By Holly Yan, Pete Muntean

(CNN) — Travel plans for millions of Americans could come to a screeching halt this month as more flights get canceled during the longest government shutdown in US history.

More than 1,000 Friday flights were slashed on the first day of the Trump administration’s mandatory cuts to reduce US air traffic, a majority due to the cuts mandated because of the shutdown. Officials said the cancellations are needed to maintain safety, as some air traffic controllers and federal security screeners – who aren’t getting paid during the shutdown – are not showing up to work.

But the reduction in flights has not eased the cascading delays for travelers across the country due to air traffic controller staffing shortages, as more than 4,500 flights were delayed Friday.

The cancellations and delays are mostly due to a combination of air traffic controller staffing issues and an Federal Aviation Administration-mandated 4% cut to flights because of the shutdown.

Friday’s cuts represent a 4% reduction in flights at 40 of the largest airports nationwide. If the shutdown drags on, airlines would have to gradually cancel more flights in the coming days – potentially leading to 10% of flights axed next Friday.

Here’s what to know as bewildered travelers get hit by the shutdown:

Which 40 airports are losing flights?

Airlines must cancel flights to 40 “high-volume traffic” airports, the FAA said. Each airline can decide which specific flights to cut, as long as they meet the mandatory number of cancellations.

Here are those airports, divided by region:

Northeast: Boston Logan International, Newark Liberty International, New York John F. Kennedy International, New York LaGuardia, Philadelphia International and Teterboro in New Jersey.

Midwest: Chicago Midway International, Chicago O’Hare International, Cincinnati/Northern Kentucky International, Detroit Metropolitan Wayne County, Indianapolis International, Louisville Muhammad Ali International and Minneapolis-St. Paul International.

South: Charlotte Douglas International, Dallas-Fort Worth International, Dallas Love Field, Fort Lauderdale-Hollywood International, George Bush Houston Intercontinental, Hartsfield-Jackson Atlanta International, Houston Hobby, Memphis International, Orlando International, Miami International and Tampa International.

DC area: Baltimore/Washington International, Ronald Reagan Washington National and Washington Dulles International.

West: Denver International, Harry Reid International Airport, Los Angeles International, Oakland International, Ontario International, Phoenix Sky Harbor International, Portland International, Salt Lake City International, San Diego International, San Francisco International and Seattle/Tacoma International.

Hawaii and Alaska: Daniel K. Inouye International and Ted Stevens Anchorage International.

Many smaller airports could also be impacted, as some flights from those 40 high-traffic airports fly to those locations.

How many flights will be cut, and when?

The FAA ordered airlines to cut traffic by 4% at 40 major airports, resulting in about 800 flights slashed, which could increase significantly in the coming days.

Overall, more than 1,000 flights were canceled Friday in the US, which includes cancellations that are not shutdown-related, according to FlightAware.

Airlines might have to cancel 6% of their flights to those airports on Tuesday, 8% Thursday and 10% next Friday if the stalemate continues, the FAA says.

If the impasse drags on – and not enough air traffic controllers go to work – the Department of Transportation might hike cancellations to 15%-20%, DOT Secretary Sean Duffy told Fox News on Friday.

“I don’t want to see that,” Duffy said. “If controllers start coming to work and the pressure goes down, we can move those numbers in the other direction.”

But travelers can still see flight cancellations even if the government were to reopen next week, as the FAA’s emergency order requires airlines to cancel flights days in advance.

And air traffic controllers probably wouldn’t return to work right away, Duffy said.

What airports are seeing flight delays?

The control towers at nine airports were short-staffed Friday, including airports in Austin, Texas; Burbank, California; Newark, New Jersey; New Orleans; Ontario, California; San Francisco; Tampa; and near Washington, DC.

The FAA reports 32 facilities were understaffed, even as airlines were forced to cut flights.

Flights headed to Ronald Reagan Washington National had to wait an average of four hours before taking off Friday evening, according to an advisory from the FAA.

San Francisco International Airport’s control tower has also been facing staffing shortages, forcing inbound travelers to wait on the ground about an hour for takeoff.

Flights headed to Phoenix were delayed 90 minutes and those headed to Chicago’s O’Hare International Airport were delayed 61 minutes. Manchester, New Hampshire-bound flights had delays of 45 minutes.

Flights headed to Houston’s William Hobby Airport are delayed an average of nearly three hours. The city’s other main airport, George Bush Intercontinental, is seeing incoming delays of 46 minutes, and departures are delayed 30 minutes. Dallas Love Field’s delays began growing to 30 minutes and Austin, Texas, has 47-minute average delays.

How many Friday flights did the biggest airlines cancel?

Delta Air Lines has canceled about 170 regional and mainline flights scheduled for Friday, a spokesperson told CNN.

United Airlines canceled about 200 flights Friday – mostly regional flights. It will cancel another 4% of its flights Saturday and Sunday, United said.

American Airlines’ 4% reduction equals about 220 flights canceled each day from Friday through Monday, spokesperson Sarah Jantz told CNN. “Even with these cancellations,” Jantz said, “we plan to operate around 6,000 daily flights.”

About 120 Southwest Airlines flights are canceled for Friday, the airline said.

What should I do if I plan to fly soon?

“If you are flying Friday or in the next ten days and need to be there or don’t want to be stranded I highly recommend booking a backup ticket on another carrier,” Frontier Airlines CEO Barry Biffle posted on Instagram.

“Don’t book a basic ticket. For example, book Economy on Frontier so you can reuse the ticket value as changes are free or you can get a credit,” Biffle wrote. “If your flight is cancelled your chances of being stranded are high so I would simply have a backup ticket on another airline.”

American, Delta, Southwest, United and Frontier are all offering waivers to allow passengers who do not want to fly to change their tickets without fees.

It’s also helpful to book directly with an airline instead of through a third-party website. If you book through another company and your flight gets canceled, you might have to deal with a third party to get your problem resolved.

And try to avoid booking flights with layovers, the US Public Interest Research Group says. The more planes you have to jump on to reach your destination, the more chances you have for a flight to be delayed or canceled.

What if I’m stuck at an airport and my flight gets canceled? Will anyone pay for a hotel?

Airlines must give passengers full refunds for flight cancellations, the DOT says. But they don’t have to pay for hotels, meals or other expenses if a delay or cancelation isn’t their fault.

Even though airlines can choose which flights to cancel as part of the mandatory cuts, the cancellations are not considered to be the airlines’ fault.

Will international flights be impacted?

Probably not. The flight reduction order “does not require a reduction in international flights,” a statement from the FAA and DOT said. However, “Carriers may use their own discretion to decide which flights are canceled to reach the order’s goal.”

In other words: Airlines get to pick which flights they will cut. But it’s unlikely they will cancel international flights, since they generally bring in more revenue than domestic flights and are subject to international agreements.

Why is all this happening?

The government shutdown, which started on October 1, has prevented many federal employees from getting paid.

Air traffic controllers and Transportation Security Administration screeners are considered essential employees and are expected to go to work during the shutdown. But they’re not getting paid, and some have needed to find other sources of income to pay the bills.

“Controllers are resigning every day now because of the prolonged nature of the shutdown,” said Nick Daniels, president of the National Air Traffic Controllers Association. “We’re also 400 controllers short — shorter than we were in the 2019 shutdown.”

More than 480 staffing shortages have been reported at FAA facilities since the shutdown began, which is more than four times what was recorded on the same dates last year, according to FAA operations plans.

If airports are short-staffed, is it still safe to fly?

The longer the shutdown continues, the more risks could emerge – especially as controllers go longer without pay, the union leader said earlier this week.

“Every single day that this goes on, tomorrow is now less safe than today,” Daniels told CNN before the flight reductions.

But the new flight cancelations will help maintain safety, the head of the National Transportation Safety Board said Thursday.

“THIS is safety management, the very foundation of our aviation system, and it’s the right thing to do,” NTSB Chair Jennifer Homendy posted on X.

“NTSB has repeatedly stated low air traffic control staffing levels, mandatory overtime, and six-day work weeks have a direct impact on #safety.”

CNN’s Alexandra Skores, Aaron Cooper, Forrest Brown, Rebekah Riess, Tori B. Powell and David Williams contributed to this report.

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