American Airlines has been left scrambling to find pilots to operate thousands of flights due to a glitch in the system used to schedule crews.

At this point, American Airlines hasn't said just how many flights will be affected, but the pilot's union says around 15,000 flights were scheduled without a captain, a co-pilot or both because they were given off the week of Christmas.

American is the world's biggest airline, with about 15,000 active pilots and expects to operate more than 200,000 flights next month.

A spokesperson for American Airlines says, “We have reserve pilots to help cover flying in December, and we are paying pilots who pick up certain open trips 150-percent of their hourly rate, as much as we are allowed to pay them per the contract."

The airline has also said they will work with the pilot's union to make sure customers are able to fly where they want to go, but the pilot's union is urging its members to not take on these flights as it could violate their contracts.