America’s airports are slowly grinding to a halt as thousands of flights sit grounded and exhausted air traffic controllers deal with skeleton crews — collateral damage from a federal shutdown now dragging into its second week.

More than 3,500 flights were delayed at airports across the country on Tuesday night, according to FlightAware, a flight tracking website. Among them were more than 200 flights out of Nashville International Airport and a nearly 600 at Chicago’s O’Hare International Airport.

Staffing issues also caused delays at Philadelphia International Airport and Las Vegas’s Harry Reid International Airport as well as air traffic controlled by centers at Atlanta, Boston, Houston, and Dallas, according to an advisory notice from the Federal Aviation Administration.

“There have been increased staffing shortages across the system,” an FAA representative said in a statement to Axios, adding that traffic has been slowed into select airports to “ensure safe operations.”

Classified as essential workers, air traffic controllers are required to report for duty during the shutdown — but do not receive a paycheck for their shifts, though they may be reimbursed later. Many are calling in sick rather than work for free, leaving critical gaps in a system that was already troubled by chronic staff shortages before the shutdown.

 An official with the National Air Traffic Controllers Association — representing 20,000 controllers nationwide — says the union is scrambling alongside the FAA to contain disruptions rippling through the National Airspace System.

“It is normal for a few air traffic controllers to call in sick on any given day, and this is the latest example of how fragile our aviation system is in the midst of a national shortage of these critical safety professionals,” the NATCA official told Axios.

On Monday evening, Burbank Airport’s air traffic control tower was left unmanned for nearly six hours. Operations were handled remotely by Southern California TRACON, an approach and departure team based at San Diego that already handles much of the air traffic in the region, according to a report from ABC7 Los Angeles.

“It’s a little scary, right?” Salice Rose, who was at Burbank airport dealing with multiple delays on her flight, said to the news outlet. “I’m about to run it myself.”