Southwest Airlines has raised the stakes in a showdown with its union of aircraft mechanics by filing a lawsuit over what it claims is an illegal work slowdown that is grounding planes and disrupting flights.

It is 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 did not respond to requests for comment late Thursday or Friday. In previous interviews, they have denied organizing a work slowdown and have countered that Southwest maintenance supervisors have pressured mechanics to approve planes for flight, which they say is a safety hazard.

Southwest said in the lawsuit that it normally has 14 planes out of service on an average day, but the number has spiked as high as 62 in recent days.

The airline said the rising rate of mechanical issues began after a recent bargaining session between the company and the union. A federal mediator is trying to bring the sides together.

Dallas-based Southwest sued the same union in February 2017, claiming that it was behind a refusal by mechanics to work overtime. That lawsuit is still pending.

Southwest has struggled to respond effectively to the latest dispute with the union, known by its acronym, AMFA, which represents the carrier's 2,400 mechanics.

Last month, chief operating officer Mike Van de Ven publicly apologized to customers for flight delays and cancellations, blaming them on mechanics who were grounding a high number of planes. The airline's chief legal officer, Mark Shaw, warned union leaders that they must stop "this unlawful job action" by union members.

The airline imposed emergency procedures that included threatening to fire mechanics who didn't get a doctor's note after calling in sick or refused to work overtime. Vacation requests were put on hold.

Some employees believed the emergency measure was retaliation for mechanics going to federal regulators and the media with their safety concerns. One of the union's lawyers said Southwest had provided no evidence of a work slowdown, and that mechanics were doing their jobs as required by federal safety regulations.

The airline's CEO might have thought that some fence-mending was needed.

In a Feb. 22 letter to all Southwest employees, CEO Gary Kelly called the mechanics "extraordinary" and said "they have been especially heroic in getting aircraft returned to service over the last two weeks." He said the mechanics deserve a new contract and blamed all sides for the lack of one.

Over the years, Southwest has prided itself on better labor relations than most airlines, even though it has the most heavily unionized workforce among major U.S. carriers.

That spirit of working together has been strained in recent years, however, as pilots and other employees have picketed in protest of what they considered inadequate wages.

There is a long history of public spats between airlines and their unions. Some of them were far more damaging than this one, including a strike that helped bring down Eastern Airlines in the 1980s.

"The airline industry is a special case. Airline workers have a lot of leverage because of their role connected with safety," said Lance Compa, a senior lecturer in labor law at Cornell University.

Federal law prohibits airline union employees from job actions including strikes, and bans airlines from locking out union workers unless a federal mediation board declares an impasse in negotiations, which has not happened in this dispute.

The same law, the Railway Labor Act, provides that contracts in the airline industry never expire; they are only renewed. Compa said that leads to drawn-out negotiations that can be frustrating. He sees little long-term risk to Southwest from the current fight.

"These things," he said of airline contract disputes, "tend to work themselves out."