The impact of the federal government shutdown is beginning to be felt at the airport and in the sky.

The partial shutdown, now in its third week, is already the second-longest ever, and after another dead-end meeting between President Trump and congressional leaders Wednesday, no end is in sight. Meanwhile, associations and other groups are warning of the risks if immediate action isn’t taken to fund basic aviation services like security checkpoints and air traffic control facilities.

A major federal employee union warned Wednesday that Transportation Security Administration agents have started to quit their jobs rather than continue working without pay, posing a potential security threat.

Hydrick Thomas, president of the American Federation of Government Employees’ TSA Council, said he had heard from agents who had decided to resign rather than struggle with the financial hardship of a paycheck lapse.

“The loss of officers, while we’re already shorthanded, will create a massive security risk for American travelers since we don’t have enough trainees in the pipeline or the ability to process new hires,” Thomas said in a news release.

The union’s statement was echoed by associations that have warned repeatedly about the impact the shutdown could have on aviation infrastructure, both in the near term and the long term.

Captain Joe DePete, president of the Air Line Pilots Association, International, wrote in a letter to President Trump and congressional leaders that TSA agents, Federal Aviation Administration employees, and air traffic controllers are providing important services to travelers at great personal cost.

“They are dutifully providing safety of life services while facing increasingly difficult financial pressures to provide for those dependent on their paycheck,” DePete wrote last week [PDF]. “The pressure these civil servants are facing at home should not be ignored.”

The National Air Traffic Controllers Association noted that the shutdown could disrupt implementation of a new safety technology intended to modernize the way that pilots and air traffic controllers communicate. In a news release, NATCA Director of Safety and Technology Jim Ullmann said deployment of the Controller Pilot Data Link Communications system, also known as DataComm, could be significantly delayed.

“Because the timeline is so structured and the training so critical and time consuming, it places additional pressure on employees who are operating with only 83 percent (on average) of the staffing target at their facilities,” Ullmann said in a statement. “This shutdown will delay the implementation of DataComm in many centers by up to a year.”

In an appearance on CNN, NATCA Southwest Regional Vice President Andrew LeBovidge said nearly 20,000 federal workers are doing important air traffic control work despite an existing staff shortage and with the added stress of financial uncertainty.

“We are already in a critically staffed situation, and with the shutdown, the training pipeline has been cut short and there is no relief in sight,” he told CNN Newsroom on Monday.

In addition to safety risks, the travel-related economic costs of the shutdown are mounting, according to the U.S. Travel Association. The group estimated this week that the shutdown is costing the U.S. economy more than $100 million daily—nearly $50 million in domestic travel spending and more than $50 million in output related to travel.

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