Dallas • The partial government shutdown is starting to affect air travel.

Over the weekend, some airports had long lines at checkpoints, apparently caused by a rising number of security officers calling in sick as they face the prospect of missing their first paycheck this week.

Safety inspectors aren't even on the job. A Federal Aviation Administration spokesman said Monday that inspectors are being called back to work on a case-by-case basis, with a priority put on inspecting airline fleets.

So far, the most visible signs of the shutdown — in its 18th day Tuesday — include the closure of some government buildings and national parks and trash overflowing bins on the National Mall in front of the Capitol.

By increasingly affecting air travel, however, the pain will be felt more widely.

Here are some common questions about the shutdown's impact on airports and travel, along with the answers:

Who is supposed to keep working?

About 10,000 air traffic controllers who work for the Federal Aviation Administration, about 51,000 Transportation Security Administration officers, and an undisclosed number of federal air marshals have been told to keep reporting to work because they are deemed essential. Government employees have always been paid after past shutdowns ended, and that is the widespread expectation this time too.

Are they showing up?

TSA acknowledges that more screeners are calling in sick at some airports, including Dallas-Fort Worth International. It gave few numbers but issued a statement Friday saying that more have been missing work since the Christmas and New Year's holidays. The TSA said the effect was "minimal."

Then over the weekend, travelers reported longer checkpoint lines at some airports, including LaGuardia in New York.

On Monday, TSA tweeted that agents screened 2.22 million passengers nationwide on Sunday, which it called a "historically busy day due to holiday travel." TSA said only about 220,000 travelers waited at least 15 minutes at checkpoints, while 0.2 percent — fewer than 5,000 — waited at least 30 minutes.

How will TSA respond to no-shows?

Airport screeners start around $24,000 and most earn between $26,000 and $35,000 a year, according to TSA. That is far less than many other government employees, making them more vulnerable if they don't get paid.

TSA spokesman Jim Gregory said officials are managing. "If we don't have appropriations by midweek or so, (officers) will miss their first paycheck. That's obviously where it becomes more difficult," he said.

Gregory said the agency has a team of officers who can go to airports facing a shortage, a tactic developed in case natural disasters prevented screeners from getting to work.

What about traffic controllers?

About 1,900 air traffic controllers — nearly one in every five — are eligible to retire right now.

"I don't know how long they're going to stay on the job if they are not getting a paycheck," said Paul Rinaldi, president of the National Air Traffic Controllers Association.

There is an even larger group of recently hired trainees and apprentices, and Rinaldi said a long shutdown could cause some of them to take other jobs.

Will the shutdown lead to flight delays?

Rinaldi said safety is not being compromised but that capacity to manage traffic could be reduced if the shutdown worsens an existing shortage of controllers. That could lead to flight delays, he said. Others are not so sure.

"It would have to get pretty bad before the government said (to airlines), 'Hey, start scaling back your plans for service,'" said Richard Aboulafia, an aviation analyst. "You could see that in a worst-case scenario."

An early test of the air traffic system could come around the Feb. 3 Super Bowl in Atlanta, when an influx of corporate jets and private planes will further crowd the sky above the nation's busiest airport. Planning for handling that traffic has been put on hold, Rinaldi said.

What about safety?

The largest pilots' union wrote to President Donald Trump last week urging a quick end to the shutdown, which it said was threatening the safety of the nation's airspace.

On Tuesday, the new Democratic chairman of the House Homeland Security Committee, Bennie Thompson of Mississippi, said the Trump administration hasn't answered questions about how the shutdown is affecting the Department of Homeland Security's ability to screen passengers heading to the U.S. and to assess security at foreign airports.

Patrick O'Carroll, executive director of the Federal Law Enforcement Officers Association, said there has been no deterioration of safety, no reductions in coverage, and no reports of air marshals — armed, undercover officers on flights — calling in sick.

Who is inspecting planes?

Federal aviation safety inspectors are not deemed essential government employees; they have been furloughed.

FAA spokesman Gregory Martin said the agency is recalling inspectors for certain jobs including assignments at the airlines, as in previous shutdowns.

"We're going to continue to prioritize with the resources that we have," Martin said. "Our focus is on the commercial air carriers and volumes of people they carry."

Martin did not say how many inspectors are working or how the number of inspections being done compared with pre-shutdown levels.

Chuck Banks, one of those furloughed inspectors, said colleagues are being called in when an airline needs something, like a plane certified for flight. The routine, normal oversight of operations at airlines and repair shops is not being done, leaving companies to regulate themselves, he said.

"Do you like the fox watching the hen house?" he said. "Every day the government stays shut down, it gets less safe to fly."

What other government services are affected?

The National Transportation Safety Board is delaying accident investigations and hearings. While there have not been any fatal airline crashes, the board has delayed other investigations, including an examination of a Florida highway accident that killed five children on their way to Walt Disney World.

NTSB representatives did not answer phone calls or reply to emails Monday. A recorded message for the public affairs office said nobody would respond until the shutdown ends.

U.S. Customs and Border Protection has closed many centers where people apply for Global Entry, a program that lets travelers get expedited clearance into the U.S. It is not clear if any applications are being processed; spokespeople at the agency did not respond for comment.