Southwest Airlines Co.’s operational emergency stretched into a fifth day Tuesday, with double the normal number of planes taken out of service daily because of mechanical issues.

The carrier notified workers Feb. 15 of the situation, saying maintenance employees risked being fired if they decline to take overtime assignments or fail to show up for work as scheduled, unless they have a doctor’s note. The Dallas-based carrier has been in contract talks with the union representing mechanics for more than six years.

Southwest has canceled 519 flights since the beginning of Feb. 15, according to FlightAware.com, although the total includes flights grounded by weather and a breakdown by cause wasn’t available.

The number of planes taken out of service has continued at more than twice the normal daily average of 20, the Dallas-based airline said Tuesday, and there is “no common theme” among the reported mechanical issues. The airline has about 750 Boeing Co. 737 aircraft and makes nearly 4,000 flights each day.

The Aircraft Mechanics Fraternal Association, which represents 2,700 Southwest employees, said its members were working as usual. The mechanics union rejected a tentative contract agreement in September.

“Members are working normal schedules, and working normal overtime,” Bret Oestreich, the union’s national director, said late last week. “Overtime may have been increased by workloads.”

In 2017, Southwest accused the union of encouraging members to refuse overtime assignments in order to pressure the company in contract talks. A lawsuit filed by the airline was suspended in 2018 after an initial agreement was reached.

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Photo Credit: Southwest Airlines has more aircraft out of service than usual, so the airline called a "operational emergency." Pictured are Boeing 737s operated by the airline. Bloomberg