Air traffic operations over the United States continued to improve on Sunday after nearly all flights were grounded for more than 4 hours on Friday. Still, disruptions remained across the system with many flights canceled or delayed.
Nearly 70 percent of all flights left within 14 minutes of their scheduled departure time by late Sunday afternoon, according to Cirium, an aviation analytics company. Some 7 percent of flights — more than 1,900 trips — had been canceled, higher than a typical day, showing the continuing effects of the nationwide flight freeze.
Disruptions have occurred at major hubs — from Hartsfield-Jackson Atlanta International Airport in Georgia to Dallas Fort Worth International Airport in Texas — and at airports in Phoenix and Philadelphia.
The disruptions postponed college sports games and stranded professional basketball teams. Passengers slept overnight in airports and competed to rebook their flights; some canceled their trips entirely.
The logjam is the latest example of the vulnerabilities in the American air traffic system, which is facing twin crises: a shortage of air traffic controllers and an outdated infrastructure.
It was an upgrade to the system that officials have identified as the root cause of the problems that led to the shutdown on Friday. As the contractor L3 Harris was upgrading copper wires to fiber-optic cables, a router overloaded the system with data requests, disrupting communications.
As a result, tens of thousands of passengers endured a nightmare travel weekend. Some 9,000 flights were canceled on Friday, according to Cirium, while about 30 percent were delayed. As airlines worked to get back on track on Saturday, there were still 6,649 cancellations and more than half of all flights were delayed.
“Our air traffic control system is operating on DECADES-OLD, analog equipment that simply cannot compete with the demands of the 21st century,” Sean Duffy, the transportation secretary, wrote on social media.
Officials said Sunday that they expected schedules to return to normal by late in the day or Monday morning.
For Ashley Greiner, 45, the delays have compounded.
On Saturday, her flight from Seattle to Atlanta was rerouted to Charlotte, N.C. But her connection to Atlanta was canceled, and she spent the night in North Carolina. On Sunday, she faced two delays and a plane change.
“I’m a heart-failure patient, and my trips were for speaking engagements to share my story with health care providers,” she said in a direct-message conversation. “This has been exceptionally hard on my body.”
The disruption in the communication system forced officials at the Federal Aviation Administration to ground flights at around 4 p.m. on Friday.
It was the third such ground stop this century.
Some 3,500 planes were airborne when the order went out, Secretary Duffy said on Friday.
Overall, more than 17,000 flights have been canceled since Friday, according to Cirium’s latest tally. Nearly 40 percent of the 74,696 scheduled flights since Friday have been delayed.
On Sunday, Secretary Duffy said the affected facilities in Texas and Arkansas were “secured” and diagnostic testing was underway.
“We have no indication of a foreign cyberattack, and we will continue to investigate alongside the F.B.I. and our federal law enforcement partners,” he said.
Brenda Folkerts, 59, arrived in New Orleans on Thursday for a marathon but now faces the prospect of a long trip to her home state of Washington on sore and tired legs. Her connection through Dallas was delayed by two hours on Sunday, and she missed her flight to Seattle. She does know when or how she’ll make it home.
“My case is not unique,” she said in a direct message as her plane taxied. “Most people here have missed connections and are scrambling to figure out what to do.”
