Hundreds of flights were cancelled across the United States on Friday after the Trump administration ordered reductions to ease the strain on air traffic controllers who are working without pay amid congressional paralysis on funding the US budget.
With Republicans and Democrats in a bitter standoff over spending priorities, including over healthcare, Congress has ground to a standstill, leaving the entire federal funding spigot closed.
Vast numbers of government employees, including vital airport staff, are working either without pay or are at home furloughed, waiting for the now nearly six-week crisis to end.
The impact remained unclear early on Friday, with the flight reductions due to take effect gradually, starting at four percent and rising to 10 percent next week if Congress still hasn't reached a funding deal.
More than 800 flights scheduled for Friday were cancelled, according to tracking website FlightAware, which would be more than the previous three days combined.
Aviation analytics company Cirium said three percent of US flights had been cancelled so far.
The upheaval means ordinary Americans are now directly aware of the impacts from the fight in Washington, where the funding shutdown began on October 1, increasing pressure on both parties.
"This isn't about politics, it's about assessing the data and alleviating building risk in the system," said US Transportation Secretary Sean Duffy, pushing back against criticism that the order aims to increase pressure on Democrats to end the shutdown.
The most affected airports so far were Chicago O'Hare, Hartsfield-Jackson in Atlanta, Denver and Dallas-Fort Worth, according to data analysed by AFP.
American Airlines said in a statement that its scheduled reduction amounted to 220 flight cancellations each day.
Delta Airlines said it was axing about 170 flights scheduled for Friday, while broadcaster CNN reported Southwest Airlines cut around 100 flights set for that day.
More than 6,800 US flights were delayed on Thursday with some 200 cancellations, FlightAware data showed, with passengers facing long lines at security checkpoints.
Travellers at Boston and Newark airports also faced average delays of more than two hours, and those at Chicago's O'Hare and Washington's Reagan National more than an hour.
Authorities said they wanted to act before an accident occurred.
"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.
The reduction measures come as the country enters its busiest travel time of the year, with the Thanksgiving holiday just weeks away.
Millions of Americans are likely to face travel chaos amid a shortage of air traffic control personnel, although President Donald Trump's administration sought to reassure people that flying remains safe.
"It's safe to fly today, tomorrow, and the day after because of the proactive actions we are taking," Duffy said on social media late Thursday.
But many in high-stress aviation-related jobs are now calling in sick and potentially working second jobs to pay their bills, Duffy said Wednesday.
FAA Administrator Bedford said the situation was unprecedented.
"I am not aware in my 35-year history in the aviation market where we've had a situation where we're taking these kinds of measures," he said Wednesday.
Bedford added: "Then again, we're in new territory in terms of government shutdowns."
