Southwest Airlines' pilots union is suing Boeing for damages caused by the prolonged grounding of the Boeing 737 Max, saying a steep reduction in flights has cost pilots an estimated $115 million in compensation.

In a 79-page lawsuit filed in Dallas on Monday, the Southwest Airlines Pilots Association, which represents nearly 10,000 pilots at the nation's largest domestic carrier, slammed Boeing for what it called negligence and fraudulent misrepresentation and nondisclosure.

All stem from what the union says was Boeing's sales pitch that the Max was "essentially the same as the time-tested 737 aircraft'' Southwest pilots knew well.

The airline has an all-Boeing fleet and was announced as a launch customer for the Max in 2011. Southwest took delivery of its first Max in 2017 and had 34 in its fleet when the plane was grounded March 13 after two fatal crashes in less than five months.

"Boeing made a calculated decision to rush a re-engined aircraft to market to secure its single-aisle market share and prioritize its bottom line,'' the lawsuit says. "In doing so, Boeing abandoned sound design and engineering practices, withheld critical safety information from regulators and deliberately mislead (sic) its customers, pilots and the public about the true scope of design changes to the 737 MAX.''

Spokesman Peter Pedraza said Boeing believes "this lawsuit is meritless and will vigorously defend against it. We will continue to work with Southwest Airlines and its pilots on efforts to safely return the MAX to service."

The preliminary investigations of the crashes of Lion Air and Ethiopian Airllines that killed 346 people zeroed in on a new automated control system with which pilots were unfamiliar. Last week, the National Transportation Safety Board said the system didn't allow pilots to easily understand what was happening in a critical situation, and recommended that the federal government require Boeing redesign the Maneuvering Characteristics Augmentation System, or MCAS, so it is more intuitive.

Boeing has been working on a software fix for months and has repeatedly said it plans to submit the plans soon to the FAA.

The pilot union lawsuit, filed in the District Court of Dallas County, Texas, says the union sought compensation from Boeing and "Boeing refused.''

The Max grounding has caused Southwest to cancel more than 30,000 flights since March, according to the lawsuit. Southwest Chief Financial Officer Tammy Romo has previously said the airline's seat capacity will be down 8% from original expectations at the end of the year because of the Max grounding. In addition to the 34 Max aircraft Southwest had when the plane was grounded, the airline expected another 41 to be delivered and had built its flight schedule around that.

Southwest has taken the plane off its schedule through Jan. 5, longer than competitors American and United, because it didn't want to have to make last-minute flight changes during the busy holiday travel season if the grounding doesn't lift by the end of the year as Boeing executives have said is the plan.

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The Southwest pilots won't be the only group seeking compensation from Boeing. Southwest CEO Gary Kelly, along with the top executives at American and United, have said they will seek compensation from the company. Kelly has said he hopes to share any compensation with employees as the airline's profit sharing will be down this year due to the flight cutbacks.

The union representing Southwest flight attendants sent a note to members Monday saying it continues to talk to its legal advisers.

"We will persist in exploring all legal options available to our organization, and vow to advocate on your behalf vigorously,'' Chad Kleibscheidel, first vice president of Transport Workers Union Local 556 said in the memo.