Travelers across the United States are bracing for potential chaos ahead of mass flight cancellations ordered by authorities due to strains from the longest government shutdown in history.

On Friday, airlines will begin implementing the US Federal Aviation Administration's order to reduce domestic operations at 40 high-traffic airports for safety reasons.

In its official order published Thursday evening, the FAA said that the reductions, which will impact all commercial airlines, will start at 4% on Friday and ramp up to 10% by November 14.

The cuts were expected to affect hundreds of thousands of passengers with little notice.

The measures come just weeks ahead of the Thanksgiving holiday — the busiest time for air travel in the US. 

Which US airports will be affected under FAA directive?

Affected airports include some of the busiest hubs, for example, Atlanta, Miami and San Francisco.

Along with the Atlanta airport, Dallas/Fort Worth International, Denver International, Chicago O'Hare, and Los Angeles International are the five busiest airports in the US.

In big cities such as New York, Houston and Chicago, multiple airports will be affected.

How US airlines are handling flight cuts

Delta was axing about 170 US flights on Friday, with fewer expected on Saturday due to lower travel volume.

American Airlines reduced its schedules by 4% across 40 airports, amounting to around 220 flights canceled each day from Friday through Monday.

United announced it plans to cut 4% of its flights from Friday through Sunday, resulting in fewer than 200 daily cancellations.

Southwest Airlines will cancel about 120 flights on Friday.

In line with the federal directive, all major airlines were offering customers greater flexibility to change or cancel flights. 

Why is a reduction in US flights necessary?

The FAA is imposing the reductions because of safety concerns due to the government shutdown, which began on October 1.

"We're not going to wait for a safety problem to truly manifest itself, when the early indicators are telling us we can take action today to prevent things from deteriorating," said FAA Administrator Bryan Bedford.

During the shutdown, 13,000 air traffic controllers and 50,000 security screeners have not been paid and have been increasingly taking sick days.

Most controllers work mandatory overtime six days a week, leaving little time for side jobs to feed their families unless they call in sick.

Is it safe to fly in the US at the moment?

Aviation expert Michael Baiada told DW News that while the situation facing air traffic controllers is "very challenging," he would not "call it dangerous."

Baiada explained that aviation authorities "err on the conservative side and cut back the traffic" as a protective measure, adding that "I think the system is safe."

"This is no different than a winter snowstorm where flights get canceled and everything gets rerouted... It's just long, long delays where you may get your flight get canceled, and you may not be able to get on a flight for, say, three days," he said.

The aviation expert warned that if the shutdown continues, "it's going to become very, very chaotic."

"The people that pay the price are the passengers, and things will slow down even more than they are today," Baiada said.

Edited by: Srinivas Mazumdaru and Kieran Burke