I fly about once a month or so over the past 2-3 years, a number of different airlines. United is the only one that consistently asks for volunteers to get bumped on every flight.
We're talking about ~0.1% to 1% probabilities here of overbooked flights. (EDIT: Apparently we're talking even smaller: 0.01% to 0.1% probabilities... actually)
That's not every flight. Small enough that its too small for anecdotal evidence to be useful, big enough that it causes media stories. Every airline has some degree of overbooking necessary because flights get delayed, connecting flights lose passengers, and sometimes people just simply don't show up.
Especially in large airports (ie: Chicago's) where the airport is basically a hub for other airports to go to.
That's only the involuntary bumps. If you divide out the numbers for the voluntary denied boardings, United is #3 behind Skywest and Expressjet - far worse than other major carriers for overbooking flights.
This is naturally going to create a systemic problem when there's too many overbooked flights. If you have to bump some passengers from flight 1, then they try to get on flight 2, but flight 2 is ALSO overbooked, so now you have to bump or pay off even more passengers from flight 2 and put them on flight 3, which is ALSO overbooked and so on. You end up with a cascading snowball of bumped passengers.
Inevitably you run into a situation where nobody wants to volunteer, then you get a situation like the one in the news. It's not a coincidence that happened on United.
Voluntary is a bit harder to compare though. For one, because its not necessarily wrong for people to give up their seat for money. If I were going on a vacation and someone offered me $400 to take a flight 2-hours later, I probably would take the $400.
In any case, United is not the worst offender in either voluntary nor involuntary "bumps".
I'm just saying in order to estimate how many flights they overbook, you'd have to look at the total of voluntary and involuntary bumps. People wouldn't be volunteering to take a later flight and United wouldn't be handing out vouchers if the flights weren't oversold.
I don't think it's wrong per se, in a moral sense. I just think it creates problems. The more overbooked flights you have, the less able you are to respond to delays or to re-seat passengers who got bumped, or to find new flights for people whose plane suddenly got smaller or who missed a connection or whatever, and there are ALWAYS going to be things like that happening.
The passengers who get bumped don't usually just give up altogether. They have to get on some flight. The more overbooked flights there are, the fewer chances there are for them to get on a different flight, and the more people there are looking for different flights in the first place. The problem compounds and you get the cascading spiral like I described above.
A constrained system like the airline is going to respond nonlinearly to perturbations. When you double the number of overbooked flights, the problems they cause aren't just going to get twice as bad, they'll get four times or six times as bad or more. (made up numbers, I'm just saying it's not one-to-one).
Flying is already a stressful experience thanks to the TSA and just the general stress of making sure you get there on time with all the uncertainty in what can go wrong (traffic, long security line, long wait to check a bag, whatever). If I book a flight at a certain time, it's because I want to leave and arrive at the times I chose, not a couple hours later or the next day or two days later, or never. This is why I pretty much refuse to fly United anymore. I don't like the stress of having to wonder if I'm going to be allowed on the flight I've paid for.
That's not every flight. Small enough that its too small for anecdotal evidence to be useful, big enough that it causes media stories. Every airline has some degree of overbooking necessary because flights get delayed, connecting flights lose passengers, and sometimes people just simply don't show up.
Especially in large airports (ie: Chicago's) where the airport is basically a hub for other airports to go to.
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United Airlines is solidly average on overbooking: https://www.transportation.gov/sites/dot.gov/files/docs/reso...
At 0.4 passengers per 10,000 bumped, United Airlines is actually a lot better than Jetblue or Southwest.
See page 33.