Southwest Airlines Co., which acquired most of the flying rights sold by American Airlines Group Inc. at Washington’s Reagan National Airport, said it will return one pair of the slots that it received for free and didn’t plan to use.

Southwest decided to return the slots to federal regulators because the one Sunday round trip they allowed at Reagan didn’t fit the airline’s schedule, Whitney Eichinger, a spokeswoman for the Dallas-based carrier, said in an e-mail today.

The slots were the final pair among 52 that American agreed to divest to settle a federal antitrust lawsuit that sought to block its merger with US Airways Group Inc. The slots are prized by airlines because of flight limits at Reagan, which is used by business fliers who often pay higher fares.

A slot allows for one takeoff or landing, and a pair is needed for a round trip. Reagan is one of four major domestic airports where the U.S. limits access to control congestion.

Southwest agreed to purchase a bundle of Reagan slots that included rights for 27 daily round trips and the unwanted Sunday pair, which “we did not pay for,” Eichinger said on Feb. 21. JetBlue Airways Corp. is buying rights for 20 pairs and Virgin America Inc. for four. Terms of the sales, which must be approved by the U.S. Justice Department, haven’t been disclosed.

Southwest and Virgin America agreed to buy 17 slot pairs American was required to divest at New York’s LaGuardia airport.

Editors: Molly Schuetz, James Callan. To contact the reporter on this story: Mary Schlangenstein in Dallas at To contact the editor responsible for this story: Ed Dufner at 

Photo Credit: A jet departs Washington's Reagan National Airport next to the control tower outside Washington. Larry Downing / Reuters