What to know about canceled flights across the U.S.:

Airlines were expected to cancel 4% of their flights at 40 of the busiest airports in the U.S. on Monday to comply with an order from the Federal Aviation Administration amid the government shutdown .

Officials said the FAA order was an effort to address strain on the national airspace system as the agency dealt with a shortage of air traffic controllers.

More than 2,100 flights were canceled Monday.

The FAA wants airlines to increase the cancellations at the 40 airports to 6% by Tuesday and ultimately ramp up to 10% by Friday.

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