Southwest Airlines Co. was sued by its pilots union and accused of violating federal labor law by unilaterally changing working conditions, rules and pay rates as travel collapsed because of the coronavirus pandemic.

The carrier violated the Railway Labor Act by making changes in its contract, the Southwest Airlines Pilots Association said in a lawsuit filed Monday in a Dallas federal court. The union, which represents 9,000 pilots, said it seeks to block further similar actions and asked a judge to order Southwest to make every effort to negotiate an agreement.

The lawsuit is the latest sign of rising tensions between the Dallas-based airline and its pilots as talks on a new contract have stalled.

The union has threatened to picket Southwest ahead of winter holiday travel over its failure to negotiate contract terms for items affected by the pandemic and the possibility of mandated vaccines. Southwest will trim its flight schedule for the last four months of the year in part after pilots, flight attendants and other employees complained about overwork and understaffed operations.

“Southwest Airlines has issued and implemented an Infectious Disease Control Policy during the COVID-19 pandemic, that significantly altered the working conditions, rules, and rates of pay for pilots,” the union said in the lawsuit. The changes violate a “status quo” provision of the Railway Labor Act, which controls airline-union relations, by not maintaining terms of an existing contract during negotiations, the union claimed.

The pilots said they remain among the most at-risk groups for Covid during the pandemic, “while management employees protected themselves by closing down headquarter offices to work from home and meeting virtually.”

The airline, in a statement, said negotiations with the union weren’t required for any of the changes made to respond to the “unpredictable challenges” presented by the pandemic.

“Southwest remains committed to pilots’ health and welfare and to working with SWAPA, and our other union partners, as we continue navigating the challenges presented by the ongoing pandemic,” said Russell McCrady, vice president for labor relations.

The carrier has increased by 16% to 5,200 the number of workers it wants to hire to help relieve flight disruptions and ease overtime.

The case is Southwest Airlines Pilots Association v. Southwest Airlines Co., 3:21-cv-02065-M, U.S. District Court, Northern District of Texas (Dallas)