Airline pilots are very concerned by United States flight delays and cancellations amid the government shutdown and believe the effect on the industry is “only going to get worse” in the coming days, according to an airline union official.

Capt. Dennis Tajer, spokesman for American Airlines’ Allied Pilots Association, told the Washington Examiner that the thousands of pilots who lived through the 2019 government shutdown are living through “PTSD” right now, knowing full well that things will get worse in air travel before they get better, unless the Senate passes funding.

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“One thing that we learned in the last shutdown was that the longer it goes, the worse it gets. Delays will increase, and the impact, not only the human impact on those who are working without paychecks, but on the air traffic control system, it will take even longer to recover, and you’ll see a domino effect,” Tajer said in a phone call Thursday.

“Everyone has some PTSD, knowing it’s only going to get worse, unless they get the government open again,” Tajer said. “We’re respectfully sending out a warning. We’ve been here before. We all know what can happen. Let’s not go there again, and let’s get it fixed.”

Transportation Secretary Sean Duffy warned on Monday that the Federal Aviation Administration’s air traffic controllers were taking slightly more sick leave than normal as the 13,000 employees were forced to work without pay since Oct. 1.

As air traffic controllers skip work, the FAA is forced to impose ground delays to ensure it has enough eyes in the tower to track planes. The ground delays then affect pilots, flight attendants, and staff. This week, more than 13,000 U.S. flights have been delayed thus far.

Pilots are scheduled by day and can only accrue so many hours on duty before they are maxed out and unable to work, including hours spent delayed in airports.

“It’s a linear inconvenience, but it becomes exponential as you go forward. So at the 15-day point, the 20-day point, it’s anybody’s guess, but it gets exponentially worse the longer it goes,” said Tajer, whose union represents 15,000 American Airlines pilots.

Delta CEO Ed Bastian told CNBC that the situation could quickly worsen by next week.

A more than monthlong government shutdown from late 2018 to early 2019 ended hours after an increase in sick calls from air traffic controllers snarled travel in the New York area.

“I would say that if this doesn’t get resolved, say beyond another 10 days or so, you probably will start to see some impacts,” Bastian told CNBC’s Squawk Box on Thursday.

Delays and cancellations should not be indicative of danger in the skies, Tajer added.

“Seeing airports slowed down for unique staffing issues is actually a clear sign that the metering, the reduction of air traffic, is happening to ensure that the safety margin is maintained,” Tajer said. “They are not going to let it get to a dangerous place. They’re just going to slow it down.”

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Airline passengers will not be the only ones frustrated by the delays and cancellations if the shutdown continues.

“For every pilot that’s missing a day off at home, you’ve got passengers who paid for tickets that are missing important events,” Tajer said. “If you’re delayed, canceled on a flight because of the shutdown, everybody’s on the phone giving the bad news to family. Chances are, pilots are on that same call with family.”