United had accused some pilots of abusing sick time and refusing to fly extra hours. Sick-outs in particular are not allowed under the Railway Labor Act, the federal law that governs airline labor relations.

United said the judge in Chicago found that the actions of the Air Line Pilots Association had violated the act, and issued a preliminary injunction on Monday against four pilots and the union. United said it would next seek a permanent injunction.

Over the summer United blamed the pilots for the cancellation of 329 flights between July 19 and July 27. The carrier said that cost it about $8 million in lost revenue and $3.9 million in operating profit. United filed the lawsuit on July 30.

A spokesman for the United branch of ALPA did not immediately return a phone message seeking comment.

"While there is always room for discussion and tough give-and-take about our business, deliberate actions that unfairly or unlawfully impact our customers and employees — and that keep us from achieving our full potential — will not go unchallenged," Chief Operating Officer John Tague said in an e-mail to United employees.