Traffic management specialists monitor airline traffic at the Air Traffic Control System Command Center on Nov. 16, 2018, in Warrenton, Va. The facility balances air traffic demand with system capacity in the National Airspace System and is a part of the Federal Aviation Administration air traffic control system. (Pete Marovich/For The Washington Post)

The U.S. aviation system safely delivered 46 million passengers over the holiday season, but union leaders say cracks are showing as 3,000 support workers have been furloughed and about 10,500 air traffic controllers continue to work without pay due to the government shutdown.

As the partial federal shutdown that began Dec. 22 continued Thursday, Paul M. Rinaldi, president of the National Air Traffic Controllers Association, warned Congress that his controllers work in “a system that has no room for error.”

“Right now, we are asking our controllers to maintain the safety and efficiency of the system without the necessary contribution of 3,000 safety professionals,” Rinaldi said in a letter to House and Senate leaders. “We wouldn’t ask a surgeon to perform an operation without the assistance of a support team, and we shouldn’t be asking air traffic controllers to continue working without support staff.”

Federal Aviation Administration workers on Thursday articulated Rinaldi’s fears.

They said about 6,300 projects, many of them safety-related, have been stalled by the shutdown. Workers who supply technical expertise in support of day-to-day controller operations also have been furloughed.

The shutdown also means the FAA and the National Transportation Safety Board have had to prioritize their crash investigations. Small-plane crashes in Florida and South Dakota and two crashes in Michigan that killed a total of five people have not received NTSB scrutiny. Nor have vehicle crashes that killed seven people in New Jersey and Oxon Hill, Md. And three fatal rail accidents in New York City and Kansas that killed three people have yet to come under NTSB investigation.

But the most notable of the union’s fears — and the one that ultimately could slow air traffic — is the supply of certified air traffic controllers.

A pivotal moment in aviation history came in 1981, when President Ronald Reagan fired 11,345 striking air traffic controllers. The modern-day consequence of that moment is that thousands of young controllers were hired after that date and have now reached the mandatory retirement age of 56. More than 21,000 of them — twice the current controller workforce — had reached that age by 2017, making the turnover rate probably higher than at any other federal agency. And 1,842 were eligible for retirement as of Dec. 31.

“We shut down the [air traffic control] academy, we lose the pipeline of new controllers coming into the system,” Dean Iacopelli, Rinaldi’s chief of staff, said in an interview. “The whole staffing pipeline is built on hiring people, getting them into the system, getting them trained in the facilities.”

Iacopelli said, “a shutdown of one month will take about five to six months to get back up and running.”

And he said it influences people who have reached retirement age.

“It makes those closer to retirement throw their hands up in the air and say, ‘I’m not going to work for free. I’d rather just put my retirement paperwork in,’ ” Iacopelli said.

Despite their frustration, he said, those workers are in “limbo” because the people who would receive that retirement paperwork have been furloughed.

The nation’s aviation system appeared to behave normally during the busy holiday season as FAA controllers and workers at the security checkpoints staffed by the Transportation Security Administration stayed on the job, but the shutdown has cut off their paychecks.

“The controllers who are on the job now, much as other similarly situated employees, they’re working without knowing when they’re going to get paid,” Iacopelli said.

Controllers are paid every two weeks, and their next paycheck is due on Jan. 14. If the government doesn’t reopen by Jan. 11, there won’t be time to process their pay.

“That’s weighing very heavily on people who should not have to deal with that,” Iacopelli said. He said he’s unsure when or how the confrontation over a border wall with Mexico that pits House Democrats against the White House will end.

“We have our folks out in the Capitol trying to find out, but in this case it’s very eerie,” he said. “We have no idea. We’re watching this unfold as anyone else is.”