ORLANDO, Fla. — Southwest Airlines is accusing the mechanics union of grounding planes unnecessarily, and they are asking a federal judge to step in.

The airline filed suit Thursday in federal court in Texas against the Aircraft Mechanics Fraternal Association.

The suit comes amid several weeks of maintenance issues in which a large number of Southwest planes have been grounded. 

Southwest Airlines and AMFA have been in contract negotiations for over six years. 

In the suit, Southwest says an "abnormal increase" in maintenance write-ups started around February 11, which is around the same time that the latest round of contract negotiations broke down.

Southwest says maintenance stations at Orlando International Airport, and airports in Houston, Las Vegas and Phoenix, began reporting Unscheduled Aircraft Downtime "substantially exceeded the stations' average UAD."

The number of aircraft out of service has increases steadily since then. 

"Southwest began experiencing an uptick on cosmetic and other minor maintenance write-ups,” the airline said in the lawsuit. “It quickly became clear as these unusual write-ups spread across Southwest's stations that something coordinated was beginning to occur."

On February 15, Southwest declared an operational emergency for Orlando, Houston, Las Vegas, and Phoenix, in order to stabilize operations. After that, Southwest said the number of UADs grew and the issue spread to other airports.

Southwest is accusing the union and mechanics of purposely and unecessarily writing up planes for maintenance, under the guise of safety, to force the airline to reach a better deal with the union.

“This action does not alter our goal of reaching an agreement that benefits our hardworking Maintenance Employees nor does it change the Company’s unwavering commitment to Safety,” said Vice President Labor Relations Russell McCrady. “Southwest is--hands down--one of the best companies in the world to work for and we will not stray from our focus on rewarding our Mechanics, while we work to shield our Employees and Customers from unnecessary disruptions within the operation.”

The union, however, says mechanics have long felt pressure by the airline to overlook potential safety issues.

On February 12, Sens. Ed Markey, D-Massachusetts and Richard Blumenthal, D-Connecticut, asked the Federal Aviation Administration to investigate a published report on the matter.

In response, the FAA increased oversight on maintenance at Southwest facilities. 

With the increased oversight, AMFA says its members are no afraid to write up more legitimate issues.

"The truth is, this is not an “emergency,” but the new normal. [Aircraft Maintenance Technicians] are raising legitimate safety concerns," the union posted in a February 20 statement.

The union says the level of workers calling out sick or declining overtime has also not increased throughout the Operational Emergency.

We reached out to the union for a statement on the lawsuit.