The nation's airports remain fully operational despite the government shutdown, but with federal aviation workers expected to miss their first paycheck Friday, union leaders said the system may begin to feel the strain.

Leaders say more workers - who continue to work because their jobs are considered essential - may decide to stay home rather than continue to work without pay.

The union that represents air traffic controllers fired off a letter to the White House and congressional leaders warning that "the human and economic consequences are increasing and doing greater harm."

"Training of air traffic controllers has been suspended, slowing the arrival of new workers in a system that is already at a 30-year low," said the letter from the National Air Traffic Controllers Association (NATCA) that was signed by more than 30 aviation-related groups.

More than 1,800 of the 10,483 controllers are eligible for retirement, and close to 15,000 retired in the previous five years, the legacy of 1981, when President Ronald Reagan fired all of the striking controllers.

Hydrick Thomas, president of the union that represents Transportation Security Administration (TSA) workers who have remained on the job at airport checkpoints, said Congress and the White House should "stop using federal workers . . . as pawns in their political games."

"Every day I'm getting calls from my members about their extreme financial hardships and need for a paycheck," Thomas said in a statement. "Some of them have already quit and many are considering quitting the federal workforce because of this shutdown."

At an afternoon rally, NATCA president Paul Rinaldi and Mike Perrone, president of Professional Aviation Safety Specialists, were among those who addressed a crowd of federal workers, many of them furloughed for the duration of the shutdown.

"We do it because it's the safety of the flying public," Perrone said. "The people that fly are doing what they need to do. Nobody is overseeing that. [But] We do not have inspectors looking and making sure that this system continues to be safe. We're not saying that it's unsafe at the moment, but every day the government is shut down, safety is going to be compromised."

Most federal workers are paid every two weeks. As a result, the checkpoint officers to whom Thomas refers to have not yet missed a payday. Like federal air traffic controllers, who also are working without the promise of a pay check, the TSA workers will miss their first paycheck Friday.