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U.S. Department of Transportation Secretary Sean Duffy on Thursday raised concerns about air traffic controllers taking on second jobs to pay their bills during the government shutdown.

"If you have a controller that's working six days a week, but has to think about, 'How am I going to pay the mortgage, how am I going to make the car payment, how am I going to put food on my kids' table,' they have to make choices," Duffy said at the House GOP's press conference at the Capitol. "And the choices they are making is to take a second job."

Duffy's statement came after U.S. House Speaker Mike Johnson highlighted that air traffic controllers -- essential government employees who are required to work without pay during the lapse in funding -- are working long shifts in towers and centers, then "hopping in a car and driving for Uber or delivering for DoorDash all night."

The transportation secretary discouraged air traffic controllers from seeking additional employment on top of an already demanding, high-stress job.

"Well, I don't want my air traffic controllers to take a second job -- I want them to do one job. I don't want them delivering for DoorDash, I don't want them driving Uber. I want them coming to their facilities, and controlling the air space," Duffy said, while acknowledging the financial pressures they are under to "put food on their table, feed their kids and support their family members."

Duffy also said he "can't guarantee" that passengers' flights will take off on time or that they won't be canceled as the second-longest government shutdown in U.S. history drags on.

Johnson said that 19,000 flights were delayed across the U.S. from Saturday to Monday, citing data from flight tracking website FlightAware. Another 1,600 flights were canceled over the same period.

Duffy explained that sporadic air traffic controller staffing shortages could lead to flight delays and cancellations, with the department deliberately slowing down flight traffic to ensure that passengers remain safe.

"It's not moving as many flights as possible, it's moving as many flights as possible safely," Duffy said. "And if we can't do that, you'll see the flights come down. You'll see the delays, you'll see the cancellations."

U.S. transportation chief doesn't want air traffic controllers driving for Uber during shutdown