Thousands of U.S. flights remained canceled or delayed on Saturday as airlines and airports worked to recover from a nationwide ground stop that halted nearly all departing flights for almost five hours. The disruption exposed continuing vulnerabilities in the U.S. air traffic control system, while officials moved to secure significantly more funding for infrastructure upgrades.

Airlines Face a Difficult Recovery

Major U.S. airlines canceled more than 3,700 flights and delayed another 2,000 on Saturday as tens of thousands of passengers remained stranded. The disruption followed Friday’s nationwide ground stop, which forced airlines to cancel nearly 10,000 flights and delay more than 7,000 others.

Although the FAA lifted the ground stop Friday night, the operational impact continued as airlines attempted to reposition aircraft, flight crews and other personnel. United Airlines imposed a ground stop at Chicago O’Hare, while Delta Air Lines did the same at New York LaGuardia to manage congestion and gate constraints. Several major airports also said they could not accept additional diverted aircraft.

Air Traffic Control Network Failure Under Scrutiny

The FAA issued the ground stop after an outage appeared to affect a network responsible for handling critical radio frequencies used by air traffic controllers to communicate with pilots, according to two airline officials. Transportation Secretary Sean Duffy said the problems began at 3:36 p.m. ET when air traffic control centers experienced instability in radar and communications systems.

Duffy said switches and routers became overloaded with data, producing inconsistencies that prompted officials to shut the system down. He said air traffic personnel never completely lost radar or communications while guiding aircraft to landings and that the immediate problem had been fixed.

However, the incident has intensified scrutiny of an air traffic control system facing aging technology, equipment and communications problems, as well as a shortage of several thousand controllers. The contractor overseeing the affected network, L3Harris, did not immediately respond to a request for comment.

$50 Billion Infrastructure Push

Duffy called the latest disruption an emergency and increased his funding request for air traffic control upgrades to $50 billion, from $30 billion previously sought. Earlier Friday, the White House and Department of Transportation had urged Congress to approve $30.1 billion for upgrades to aging airport and air traffic control infrastructure.

Officials said there was currently no evidence that the outage resulted from a cyberattack or a rogue AI system, although investigations had not ruled out either possibility. The National Security Agency was working with the Department of Homeland Security on the investigation.

Airlines Face Financial and Operational Pressure

The disruption is also creating financial pressure for carriers already operating in a highly competitive industry. Shares of American Airlines, United Airlines and Delta fell between 1% and 2% in after-hours trading Friday, while JetBlue Airways declined about 3% as investors began pricing in the potential impact.

For investors, the key issue extends beyond the immediate restoration of flights. The episode raises questions about the resilience of critical transportation infrastructure and the scale and timing of government investment required to modernize the system. Airlines may recover operationally as aircraft and crews are repositioned, but the broader infrastructure debate is likely to remain central as policymakers assess how to prevent another nationwide disruption.