Southwest cancelled approximately 100 flights Tuesday as it expanded the operational state of emergency it invoked late last week.

The carrier said that more than twice its typical number of 20 aircraft were out of service on Tuesday out of a fleet of approximately 750 Boeing 737s.

"We are requiring all hands on deck to address maintenance items so that we may promptly return aircraft to service and take care of our customers," the airline said in a statement. Southwest operates approximately 4,000 flights daily. 

The maintenance issues have been caused by an unusually high number of call-ins by Southwest maintenance workers, the Chicago Business Journal reported over the weekend. In an internal memo obtained by the CBJ, senior director of technical operations Lonnie Warren warned maintenance crew that they could be fired if they called out without a valid reason. Workers claiming to be ill would be required to provide a doctor's note upon their return to work. 

Southwest has been mired in negotiations with the union representing its maintenance workers for six years. 

On Tuesday, the carrier added its Dallas Love Field maintenance base to the operational state of emergency it had already put in effect in Phoenix, Las Vegas, Houston Hobby and Orlando. 

"We're asking that customers check for the latest on their specific flights as our operational planners are actively working in the background to minimize impacts to their travel plans," the carrier said.

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