Southwest Airlines Co. pilots' union asked a federal court to block the carrier from flying Boeing Co.'s newest 737 until the plane is negotiated into a new contract for the 8,300 aviators.

The union also wants the court to order Southwest to stop "delaying and frustrating bargaining" and fulfill its legal duty to make every effort to reach a contract if the airline wants to fly the 737 MAX when it's delivered next year. The Southwest Airlines Pilots Association sued Monday in Dallas federal court.

The prospect of a rare and unexpected early delivery of the MAX is adding to tensions between the carrier and the pilots, who have been negotiating a new contract for more than four years. The first MAX may arrive as soon as March, giving pilots extra leverage in the push to reach a deal or see the plane sit idle at Southwest.

The airline is attempting to force the union "to negotiate the MAX dispute with an illegal gun to the head of SWAPA," according to the filing. "Southwest Airlines, in short, is presently engaging in a high stakes and illegal game of 'chicken.'"

Same Plane

The lawsuit is "unnecessary and premature" because issues involving the MAX could be resolved before the plane arrives, Chief Executive Officer Gary Kelly said in an e-mailed statement Monday. If a contract isn't reached, the union "must use the grievance arbitration process outlined under the agreement between Southwest and SWAPA for a resolution, not initiate legal action," he said.

The airline is confident that the new plane is like others in the existing contract except for an updated engine and can be flown, Kelly said. Listing aircraft in a pilot contract and specifying rates of pay and related items is a common practice in the industry.

The MAX 8 version of the plane is on track to enter the market in the first half of 2017, Ray Conner, CEO of Boeing's commercial airplane unit, said on May 11. Southwest has said it expects the Max to arrive in the third quarter of next year, while the pilots' lawsuit set the date in March. The legal challenge also includes the MAX 7 variant, which Southwest won't get until 2019.

Status Quo

The lawsuit followed the union's request last week that Southwest guarantee it won't operate the plane before it's named in an approved contract. Doing so would violate a provision of the Railway Labor Act that requires both sides to maintain the status quo during talks, according to the lawsuit. The act governs negotiations involving airlines and organized labor, setting standards under which a strike can occur.

Southwest's "intent and expectation" is that a new agreement will be in place by the time the MAX enters service next year, Randy Babbitt, senior vice president for labor relations, said in a May 13 response to the union.

The carrier historically has negotiated specific aircraft into contracts, the pilots said, and didn't fly the Boeing 737-800 until it was listed in a side letter to the existing labor accord in 2011. Provisions for operating the MAX were included in an earlier agreement rejected by pilots in November.

Development Delays

If nothing goes amiss, the MAX's early debut would mark a reversal from many recent commercial programs, which have sustained lengthy development delays: Airbus Group's A380 superjumbo and Boeing's 787 Dreamliner had multi-year holdups, while Bombardier Inc.'s CSeries is three years behind schedule.

The Max began flight-testing ahead of schedule in January and is ticking off key milestones toward certification for commercial service. The first plane intended for Southwest has rolled out of Boeing's Seattle-area factory, but it may not be the first delivered if the dispute is protracted.

"We have strong indications that some Max customers want to start utilizing the airplane in their fleets as early as possible," said Doug Alder, a Boeing spokesman. "Given the airplane's performance in flight test to date, we believe that we can accommodate some customers in the first half of 2017."

He declined to name the customers or confirm the March delivery cited in the pilot lawsuit.

The legal action comes as Southwest prepares for its annual shareholder meeting in Chicago May 18. Pilots plan to picket at the gathering and at Midway Airport.

The case is Southwest Airlines Pilots' Association v. Southwest Airlines Co.,

16-cv-1346, U.S. District Court, Northern District of Texas (Dallas).