The National Air Traffic Controllers Association (NATCA) said Wednesday that the government shutdown will weaken the country’s airspace system.

“When the federal government shuts down it introduces unnecessary distractions and our entire aviation system is weakened,” NATCA President Nick Daniels said in a release. “Congress must restore federal funding so that the safety and efficiency of our National Airspace System is not compromised.”

NATCA represents nearly 20,0000 air traffic controllers, engineers and other aviation safety employees.

In its shutdown contingency plan released Tuesday, the Department of Transportation (DOT) said that more than 11,000 Federal Aviation Administration employees, roughly a quarter of the agency’s workforce, will be furloughed due to the shutdown.

The DOT noted that more than 13,000 air traffic controllers will keep working without pay, and hiring and training of controllers will continue.

However, a group representing U.S. airlines told Congress on Monday that flight delays could occur due to air traffic controllers and Transportation Security Administration employees having to work without pay.

Daniels also expressed concern over working conditions for controllers, some of whom are already working six days per week and 10 hours per day, according to NATCA.

“The increased stress and fatigue that comes from working long hours without pay cannot be overstated,” he added. “Despite the shutdown, there is one thing that will not change — air traffic controllers are incredibly proud of the public service they provide, and they will answer the bell without fail.”