The FAA is imposing the reductions to take pressure off air traffic controllers, who are federal employees and have been working without pay during the shutdown.

In addition to reduced flight traffic, the shutdown has led to more flight delays, with experts advising travelers to check flight statuses before heading to the airport, among other tips.

Here’s a closer look at what to know about the reductions.

Which airports are affected?

The 40 airports selected by the FAA for reductions span more than two dozen states and include hubs such as Atlanta, Dallas, Denver, Los Angeles, Miami and Newark, according to an order published by the agency Thursday evening. A full list of affected airports can be found here.

Tips for passengers whose flights are delayed or canceled during the shutdown can be found here.

More than 800 flights were canceled for Friday, according to FlightAware, a website that tracks flight disruptions. Many of the cancellations for Friday were Delta Air Lines, United Airlines and American Airlines flights.

Why is this necessary?

Air traffic controllers have gone without paychecks during the shutdown of the federal government. That has led to controllers calling in sick and contributed to staffing shortages that have been affecting air travel for weeks.

Most controllers work mandatory overtime six days a week during the shutdown without pay, the National Air Traffic Controllers Association has said.

That leaves little time for a side job unless controllers call in sick to the FAA.

When will the reductions begin and end?

The reductions began Friday and airlines have said people could see weekend travel plans disrupted with little notice.

Airlines will phase in reductions at the direction of the FAA, starting by eliminating 4% of flights at the 40 targeted airports Friday and building to 10%, according to the agency’s order.

United Airlines will cut 4% of its flights this weekend based on guidance from the FAA, said company spokesperson Josh Freed.

It is unclear when the reduced flights could end. Airlines, unions and the travel industry have urged Congress to end the shutdown, which on Wednesday became the longest on record.

What will the effect of the reduction be?

The cuts could include up to 1,800 flights and about 268,000 seats combined, according to one estimate.

United Airlines and Delta Air Lines both said they would offer refunds to travelers who opt not to fly, even if they have tickets that aren’t normally refundable.

The cuts also could disrupt package deliveries because two airports with major distribution centers are on the list. FedEx operates at the Memphis, Tennessee, airport and UPS in Louisville, Kentucky, where there was a deadly cargo plane crash this week.

Transportation Secretary Sean Duffy warned this week there could be chaos in the skies if the shutdown drags on long enough for controllers to miss their second full paycheck next week.

Has this happened before?

FAA Administrator Bryan Bedford said this week that he has never seen these kinds of measures taken before in his nearly four-decade experience in the aviation field.

“We’re in new territory in terms of government shutdowns,” he said.

Staffing problems led to delays throughout October, but they were mostly isolated and temporary. Last weekend, though, saw a change.

From Friday to Sunday evening, at least 39 air traffic control facilities reported potential staffing limits, according to an Associated Press analysis of operations plans shared through the Air Traffic Control System Command Center system. The figure, which is likely an undercount, is well above the average for weekends before the shutdown.

During weekends from Jan. 1 to Sept. 30, the average number of airport towers, regional control centers and facilities monitoring traffic at higher altitudes that announced potential staffing issues was 8.3, according to the AP analysis.

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Associated Press writers Mike Catalini in Trenton, New Jersey, and Hannah Schoenbaum in Salt Lake City contributed reporting.