Southwest Airlines has accused its mechanics union of "coordinating" to undermine the airline's business by writing improper safety assessments that have taken a large number of aircraft out of service over the past three weeks, according to claims made in a new lawsuit filed in Dallas on Thursday.

Southwest filed suit in U.S. District Court in Dallas against its mechanics union, claiming the American Mechanics Fraternal Association (AMFA) and its leaders have not done enough to stop what Southwest alleges is an illegal job action aimed at disrupting the airline's operations.

Southwest Airlines and the AMFA — which represents about 2,400 Southwest mechanics — have been locked in tense contract negotiations for six years. Central to the contract negotiations is Southwest's use of outside maintenance contractors, which factored into the mechanics rejecting a contract offer in the fall.

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According to documents obtained by CBS News, the suit alleges mechanics are conducting a coordinated effort to disrupt Southwest's operations and thus exert concessions from Southwest Airlines during ongoing labor negotiations.

"On February 12, 2019 — immediately following AMFA's blow up at the NMB [National Mediation Board] mediation session — Southwest began to experience an unprecedented number of aircraft out of service, despite no change in leadership and no change in policies or procedures," the lawsuit reads. "Given the timing of the recent NMB negotiations and the nature of the write ups, it appeared that AMFA and its members were organizing and encouraging Southwest mechanics to unnecessarily write up maintenance issues in order to remove aircraft from service and disrupt Southwest's operations in an effort to gain an advantage in contract negotiations."

According to the complaint, Southwest began experiencing an "uptick on cosmetic and other minor maintenance write-ups that do not have an effect on the safety of the flight," like a missing row number for an airline that doesn't assign seats, which spiked almost 400 percent to 500 percent after Feb. 10, 2019.

These write-ups led to multiple aircraft being called out of service, leading to flight cancellations. The lawsuit argues that on a typical day, Southwest Airlines "will have approximately 14 aircraft out of service," and that anytime more than 35 planes are out of service it's "impossible" for the airline to meet its customer service obligations. According to the lawsuit, between Feb. 11 and Feb. 22 the airline averaged 46 out of service aircraft per day due to maintenance concerns. On Feb. 19, 62 Boeing 737s were out of service, more than triple a normal day.

On Feb. 15, 2019, Southwest declared an operational emergency for Phoenix, Las Vegas, Houston, and Orlando stations, hoping to to stabilize its operations. The state of operational emergency allows Southwest the option of assigning longer work hours, changing assignments and shifts, denying vacation and schedule-swap requests, and it permits expanded use of outside maintenance contractors. According to the lawsuit, "The operational emergency essentially called for all hands on deck to address the increased workload created by the spike in maintenance write ups."

The blame game

In its lawsuit, Southwest Airlines accused its aircraft mechanics of deliberately assessing planes as unsafe to fly, thus directly blaming them as the cause for the cancellation of flights.

"It quickly became clear as these unusual write ups spread across Southwest's stations that something coordinated was beginning to occur — a fact borne out by the empirical data regarding Southwest's maintenance activities before and during this period," the lawsuit reads.

AMFA has denied organizing or condoning any work action against Southwest. The union declined to comment, saying it is still "assessing" the lawsuit.

Last week, the union told CBS News, "All of the out of service aircraft were written up for legitimate problems." AMFA National Director Bret Oestreich called critical claims previously made by Southwest Airlines against the association "scapegoating" and "simply an attempt to divert attention away from the airline's safety issues."

More than two dozen Southwest mechanics interviewed by CBS in recent weeks insist all of the write-ups have been legitimate and done in good faith. They wished to remain anonymous for fear of retaliation, but cited examples of problems they've identified on airplanes that were later put out of service — things like the pins holding lavatory doors in place. When those migrate upwards they can lead to punctures in the plane's fuselage.

According to the Southwest mechanics CBS spoke with, they have also seen holes in tires, loose rivets holding the skin of the airplane intact, and damage to fan blades. Mechanics say the issues they are reporting are not just cosmetic and could, under the wrong circumstances, endanger the airworthiness of the airplanes.

Photos obtained by CBS News show a deep cut on a main tire (left), and loose rivets on an engine cowl (right).

Fan blades have received extra scrutiny since an accident last year which killed a woman on a Southwest flight. An investigation by the National Transportation Safety Board found evidence that metal fatigue caused a fan blade to break loose from the engine. Southwest Airlines was current with all of the manufacturer's requirements at that time, but there has been an increased push by the Federal Aviation Administration since the accident to inspect engine blades.

Another accusation made

The surge in aircraft being taken out of service for maintenance issues followed a CBS News investigation into allegations by Southwest mechanics of being pressured to keep planes in service and concerns over the airline's safety culture.

CBS News has obtained the internal email sent to employees explaining the lawsuit from Senior Vice President of Technical Operations Landon Nitschke, who, as we reported, told mechanics at the end of 2017 that "compliance, compliance, compliance" with FAA regulations was their priority.

"The number of write-ups — mostly non-operative items that do not impact Safety or flight — has continued to be abnormally high," Nitschke wrote. "Since we have not seen a clear improvement... we are continuing the path to seek legal recourse."

The message provides a previously unreported update on Southwest's state of operational emergency, disclosing that the airline's maintenance facility in Las Vegas has been removed from the list of affected stations, but Los Angeles has been added. The state of operational emergency order remains in effect at four other maintenance hubs: Dallas, Houston, Orlando, and Phoenix.

As we reported, the airline's chief counsel laid the groundwork for the lawsuit with a letter to the union last Friday. In the letter, Mark Shaw wrote that a group of about 100 mechanics (out of 2,400) were responsible for the bulk of the write-ups that have taken up to more than 60 planes a day out of service.

The airline is still seeking to resolve this operational nightmare while pursuing the lawsuit.

"Today's 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 in a statement provided to CBS News. "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."

Southwest Airlines pilots get involved

The dispute between Southwest and its mechanics has reached such a point that it's threatening to bring in another group integral to the airline's overall mission and operation: its pilots.

Earlier this week, the Southwest Airlines Pilots Association (SWAPA) joined the mounting war of words with a message titled "State of Managerial Emergency," calling the airline's state of operational emergency "a veiled attempt designed to intimidate our mechanics that has instead caused unnecessary fear and safety concerns in our passengers and the flying public."

The pilots union president, Captain Jon Weaks, praised mechanics and accused Southwest executives of "the most egregious display… of tribalizing and scapegoating our employees in the history of our company."

"The last few weeks have highlighted how poorly upper management at Southwest Airlines is performing, how it truly views labor, how ineffective its communication and execution of our daily operation are, and how everyone at OUR airline should be concerned," Weaks wrote. "Let me be clear, our aircraft are safe, and a large part of that is because the men and women of AMFA continue to do their jobs in the face of increasing pressure, intimidation and scrutiny from Southwest management. They have our eternal gratitude for a job well done."

The mechanics association seemed to appreciate the support offered by the pilots.

"We are grateful for the support of Southwest's pilots, in SWAPA's statement today regarding, as they call it, the 'state of managerial emergency' that all of us at Southwest Airlines are facing," the AMFA said in a statement Tuesday, prior to Southwest filing the lawsuit. "They, better than perhaps anyone at the Company, can appreciate that this is about safety and safety alone, and not about contract negotiations."