The nation's airports remain open despite the partial government shutdown, and air traffic controllers are working even though they aren't receiving a paycheck Friday, the first missed payday for federal workers since the start of the closure 21 days ago.

At San Francisco International Airport, controller Frederick Naujok told KQED the staff is beginning to feel the strain in a region where housing is expensive and many workers live paycheck-to-paycheck.

"I've even had a couple of people come to me and say that they want to work for Lyft or Uber, in addition to coming to work and working air traffic," Naujok said.

While Lyft can't confirm whether any air traffic controllers have joined their driving fleet and Uber declined to comment entirely, Lyft released a statement welcoming all furloughed employees in need of extra income to drive for the ride share company.

"Driving is a reliable way to make ends meet and can help these workers fill in the gap during this uncertain time," the statement reads.

ALSO: Aviation system hits crunchtime as workers face payless 'payday'

Air traffic controllers and technicians employed by the Federal Aviation Administration are expected to work without pay during the shutdown because their jobs are considered essential.

While news reports have indicated airport security workers are calling in sick, the FAA assures passengers they haven't seen an unusual number of people skipping work. The agency issued a statement on Thursday stating safety remains a priority despite the shutdown: "We are allocating resources based on risk assessment to meet all safety critical functions. If we identify an issue, we recall inspectors and engineers to address it. We sincerely thank FAA employees who are working to keep the traveling public and our skies safe."

MORE: Air traffic controllers get their first shutdown pay stub: $0.00

The union representing air traffic controllers is suing the Trump administration over the pay freeze, and they sent a letter to the White House and congressional leaders Thursday, warning, "No one should be under the illusion that it's business as usual for aviation safety during a shutdown. Every day the shutdown continues, the negative consequences to the National Airspace System (NAS) and its employees are compounding."

The National Air Traffic Controllers Association represents more than 19,000 FAA employees, including 14,000 air traffic controllers.

SFGATE reached out to the National Air Traffic Controllers Association for additional comment and will update the story if we hear back.