline, easyJet, says: “On any given day large numbers of passengers don’t show up for their flight. Filling our planes and minimising the number of empty seats we fly is one of the ways we can reduce the price you pay for your flight.”

There are other benefits, for passengers and the planet.

What should happen if too many people turn up for a particular flight?

When there are more passengers than seats, the law and good sense align. In the UK, the European Union and North America, the airline is required to seek volunteers to travel on a later flight (or possibly a departure to a nearby location, eg Cologne rather than Dusseldorf or New York Newark instead of JFK).

The carrier should offer inducements until enough volunteers are found. These can include:

If the later flight is the following day, the airline must provide a hotel (together with transport to reach it) and meals as appropriate for the time spent waiting.

How can the airline be sure that it will get enough volunteers?

By throwing money at the problem. Everyone has their price. People who need to travel urgently place a high value an immediate departure; other passengers may not, and can be bribed to postpone their flight with promises of cash or future tickets.

On that British Airways “Taylor Swift Express” from Heathrow to Lyon, the offloaded passenger, Megan Ridout, was desperate to reach the city to see the gig. But it is a fair bet that most of the passengers did not have a ticket for Ms Swift and were open to a suitable inducement. They could include:

In my experience in the US overbooking is handled very well, with airlines raising their bid until they have enough volunteers. The trouble is that in Europe there is no exact definition of what an airline must do, and the evidence I’ve seen is that they don’t try hard enough – and instead pick “victims”.

I’ve been selected for overbooking against my will. I must travel. What can I do?

Insist that the airline seeks volunteers. If this fails, assert your right to be flown as soon as possible to your destination.

If an airline denies you boarding a London-Los Angeles flight it should not be neck essary to wait six hours to that carrier’s next departure – if another airline has an earlier departure. The rules state you are entitled to “re-routing as soon as possible”. Re-routing is a clumsy term; the meaning is that you are entailed to be flown, usually on the same route, as soon as possible.

Or be creative: for example if you are booked to a provincial French or German city, note that Paris CDG, Lyon, Frankfurt and Berlin have high-speed rail stations built into the airport. So it may be that a London-Lyon journey could be made by air to Paris and train from there.

You are also due between £220 and £520 depending on the length of the flight. This should be in cash – although the airline can invite you to accept a voucher (which should be significantly more than the cash compensation) and proceed with your agreement.

But my airline insisted there wasn’t time to ask for volunteers?

This is nonsense and they should look at the good practice in the US. Passengers are often tapped up before departure, so the carrier has a list of people prepared to leave the plane if the price is right. And I was actually sitting on a plane about to leave from Seattle to Cincinnati, when, 10 minutes before departure, an appeal went out for someone to travel later in return for $300 (£235). I dinged the call bell and left the aircraft.

At busy times – typically Friday and Sunday afternoons and evenings – the bidding starts significantly higher, and the problem for the airline can become having too many passengers desperate to leave the aircraft in return for a multiple of the fare they paid.

Isn’t there a better way to handle this, though?

Some airlines, notably Vueling, may offer passengers inducements in advance to travel on an alternative flight. This isn’t quite the same as overbooking – it simply allows the carrier to shift low-paying passengers onto less popular flights and sell their places for much higher fares.

How can I avoid being denied boarding?

Book with one of the relatively few carriers that do not overbook flights, notably Ryanair and Jet2. Even then, though, there may be a rare case where a change of aircraft requires some passengers to be left behind. As with any “real” case of overbooking, the airline should seek volunteers.

Is anyone safe from overbooking?

There are no guarantees. A plane ticket is nothing more than a vague expression of the airline’s hope to get you from A to B, and the carrier can pick any victim it likes,

But UK and European regulations state: “Operating air carriers shall give priority to carrying persons with reduced mobility and any persons or certified service dogs accompanying them, as well as unaccompanied children.”

Passengers who have checked in baggage may be safer, because the process of retrieving luggage can add faff and delay.

Does it help if you book in advance?

No: Megan Ridout, the Lyon Swiftie, booked her seat 11 months ahead. Indeed, a cynical person would say that it made her more of a target for offloading. She paid a bargain £91 return for a flight that British Airways could have sold for hundreds of pounds.

What about checking in well ahead?

It appears that the odds of being denied boarding on an overbooked easyJet flight are much lower if you have already been assigned a specific seat through the online check-in process. This can be done up to a month ahead.

Are there other areas of travel where overbooking happens?

Some hotels will oversell routinely. Usually the manager will have in their back pocket some spare rooms in another hotel – where excess guests can be packed off in a taxi with a bit of cash or a free dinner for the inconvenience.

What about on road and rail?

Intercity trains are ripe for some overbooking – particularly on “open access” operators such as Lumo. While the London-Edinburgh rail firm is very good at selling out its trains, I have never been on one that does not have a handful of empty seats, which represent unrealised revenue. The same applies on long-distance coach operators.

Korean Railways has a terrific sort-of overbooking policy on its intercity trains. When all the seats sell out, a certain number of standing tickets are sold at roughly half-fare. If seats are unclaimed, standees can take those places.