Last week, the F.A.A. announced it was bringing back furloughed inspectors and other employees in order to ensure safety. Its revised shutdown plan called for having 3,113 employees responsible for aviation safety designated as essential to protect life and safety, meaning that they would work without pay during the shutdown rather than be furloughed.

The F.A.A.’s original shutdown plan called for only 216 aviation safety positions to be considered essential for life and safety. The union representing inspectors had warned that furloughing those workers was hurting the safety of the air travel system.

There are still plenty of F.A.A. employees who have been sidelined. Over all, about 14,000 of the F.A.A.’s 45,000 employees are furloughed under the revised shutdown plan.

What about airport security?

More than 40,000 transportation security officers — employees of the Department of Homeland Security who screen passengers at the airports — have worked through the shutdown. But they have been failing to show up for their shifts at a rising rate — about one in 10 were absent on Sunday. On average, they make less than $40,000 a year and many of them have had to borrow money, seek side jobs or turn to food pantries to get by.

Despite reassurances from Transportation Security Administration officials, the agency did have a lapse in early January that frightened travelers. A woman passed through a screening checkpoint at Hartsfield-Jackson Atlanta International Airport with a gun that she carried onto a flight that landed Jan. 3 in Tokyo.

The agency later said that any perception that the shutdown caused the failure to detect the gun, which was in a carry-on bag, “would be false.” No similar lapses have been reported during the shutdown.

Why are air traffic controllers so worried?

Staffing was already an issue even before the shutdown, their union said.

The number of certified controllers is at a 30-year low, according to the National Air Traffic Controllers Association. The centralized radar facility for the airports that serve New York City, which is known as a Tracon, has only about 130 controllers, far short of its full complement of 228, said Rich Santa, a regional vice president of the controllers’ union. And 50 of them are eligible to retire now, he said.