All flights in the US were grounded for several hours on Friday, causing widespread disruption that is expected to last for days.
No flights were allowed to take off from the continental US due to a problem with air traffic control systems for more than four hours from 15:54 EST (19:54 GMT) - the longest such incident since the 11 September 2001 attacks.
Officials attributed the issue to outdated equipment and said there was no evidence it had been caused by a cyber-attack.
The ground stop was lifted at 20:15 EST, allowing flights to depart, but it is expected to have a knock-on effect on air travel for days as airlines clear a backlog of flights.
More than 15,000 flights in the US were delayed or cancelled, according to data from flight tracking sites, affecting an estimated 1.5 million passengers.
In some cases, passengers were left stuck inside planes on the tarmac, unable to take off or to return to the gates. Some international flights were allowed to depart US airports, in an apparent effort to clear space.
Disruption has continued across the country into Saturday, with hundreds of flights expected to be delayed or cancelled, according to tracker Flightradar24.
The impact of the ground stop rippled around the world, with many US-bound flights ordered not to take-off, including from Europe and Australia's Sydney International Airport.
In the UK, some flights have been cancelled and in-bound flights from the US to Manchester and Heathrow airports are experiencing minor delays.
US Transportation Secretary Sean Duffy told a news conference on Friday night that the impact of the ground stop would be felt for a few days.
"It's going to take us some time to unwind what's happened this afternoon," he said.
Duffy stressed that the issue that had caused the ground stop - characterised as "intermittent" instability in communication and radar systems - had not put any lives or flights at risk.
He blamed equipment that was "too old", with overloaded routers producing inconsistent data, and said the system needed a £50bn overhaul.
Homeland Security Secretary Markwayne Mullin said there was no evidence that the problem had been caused by a cyber-attack or a rogue artificial intelligence agent.
Both he and Duffy warned that a similar issue could arise again unless the US Congress approved upgrades to air traffic control infrastructure.
US President Donal Trump has requested $30.1bn for the upgrades. Both chambers of Congress are currently controlled by the Republican Party.
Trump was uncharacteristically silent during the fiasco, however Mullin and Duffy said they were in constant contact with him and he was "fully engaged".
The Federal Aviation Administration (FAA) said it had "thoroughly tested" its systems "to confirm they were stable and that operations remain reliable" before clearing flights to resume.
Former TSA Administrator John Pistole told the BBC that he was "not optimistic" about how quickly things were going to get back up and running at airports across the country.
While he acknowledged "antiquated equipment", he said it was unclear why the fault had occurred when it had and that it was a concern ahead of Thanksgiving holiday travel.
"The fact that they were able to get it restarted in four-and-a-half hours is good news - it shows a resilience in the system - but what's to say it's not going to happen tomorrow?" Pistole added.
A nationwide ground stop has only been ordered one other time since all flights were banned from take-off for several days following the 11 September 2001 attacks.
A failure in the Notice to Air Missions (Notam) system in 023 prompted a grounding that lasted an hour and 52 minutes, delaying about 9,500 flights and cancelling about 1,300.
"This looks like the worst since 9/11, but clearly not on that scale," Dr Guy Gratton, an associate professor of aviation at Cranfield University, told BBC Verify of Friday's ground stop.
