Washington: The United States lifted its nationwide flight ground stop on Friday (local time) after a communications failure at air traffic control centres brought departures across the country to a standstill for more than four hours, triggering thousands of flight cancellations and delays.

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More than 4,700 flights were cancelled and around 10,000 delayed on Friday, while another 700 cancellations had already been reported for Saturday, according to flight-tracking service FlightRadar24. Aviation analysts warned that it could take several days for airlines to restore normal schedules following the unprecedented disruption.

The Federal Aviation Administration (FAA) had ordered a nationwide ground stop on Friday afternoon after a telecommunications failure affected air traffic control operations across the country. The restrictions were subsequently lifted, allowing domestic flights to resume.

The disruption marked the longest nationwide flight shutdown in the US since the September 11, 2001, attacks. The only other nationwide ground stop since then occurred in January 2023, when an FAA computer system malfunction halted domestic departures for around 90 minutes.

Overloaded routers triggered nationwide flight disruption

US Transportation Secretary Sean Duffy said outdated air traffic control technology and overloaded network routers were responsible for the disruption.

At a White House briefing, Duffy explained that the routers had become overwhelmed with data, resulting in inconsistent information being transmitted through the system. The authorities decided to halt departures as a safety precaution.

"Our routers actually got overloaded with data, and… they started to give us inconsistency in the data from those routers. And because of that, we shut the system down," Duffy said.

The Department of Homeland Security said there were no indications that the communications failure had been caused by a cyberattack.

Homeland Security Secretary Markwayne Mullin had earlier said investigators had found no evidence of a deliberate attack, describing the incident as a telecommunications outage.

Preliminary reports also indicated that the disruption was not the result of a cyberattack.

1.5 million passengers affected as airlines struggle to restore schedules

The shutdown caused widespread disruption at airports across the US, affecting passengers travelling for business, holidays and personal commitments.

An estimated 1.5 million passengers were affected by flight cancellations, according to an analysis by aviation data firm Cirium cited by CNN.

The ground stop meant aircraft already in the air continued towards their destinations while departures were halted, leaving airports struggling to manage the resulting disruption.

Although the FAA lifted the restrictions, the impact continued as airlines dealt with cancelled services, delayed departures and displaced aircraft and passengers.

Aviation analysts warned that restoring normal flight schedules could take days, with the disruption extending into Saturday.

Duffy seeks $50 billion for air traffic control overhaul

The nationwide outage has renewed concerns over the ageing technology used by the US air traffic control system, which has faced equipment failures, communication problems and staffing shortages.

Duffy urged Congress to approve $50 billion in emergency funding to modernise the system, raising his earlier request for more than $30 billion.

The administration had announced a plan in 2025 to build a new air traffic control system by 2028, with an estimated cost of around $31.5 billion. Of this, $12.5 billion had been secured through legislation backed by President Donald Trump.

Duffy said the existing funding would cover only part of the overhaul and stressed the need for additional investment in hardware, software and telecommunications infrastructure.

The latest incident has highlighted the vulnerability of a system responsible for managing thousands of flights across the country, with Friday's shutdown demonstrating how a communications failure can bring nationwide air travel to a halt.