The US Federal Aviation Administration has ordered airlines to cut domestic flight operations by 4% at 40 high-traffic airports starting at 6am ET (11am Irish time), citing safety concerns about air traffic control during the government shutdown.
Reuters first reported details of the plan, citing a draft order.
US airlines worked to rejig schedules and field calls from customers after the Trump administration ordered flight reductions at major airports due to a shortage of air traffic controllers during the longest government shutdown in history.
The FAA said in the final order it is requiring 4% cuts at 40 high-traffic airports until Monday before requiring them to cut 10% of flights starting 14 November.
The FAA is also imposing restrictions on space launches but not imposing any cuts on international flights.
The cuts are expected to hit hundreds of thousands of travellers with little notice.
Aviation analytics firm Cirium estimated the reductions would cancel up to 1,800 flights and cut 268,000 airline seats a day in the US.
The low demand travel period made it easier for airlines to rebook passengers by cutting flight frequencies on some routes and using larger planes.
Analysts predict the impact on earnings will be modest, provided the shutdown ends before the peak Thanksgiving holiday travel period.
Transportation Secretary Sean Duffy yesterday ordered 10% flight cuts to start today at 40 major US airports - including in New York, Los Angeles and Chicago - affecting both commercial and cargo services.
The Federal Aviation Administration revised the plan and said late yesterday airlines must cut 4% of domestic flights.
The FAA also warned it could reject specific cuts if they disproportionately impacted certain communities and could cut up to 10% of general aviation flights at high-traffic airports if staffing issues arose.
Airlines face the challenge of adjusting flight schedules on short notice while minimising the impact on passengers and crew members.
In a memo to staff, American Airlines Chief Operating Officer David Seymour said: "You deserve the same level of certainty as our customers."
He noted the carrier was trying its best to avoid disrupting the work schedules of its pilots and flight attendants.
Major airlines proactively offered customers greater flexibility regarding their travel plans, a move reinforced by the Transportation Department confirming late yesterday that passengers are entitled to a full refund.
But it said airlines are not required to provide reimbursement for hotels or meals for cancellations since they are not at fault.
In line with the federal directive, American Airlines will reduce its schedules by 4% across 40 airports, amounting to around 220 flights cancelled each day from today until Monday.
It said the vast majority of these cancellations are for regional flights, allowing the airline to maintain approximately 6,000 daily flights and minimise customer impact.
Delta Air Lines announced the cancellation of about 170 US flights today, with fewer expected tomorrow due to lower travel volume.
The carrier normally operates 5,000 daily flights globally.
United Airlines said it plans to cut 4% of its flights today through to tomorrow, resulting in less than 200 daily cancellations.
The Chicago-based airline operates around 4,500 flights a day.
Southwest Airlines will cancel about 120 flights today.
Alaska Airlines has begun cancelling a limited number of flights from today.
The carrier said that most cancellations would affect high-frequency routes, allowing the majority of customers to be re-accommodated with minimal disruption.
Discount carrier Frontier said most of its flights would operate as planned, though in a LinkedIn post its CEO Barry Biffle advised customers traveling to funerals or other critical events over the next ten days to book backup tickets on different airlines.
Airlines can lessen the disruption by using bigger aircraft, a strategy they already employ to deal with congestion in New York-area airports.
Still, passengers inundated carriers on social media platforms like X with questions and comments as they tried to clarify their travel plans.
While sales of its "disruption assistance" service have risen steadily since the shutdown began on 1 October, travel app Hopper reported an overnight jump of nearly 60% after the government announced flight cuts.
The FAA cited a safety assessment of air traffic controllers in ordering the unprecedented cuts, but officials insisted it is safe.
"It's safe to fly today and it will continue to be safe to fly next week," Mr Duffy said.
FAA Administrator Bryan Bedford reiterated that the agency would not hesitate to take further action to make sure air travel remains safe.
During the closure, 13,000 air traffic controllers and 50,000 security screeners have been forced to work without pay.
Absenteeism has risen to 30% or more at some airports as the workers turn to second jobs to feed their families or are unable to afford childcare.
Even before the shutdown, the FAA was already short about 3,500 air traffic controllers and many were working mandatory overtime and six-day weeks.
Airlines have estimated that at least 3.2 million travellers have been delayed during the shutdown.
Following a dip in corporate and leisure bookings in the first half of the year, US airlines had projected a period of steady demand this quarter.
Yet, even as reduced capacity is expected to drive up airfares, the uncertainty caused by the shutdown may depress consumer travel spending.
Passenger traffic declined in the first week of November from a year ago after rising in October, data from the US Transportation Security Administration showed.
Airlines for America CEO Chris Sununu urged customers to "stick with their current travel plans" amid a drop in bookings.
But even before the new flight cuts went into effect, customers said flight delays and cancellations were causing misery.
Delaware-based Grace Logeman drove two hours yesterday to Newark in New Jersey for a Frontier flight to Atlanta that was delayed by three hours.
The delay caused her to miss a connecting flight to the Dominican Republic for her sister's birthday.
"I'm devastated," Ms Logeman said while on hold with the airline's customer service line.
"As far as the ongoing shutdown ... it's hurting me. I'm the one sitting here now," she added.
