Associated Press – Hundreds of flight cancellations spread across the U.S. on Friday as airlines began complying with the Federal Aviation Administration’s unprecedented order to reduce service nationwide because of the government shutdown.

While the FAA order left some passengers scrambling to figure out backup plans, most were relieved to find their planes still on schedule as airlines slowly phase in reductions at the nation’s busiest airports.

The 40 airports selected for the slowdown 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.

More than 800 flights were called off nationwide—four times the number canceled Thursday, according to FlightAware, a website that tracks flight disruptions. Not all were due to the FAA order.

Airports in Chicago, Atlanta, Denver, Dallas and Phoenix led the way with the most disruptions, FlightAware said.

While many travelers changed their itineraries without too much inconvenience, others were less fortunate.

Karen Soika from Greenwich, Connecticut, found out Friday morning her flight out of Newark, New Jersey, was rebooked for an hour earlier. But she later learned her plane was actually leaving from New York’s John F. Kennedy International Airport, instead, at least an hour away.

“I’m a surgeon, I’m used to chaos,” she said. She unsuccessfully tried to book a rental car to get to Utah for a weekend trip before settling on an option that seemed straight out of Hollywood.

“I’m going to U-Haul and I’m going to drive a truck cross country to get back to Utah,” said Soika, who is advising on medical scenes there for a spinoff of the TV series “Yellowstone.”

Airlines Scramble to Rebook Passengers

Both United and American airlines said Friday they were able to quickly rebook most affected travelers. United spokesperson Josh Freed said more than half were scheduled to reach their destinations within four hours of their original plan.

“We’ve had a lot of success rebooking people is the bottom line,” he said.

The airlines focused their cuts on smaller regional routes to airports where they have multiple flights a day, helping minimize the number of passengers impacted.

American, as an example, reduced the number of flights from Dallas to northwest Arkansas from 10 to 8 per day.

Delta Air Lines said it would scratch roughly 170 flights Friday while American planned to cut 220 each day through Monday.

Airlines also directed passengers with plans into the weekend to check apps to learn their flight status.

Some passengers quickly searched for alternatives. Hertz reported a sharp increase in one-way car rentals.

Many of the routes slashed on Friday were shuttle flights in the Northeast and Florida along with those between Dallas and smaller cities, according to aviation analytics firm Cirium.

The FAA said the reductions are starting at 4 percent of flights at the busiest airports and will ramp up to 10 percent a week from Friday. They will impact all commercial airlines.

Savanthi Syth, an airlines analyst for Raymond James, said right now is a slower season for travel, so that means the airlines should be better able to rebook passengers on other flights.

The biggest airlines expect there will be limited disruptions this weekend and stressed that international flights are not expected to be affected. But the upheaval will intensify in the coming days and could become chaotic if the slowdown extends into the Thanksgiving holiday, just weeks from now. Even after the slowdown ends, it will take days to return to normal.

Why Is This Happening?

The agency said the cuts 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.

“There could be a benefit if I can get the controllers to come back to work,” Transportation Secretary Sean Duffy told reporters at Ronald Reagan National Airport, just outside of Washington. “I don’t want to see the disruption. I don’t want to see the delays.”

The FAA’s order comes as the Trump administration ramps up pressure on Democrats in Congress to end the shutdown.

Ending the government shutdown would ease the situation for controllers, but the FAA said the flight cuts will remain in place until their safety data improves.

What Can Airlines Do?

Carriers are required to refund customers whose flights are canceled but not to cover costs such as food and hotels unless a delay or cancellation results from a factor 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 several airports on the list are major distribution centers for delivery companies, including in Louisville, Kentucky, the site of this week’s deadly UPS cargo plane crash.