KEY POINTS
  • Southwest declared an operational 'emergency' after the number of planes taken out of service doubled
  • The airline told its mechanics it needs 'all hands on deck.'
  • The mechanics' union, with which Southwest is negotiating a contract, said members were threatened with termination if they didn't comply.
Southwest Airlines Co. planes stand on the tarmac at San Francisco International Airport (SFO) in San Francisco, California, U.S., on Friday, Jan. 19, 2018. Southwest Airlines Co. is scheduled to release earnings on January 25. Photographer: David Paul Morris/Bloomberg via Getty Images

Southwest Airlines told its mechanics on Friday that it is experiencing an "operational emergency" due to an unusually high number of grounded jets -- and demanded they show up for work or risk termination, according to a company memo issued on Friday and seen by CNBC.

On Friday, 100 Southwest flights were cancelled, more than any other U.S. airline, according to flight-tracking site FlightAware, and more than 1,000 were delayed.The airline usually plans for having as many as 20 aircraft removed from service for unexpected maintenance issues every day.