Southwest Airlines Co. (LUV.N) declared an “operational emergency” Friday because of an unusually high number of aircraft taken out of service for maintenance, and ordered all scheduled mechanics to show up for work or risk being fired.

Workers “alleging illness” will be required to provide a doctor’s note on their first day back at work, the airline said in a memo. Some workers might be called in on overtime, the carrier said, and those refusing to report for duty could face firing.

“This is not the type of communication I (or any leader) want to issue, but it is necessary to get our aircraft back in service in order to serve our customers,” according to the memo from Lonnie Warren, senior director of technical operations.

About 40 planes have been taken out of service over each of the past several days, about double the normal rate, said Brandy King, a spokeswoman for the airline. The carrier had 750 Boeing Co. 737 aircraft in its fleet at the end of 2018.

“It does not appear to be a a single maintenance issue creating the uptick,” she said. “We’ve been able to manage our schedule with minimal operational impact.”

Contract Talks

The Dallas-based carrier has been in contract talks with the union representing mechanics for more than six years. Members of the Aircraft Mechanics Fraternal Association, representing 2,700 Southwest workers, rejected a tentative contract agreement in September.

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.

The AMFA’s national office didn’t immediately return a call seeking comment.

The maintenance issues were reported earlier by the Chicago Business Journal.