As few as three air traffic controllers were scheduled to work on Monday evening at the facility guiding planes to and from Newark Liberty International Airport, the Federal Aviation Administration said, far fewer than the target of 14 controllers for most of those hours.
The staffing crisis added strain to an already troubled aviation system, with flights to Newark delayed by as much as seven hours on Monday.
The F.A.A. said in a statement to The New York Times that it had at least three controllers scheduled every hour on Monday evening at a Philadelphia facility that manages Newark’s air traffic. But four people familiar with problems at the airport said that the number of fully certified controllers on duty was at times one or two.
Staffing shortages affected flights at the airport for much of the day, forcing the F.A.A. to hold up incoming flights from taking off. The delays primarily affected flights coming to Newark from the contiguous United States and parts of Canada, and lasted an average of more than an hour and 40 minutes and up to almost seven hours, according to an online F.A.A. advisory.
Monday’s delays were the latest in a string of setbacks for Newark, one of the nation’s busiest airports and a large hub for United Airlines. On Friday, the air traffic control facility in Philadelphia that guides planes at the airport had a brief radar outage. A similar outage late last month had left controllers unable to communicate with pilots for about 30 seconds.
That outage, on the afternoon of April 28, had followed months of glitches and other problems that have rattled the controllers who manage the Newark airspace. Some of the controllers went on leave to recover from the stress of those delays.
On Monday, during the shift that typically runs from 3 to 10 p.m., the group that manages Newark air traffic from Philadelphia was operating with one or two fully certified controllers, four of the people said. The target for that period is 14 controllers, according to an agreement between the controllers union and air traffic control officials that became effective in January.
“We plan for staffing with traffic management initiatives to ensure safety is never compromised. Tonight, at least three controllers are scheduled for each hour at Philadelphia TRACON Area C, which directs aircraft in and out of Newark Liberty International Airport,” the agency said, referring to the Philadelphia Terminal Radar Approach Control.
Last summer the F.A.A. moved the management of Newark’s air traffic to the Philadelphia facility from one in Long Island, N.Y., in the hopes that moving the team to a more affordable area would help the agency hire more controllers.But the relocation to Philadelphia angered some controllers who were asked to uproot their families. Even if the move had gone well, recruiting and training new controllers takes years.
At best, it could take a year to train controllers relocating to the facility in Philadelphia, the transportation secretary, Sean Duffy, said at Monday news conference. The average time to certify a controller for such complicated airspace is two and a half years.
“It takes them a long time to to train up,” Mr. Duffy said. “So we don’t have the ability to just snap our fingers and move controllers around.”
As of March 2024, only 59 percent of available controller roles were filled at the Long Island facility, which at the time handled flights at Newark, LaGuardia Airport, Kennedy International Airport and other New York airports.
In addition to staffing, the F.A.A. has been grappling with equipment and technology failures that could also take long to fix.
The recent radar outages were caused by software and hardware problems, which, when combined with the staffing limitations and runway construction, have contributed to lengthy delays at Newark in recent weeks and shaken faith in the airport among travelers.
Melissa Rodriguez of Bayonne, N.J., regularly flies from Newark because it’s closer than other airports in the region. But she is considering using other New York area airports after chaperoning a cheerleading team that landed at Newark on Monday.
“Everybody got either delayed or their flights were canceled, and this was going both ways,” Ms. Rodriguez said, adding, “They were very unhappy.”
Ms. Rodriguez, 46, said she had looked into flights arriving at Kennedy or LaGuardia as a backup. “With all the problems at Newark, you’ve got to look at Plans A, B, C, and D.”
In an email on Monday, United’s chief executive, Scott Kirby, sought to reassure customers, saying that all flights to and from Newark are “absolutely safe.”
“When there are F.A.A. issues, such as technology outages or staffing shortages, the F.A.A. requires all airlines to fly fewer aircraft to maintain the highest levels of safety,” he said.
United operates about two-thirds of the flights to and from the airport, with no other airline coming even close to its size there.
Mr. Duffy said the earlier radar outages had been caused by an antiquated backup telecommunications line that had been overwhelmed when a primary line failed.
“We are trying to slow speeds down to 1990 speeds for 1980 equipment,” he said.
Mr. Duffy said the F.A.A. had installed software updates on Friday to prevent future outages and plans infrastructure upgrades, including new fiber-optic cables connecting the airport and the facility in Philadelphia.
To reduce delays, Mr. Duffy said, he also plans to reduce the number of flights at the airport. The department is expected to meet with airline executives this week to discuss how best to do that. United has already cut back on some flying there.
In the news conference, Mr. Duffy also repeatedly sought to blame the Biden administration, saying it had “bungled” the move from New York last summer. He said he had also asked the department’s inspector general to investigate the shift to Philadelphia.
A spokesman for Mr. Duffy’s predecessor, Pete Buttigieg, said in a statement that “Secretary Duffy has a tough job. But he needs to spend more time doing what the American people are paying him to do — fix problems — and less time blaming others.”
Mark Bonamo and Tim Balk contributed reporting.