American Airlines is facing major disruption after accidentally giving all its pilots holidays over Christmas.
The Allied Pilots Association (APA) say this was due to a failure within the pilot schedule bidding system.
As a result, thousands of flights currently do not have pilots assigned to fly them.
The APA say there are "significant holes" in the operation - and that management at the carrier have "unilaterally" invoked a solution.
The union claim this action is in violation of pilots contracts, and have filed a grievance against the company.
The carrier is said to be offering premium pay to pilots who work on days off.
But the APA says: "Because management unilaterally created their solution in violation of the contract, neither APA nor the contract can guarantee the promised payment of the premium being offered."
It adds: "By not including APA in developing collaborative solutions to this critical holiday scheduling failure, management’s actions contrast with their handling of previous scheduling failures involving other work groups.
"Management’s actions likewise jeopardize any collaborative effort to ensure our passengers have a pilot crew to take them to their important holiday events".
American spokesman Matt Miller told Bloomberg the airline expects to correct the problem in time to prevent service disruptions.
The APA estimates some 15,000 flights from December 17th to 31st have been affected.
The carrier flies several services between Ireland and the US.