Elizabeth Weise

USA TODAY

SAN FRANCISCO — Passengers on Sunday's now infamous United Airlines flight said they were told by the airline that a computer randomly selected the passenger who was forcibly re-accomodated by airport police.

Was a computer program really to blame?

Reached Tuesday by phone, United's media staff said they couldn't go into nitty-gritty details about how its passenger check-in and seat assignment software works. However, conversations with several industry insiders give a sense of how airline systems decide who gets bumped.

As with any algorithm, it's what the humans put into the code that's key. And it probably wasn't truly random.

Once a flight is set, an airline's customer service software kicks into action. These complex software packages, generally outsourced from either Hewlett Packard Enterprise or IBM, combine all major passenger service functions, including pricing, shopping, reservations, ticketing, check-in and seat assignment, said Brett Snyder, founder of the airline industry blog Crankyflier.com.

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Software creates a list

Airlines are empowered to remove or refuse passengers for many reasons, several of which have nothing to do with whether there aren’t enough seats available.

For example, it's illegal to smoke on a flight, so that can get you booted. The airlines also have clear policies in their contracts of carriage about who they get to throw out, including the barefoot, the intoxicated or the smelly.

All of that has nothing to do with what passengers are chosen to lose their seats when a flight is oversold or needs to accommodate airline staff.

Very early in the life of any flight, the software generates a bumping list, a "just-in-case" list of the first passengers who should be bumped if for any reason there aren't enough seats for everyone who's supposed to fly that day.

"The gate agents don't do anything" and don't create the list, said Snyder.

First off, airlines don't want to break up families, because bumping Mom but leaving an 8-year-old on the flight isn't going to work. They also don't want to bump customers who will then miss their connecting flight, requiring complex and expensive re-booking.

Also off the "bump at will" list are the premium customers who make airlines the most money.

“They don’t want to be in the position of kicking off a Global member or a 1K passenger,” said Paul Touw, CEO of Stellar Aero, a digital marketplace for private jets.

“They want to kick off people who never fly United, and make sure they don’t kick off the customers they care about,” he said.

The goal in all of this is to never have to touch that list, said Crankyflier.com's Snyder.

Airlines instead ask for volunteers, offering free tickets for ever-increasing sums to get fliers to take the next available flight. And when the flight gets close to its departure time, seats assigned to passengers who haven't shown up can be given away.

Generally there are enough empty seats or volunteers to make up for overbooking situations. But not always.

In that case, the list of "bump-first" would be chosen from that flight's passengers with the lowest fare class, i.e. those who paid the least for their tickets, or who have no- or low-status with the airline's frequent-flier program.

Finally, it isn't simply a matter of who checked in last. "It's kind of like musical chairs, whoever's left standing is out of luck," said Snyder.

'Never truly random'

Most importantly, this should all happen before anyone has actually set foot onboard.

"This was an abnormal situation because usually the computer’s making the decision before everybody's on the plane," said Snyder.

In the case of the Chicago to Louisville flight from which David Dao was forcibly ejected (the airline had to "re-accomodate" customers in the words of United CEO Oscar Munoz) the entire flight had boarded when the four-person crew heading to Louisville showed up at the gate needing seats.

And here is where the random list was likely used. That list — generated hours before the plane ever got to the gate — would remain in the computer system until after the flight actually departed, said Ahmed Abdelghany, a professor of operations management at Embry-Riddle Aeronautical University's Daytona Beach, Fla., campus.

Presumably, it was that list the gate agents went to when they had to choose one unlucky passenger to kick off after three had accepted monetary offers to give up their seats.

So in the end, said Snyder, "it's never truly random, it's what they put into the algorithm."