US air travel disruptions have continued for a third day, with more than 10,000 flights affected on Sunday, but Transportation Secretary Sean Duffy predicted a return to normalcy by Monday, even as airlines scrambled to get back on schedule following a systemwide ground stop.
More than 2,300 flights nationwide were cancelled, and some 7,700 were delayed on Sunday, according to aviation data site FlightAware.
Airlines on Saturday had cancelled more than 6,400 US flights and delayed more than 8,000.
Duffy said on ABC's "This Week" programme that there was no evidence of a cyberattack or other malicious intent in the matter, but the cause remained unclear.
Duffy said that flights would come "back to normal" on Sunday "but probably fully normal" on Monday, and emphasised the Federal Aviation Administration's focus on keeping passengers safe.
On Sunday, 28 percent of JetBlue flights and 17 percent of American Airlines flights were cancelled, with United Airlines, Southwest and Delta reporting cancellations of 3 percent to 5 percent of their flights.
The FAA issued a nationwide ground stop on Friday that halted nearly all departing flights for almost five hours. It was the longest nationwide halt to US air travel since the September 11, 2001 attacks on the United States.
The installation by FAA contractor L3Harris of at least one new router at a remote facility led to a situation in which other air traffic control centres were overloaded with a flood of data that hampered communications systems, according to officials.
Duffy said on Sunday that the replacement of a router at an FAA facility in El Dorado, Arkansas, triggered problems with a recently replaced router in Amarillo, Texas, which started broadcasting and overloading routers at other air traffic control centres across the country.
The FAA said on Sunday that the "functionality of the originating routers" has been disabled, adding that it did the same for about 60 similar routers in the air traffic control system.
But the FAA still does not know what ultimately caused the problem that occurred with maintenance work that had been done thousands of times before without issue, Duffy said.
The worker who installed the router was "a good American patriot, we believe, a veteran — so we don't think that someone tried to sabotage the network on the install," Duffy said. "But we don't actually have the root cause."
Friday's disruption is the latest and most dramatic sign of strain on a US air traffic control system beset by ageing technology, equipment and communications problems and a shortage of several thousand controllers.
The routers that experienced issues were purchased in 2001 and were obsolete by 2006, Duffy said. Since Friday, Duffy has called for $50 billion in funding to upgrade air traffic control systems, up from his earlier request of $30 billion — after Congress approved $12.5 billion last year.
More spending sought
US President Donald Trump on Sunday made his first public comments on this weekend's air travel issues, also calling for additional funding from US lawmakers.
"We have already started the work, but it should be done all at once, not in drips and drabs," Trump wrote on social media.
"Congress must approve."
Democrats are set to take control of Congress from Republicans in January following their gains in the November 3 US midterm elections.
Duffy's plea for more money drew scepticism from US Representatives Rick Larsen and Andre Carson, the top Democrats on the House of Representatives Committee on Transportation and Infrastructure and its aviation subcommittee, respectively.
Of the $12.5 billion approved in 2025 for upgrades to the air traffic control system, about $11 billion remains unspent, Larsen and Carson said in a statement on Saturday.
"Explain to Congress, again, why they need more money?"
Larsen and Carson asked in their statement.
Duffy said on Sunday that the money allocated last year was earmarked only for certain spending and did not include funds for new software to manage the airspace.
