The Federal Aviation Administration ’s unprecedented order to scale back flights nationwide because of the record-long government shutdown is set to take effect Friday morning.

The 40 airports selected by the FAA span more than two dozen states and include hubs such as Atlanta, Dallas, Denver, Los Angeles and Charlotte, North Carolina, according to the order.

In some metropolitan areas, including New York, Houston, Chicago and Washington, multiple airports will be impacted, while the ripple effects could reach smaller airports as well.

Airlines scrambled to adjust their schedules and began canceling flights Thursday in anticipation of the FAA’s official order, while travelers with plans for the weekend and beyond waited nervously to learn if their flights would take off as scheduled.

More than 780 flights have been called off nationwide, according to FlightAware. Delta Air Lines said it would scratch roughly 170 flights Friday, and American Airlines planned to cut 220 a day through Monday.

The FAA said the reductions would start at 4% and ramp up to 10% by Nov. 14. They are to be in effect between 6 a.m. and 10 p.m. and impact all commercial airlines.

The agency said the cutbacks are necessary to relieve pressure on air traffic controllers who have been working without pay for more than a month. Many are pulling six-day work weeks with mandatory overtime, and increasing numbers of them have begun calling out as the financial strain and exhaustion mount.

“You can’t expect people to go in to work when they’re not getting a paycheck,” said Kelly Matthews of Flat Rock, Michigan, a frequent business traveler who has canceled most of her upcoming trips. “I mean it’s not a matter of them not wanting to do the job — but you can’t afford to pay for gas, your day care and everything else.”

The order comes as the Trump administration is ramping up pressure on Democrats in Congress to end the shutdown.

Airlines said they would try to minimize impact on customers. Some planned to focus on slashing routes to and from small and medium-size cities.

Carriers are required to refund customers whose flights are canceled but not to cover secondary costs such as food and hotel accommodations unless a delay or cancellation results from a contributing factor that is within the control of the airlines, according to the Department of Transportation.

Industry analyst Henry Harteveldt warned that the reductions will “have a noticeable impact across the U.S. air transportation system.”

The cuts could also slow package service as two airports on the list are major distribution centers for delivery companies: FedEx in Memphis, Tennessee, and UPS in Louisville, Kentucky, the site of this week’s deadly cargo plane crash.

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Associated Press journalists Hallie Golden in Seattle, Safiyah Riddle in Montgomery, Alabama, and Wyatte Grantham-Philips in New York contributed.