After pulling planes and canceling hundreds of flights, Southwest Airlines is offering an apology to travelers — and blaming the union that represents aircraft maintenance technicians.

Last week, the Dallas-based airline took more than 40 of some 750 aircraft out of service at four Southwest locations, spurring flight delays and cancellations. As a result, the company declared an operational emergency.

The unprecedented number of grounded aircraft followed a CBS investigation that highlighted the complaints of industry mechanics who said they were being pressured to overlook potential safety concerns for the sake of putting planes back into service quickly.

A company memo to mechanics, obtained by CBS, showed a call for all hands on deck and said maintenance employees could face "termination" for unexcused absences. It also gave the airline the ability to assign staff longer work hours and switch assignments.

Chief Operating Officer Mike Van de Ven said in a statement on Tuesday that the all-hands notice was meant to help get the aircraft safely back into the fleet. He said the problems had come about "despite no change in our maintenance programs, no changes in leadership, and no changes in our policies and procedures."

Van de Ven linked the chaos to the Aircraft Mechanics Fraternal Association, with which the carrier has been locked in negotiations over pay and benefits for years. He said the union "has a history of work disruptions," resulting in two pending lawsuits brought by Southwest.

The union's national director, Bret Oestreich, . "Southwest Airline's scapegoating of its expert Aircraft Maintenance Technicians does not bode well for the airline's safe operations," he said.

Oestreich contended that mechanics were working overtime but that the airline has the fewest mechanics to aircraft ratio of any major carrier.

He said the connection between the operational emergency and union negotiations amounted to an attempt to divert attention away from safety issues.

He added that "now we are threatened with the further coercive pressure of litigation."

In 2017, Southwest sued the union for boycotting overtime shifts. The lawsuit stated that a concerted refusal to accept overtime work could result in "foregone or delayed maintenance, expanded use of third-party vendors, and potential delay or cancellation of flights."

That suit was suspended amid an early contract agreement with the union, but the proposed contract was later rejected by the AMFA, Bloomberg reports.

Lynn Lunsford, a spokesperson for the Federal Aviation Administration, told NPR that oversight can increase as a "standard practice" while air carriers work through labor issues. "That is why the FAA has heightened its oversight of Southwest during this time."

The FAA began investigating Southwest in February 2018 for mistakes in calculating baggage weight and balance data. "The FAA will not close its investigation until it is satisfied that Southwest's corrective actions are consistent and sustained," Lunsford said.

An evidently disgruntled Southwest customer took to Instagram over the weekend, as delays and cancellations were mounting. "This is ridiculous service, and there will be no need for me to use this airline anymore," the person said. "At this point, horse and buggy would be faster."

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