How does overbooking work with reserved seats?

I think it is quickly resolved when you remember that

• not everybody chooses a seat when booking
• not everybody may choose a seat when booking

A case in point, two months ago I flew Kansai–Incheon and back. I had booked with ANA but the operating carrier was Asiana. Even though they are both Star Alliance members, the booking system did not suggest I choose a seat on booking (I was not surprised; that happens more often). Furthermore, I was not even offered an online check-in but had to actually line up at the airport. Thus, I did not receive my seat until some 90 minutes before the flight actually left.

If the percentage of customers that need to go down this path is large enough then the seat problem vanishes.

(Furthermore, if I do not intend to take a flight, I typically won’t care about wasting my time by selecting seats etc.)

So then how can sometimes more people show up to the airport than seats are available if it is already sorted out who sits where?

You presume that the online check-in system cannot lie to you. In practice, it easily can. I can see two options off the top of my head:

1. The system won't let you select a seat before the flight if all the seats are already taken. You would then have to line up at the airports check-in desk and someone would potentially be bumped off their pre-booked seat in order for you to take the flight, depending on your status with the airline.

2. The system lets more than one person select a given seat, but would then only allow one or neither of them check-in for the flight. It's also possible to reshuffle the seats at the gate, depending on who actually shows up for the flight.

Very easy.

Not all people show up at the airport (because reasons) and an empty seat is in fact so valuable that the airlines do reserve seats with duplicate seat numbers. There are very sophisticated computer programs based on statistics which are able to predict how many people will be on board so that the number of empty seats will be minimized.

But naturally sometimes it does not work, all people are there and the airline has two people with the same seat number. What now? The haggling round starts:

"Dear passengers, we need some more room. We offer a compensation of 200 dollars and a free flight to the destination a few hours later if someone gives up his ticket"
....
"Erm...dear passengers, we have still not enough places. In an unheard fit of generosity our airline increase the offer from 200 to 400 dollars."
....
"As our flight is behind schedule, XYZ airlines now offer 800 dollars for a free seat. Anyone?"

United Airlines made the mistake to act with violence and forced a doctor against his will to leave the plane. Needlessly to say the public backlash was formidable and I think this option will not be pursued anymore.

Now it must be said that the computers are quite good so that currently only 20 of 10 000 passengers leave their seat. Unfortunately some US citizens are smart alecks and reserve now deliberately seats to give them up for the multiple value of the original ticket price. (Article is unfortunately in German).

Whether and when you are able to select a specific seat varies a lot from airline to airline, and even from flight to flight in some cases (i.e. long-haul/international vs short-haul/domestic).

This can range from anyone can select a seat for free at time of booking, to no-one can select a seat at all (though this is becoming quite rare), with intermediate steps including:

• only premium passengers (frequent flyers with status, first/business/premium classes) may be able to select a seat at time of booking, or at all
• ditto, but they get it for free while others have to pay
• you need to pay to select a seat
• you can only select a seat when checking in, not while booking
• you can only select a seat if you book the flight directly online on the airline's site, not via another (partner) airline, another site, on the phone, via a travel agent...

Also, the booking process itself may not necessarily include seat selection immediately, it may often be a secondary process after the booking is made, which is not always very obvious for non-seasoned travelers.

So, there are many cases when seats are not assigned until check-in. Even if you can do online check-in, you can end up in situations where you can't do the check-in online, and are invited to do it at the airport (that's often an early hint of possible overbooking), or your boarding pass explicitly states that you will be assigned a seat at time of boarding.

Of course, there are also a few airlines that don't assign seats at all and let passengers pick at time of boarding.

Add to that operational issues (switching one plane for another, issues with the plane or crew which reduce its capacity, premium passengers from earlier delayed/cancelled flights...), and you can end up with quite a few issues.

There are often more issues with "traditional" airlines (as they have more "flexible" tickets and premium/elite passengers for which overbooking makes sense, and more variations in aircraft types) than with LCCs which don't really need to overbook: once a seat is sold, it's paid for, whether the passenger actually flies or not, though the line is not necessarily that clear-cut.

I guess there are many possible reasons for this kind of thing to happen. To me it happened only once, I was flying with Swiss Airlines from Zurich to Prague, I had my seat reserved during booking and I guess almost all other passengers did. They announced that by a system mistake there was an overbooking problem and they needed 1 passenger as volunteer to take the next flight.

I wasn't in hurry at all so I volunteered and they explained me they would offer a 150€ prepaid debit card (MasterCard to be specific) that could be used anywhere along with the next flight the same day after 3-4 hours. They explained to me that there was an error in the system and it accidentally booked 1 seat more than what was available. Being a software developer myself, I know that even in the most important and checked systems there can be problems sometimes, so I tend to believe them.

Also the fact that the staff was so worried and looked sorry and announced it before boarding started, along with most passengers being shocked and worried to lose the flight (1 poor guy transporting an organ for donation was really worried...) it made it seem to me that it's unusual and it indeed was a system error.

Unfortunately for me, 3 people didn't show up in the end so there was space for everyone.

On almost all my flights there were a few seats empty, so what Thorsten is saying actually makes sense and could be a reason why that happens on some airlines.