If you're booked on American Airlines or planning to fly them during the holidays, listen up.

Thanks to a computer glitch, the airline granted too many of its pilots too much vacation. As a result, with the end of the year approaching, they may not have enough pilots to man their aircraft.

American Airlines’ pilots’ union estimates that at this point as many as 15,000 flights could be affected.

Passengers at Tampa International Airport were getting on their flights Thursday, but whether they'll get back might depend upon how long they plan to stay at their destination.

Call it a humbug heads-up.

“Especially when you're going on vacation and expecting to see family and that kind of thing,” said Passenger Jeanie Ladd.

The issue, says the airline, was a computer error that allowed too many pilots to take too many vacation hours right around the holidays.

“I don't know what they're obligated to do, but they should make the transition pretty quick to what they need to correct it fairly quickly and get everybody to their final destinations,” said passenger Larry Ladd.

It wasn’t clear whether the airline could force its pilots to fly, or force them to give up vacation time some pilots plan a year in advance for.

“I mean, I would just say this is your job. You have to make a sacrifice sometimes,” said passenger Jeanine Sullivan.

“They're not going to look good at the end of this if they start leaving people stranded at the airports,” said 10News aviation expert Mark Weinkrantz.

Weinkrantz, a commercial pilot himself, says American is working on it by offering its pilots time and a half to pick up the extra flights.

But legally, some pilots may not be able to fly, said Weinkrantz, because they may have already put in too many hours ahead of the holidays to do so.

“It's not like ordering a pizza. You can't just get another one off the shelf,” said Weinkrantz. “The pilots have to be current in their particular aircraft at the need to fly in. And they need to have certain hours of limitations that they can do. That is an FAA hard stop.”

So, if you're American Airlines customer planning to fly around the holidays, what are your options?

Well, until they figure out how many flights they can cover, not many.

Weinkrantz thinks as the holidays approach, the airline will contact you if your flight is impacted, “Proactively canceling some flights. Canceling them in advance so that people aren't left stranded. And they will rebook them on to what they can do,” he said.

Weinkrantz says no airline wants to be a Scrooge around the holidays, but right now, he thinks that American Airlines is just waiting to see how many pilots it can reassign or how many will voluntarily give up their Christmas vacations.

In the meantime, he suggests passengers reach out to the airline for assurances as soon as possible, because the more pressure they face from customers, he says, the more likely American will be forced to come up with an alternate plan.

The airline disputes the union's impact estimate. In a statement, it said only a few hundred flights are currently unassigned to pilots, and that the number will decrease because of the pilots who are picking-up those flights.

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