Southwest is suing its mechanics' union over what it claims is an illegal work slowdown that is grounding planes and disrupting flights.
It's Southwest's second lawsuit in three years against the Aircraft Mechanics Fraternal Association and the result of raw feelings on both sides after more than six years of fruitless contract negotiations.
The airline filed the lawsuit late Thursday in federal district court in Dallas.
Southwest charges that some workers are pulling planes out of service for minor mechanical items to gain leverage in contract talks.
Union representatives didn't comment immediately. They have said that supervisors at Southwest pressure mechanics to approve planes for flight, which they say is a safety hazard.