The United States has lifted its order to ground departing flights nationwide as a major communications outage continues to disrupt air travel.
The Federal Aviation Administration (FAA) cited "extraordinary circumstances and reasons of safety", with the White House blaming outdated air traffic control technology for the outage.
Thousands of flights into and out of the US were delayed or cancelled, leaving airlines and passengers scrambling to work out their next steps.
The ABC understands the US "ground stop" has caused some delays to a small number of US-bound flights from Australia.
Here's what we know about the unfolding situation so far.
What caused the problem?
US Homeland Security Secretary Markwayne Mullin has attributed the disruption to a telecommunications issue.
"It's just a telecommunications outage as of right now," Mr Mullin told reporters at the White House earlier on Friday, local time.
It is not yet confirmed what specifically caused the communications failure.
Mr Mullin had earlier said investigators would examine whether there was any connection to the FAA's nationwide rollout of a new artificial intelligence (AI) system designed to help identify problems and reroute aircraft.
However, the system operates from a single location near Washington DC and provides recommendations to air traffic control centres.
US President Donald Trump has also rejected suggestions the new AI technology was responsible for the disruption.
In his latest press conference, US Transportation Secretary Sean Duffy blamed "old technology" for the outage.
"What we have is an incredibly old air traffic control system," he said.
"We have analogue telecoms, our routers actually got overloaded with data … and they started to give us inconsistencies in the data … because of that we shut the system down," he says.
Speaking to ABC News, Shawn Pruchnik, an associate professor in Ohio State University's aviation department, said officials made the right call by grounding departing aircraft as a precaution.
"We have this significant problem of airplanes at higher altitudes that cannot communicate with air traffic control, so the solution to that is less airplanes up there," he said.
"Keep everyone down, let everyone land."
Was it caused by a threat to the US?
Mr Mullin said there was currently no indication the outage was the result of a nefarious attack.
However, he later added:
"The first thing is [to make sure] everybody that is flying in is safe."
In a subsequent press conference, Mr Mullin reiterated that there were no signs pointing to a cyber attack for now.
"Typically, somebody's going to raise their hand up and say, 'Hey, we did that'. There's no chatter. We haven't seen any footprints … of people that have got into the system," he said.
"It appears this was mechanically driven. However, we are still looking."
Is it linked to the Microsoft outage?
Microsoft has denied any known connection between its recent global outage and the FAA's nationwide stoppage.
"We have no indication that the FAA's nationwide ground stop is related to any Microsoft services," the company said on X.
The statement came amid speculation about a possible link to Microsoft's 21-hour global outage, which was resolved earlier on Friday.
Mr Mullin also dismissed the suggestion of a connection, telling reporters: "I don't see any reason to connect those two at all."
Which flights were affected?
The FAA's order applied to departing flights across the US, grounding nearly all aircraft scheduled to take off during the five hours it was in effect.
The agency also asked Eurocontrol, its European counterpart, to suspend flights to the US from its 42 member states, including Türkiye, Morocco and Israel.
Aircraft that were already in the air when the order was issued were allowed to continue safely to their destinations.
The full scale of the disruption is still emerging, but the impact has already been felt by tens of thousands of travellers.
Flight-tracking website FlightAware reported about 9,814 delays on flights into and out of the US, alongside more than 4,749 cancellations. Those figures are continuing to change as the disruption unfolds.
Meanwhile, the ABC understands Australian airlines are dealing with a backlog of delayed flights.
Before the US order was lifted, a Sydney Airports spokesperson had said all US-bound flights from Sydney were grounded.
Since the order was lifted, the spokesperson said:
"Flights scheduled to depart Sydney for the United States are expected to depart as planned. Passengers should continue to check with their airline for the latest information."
The ABC is continuing to stay in touch with Sydney Airport for further updates.
Can planes already in the air land safely?
Yes. Aviation experts say pilots and air traffic controllers are trained to manage communications failures.
Skip Bailey, an aviation professor who spoke to The Associated Press, said pilots were expected to maintain the headings and instructions given by air traffic controllers before communications were lost.
If the outage continued, they could revert to their filed flight plans.
Pilots may also be able to communicate with other aircraft over the radio, even if they cannot reach air traffic controllers. Commercial aircraft also have their own radar systems.
"It's not mass chaos out there," Professor Bailey said.
"There's plenty of ways to try to communicate."
He said preventing additional aircraft from taking off was crucial to avoiding further complications.
"We don't want to put more aircraft into a system that's already having an issue."
When will flights resume?
Mr Duffy said US air space was reopened at 8:15pm Eastern Standard Time, about five hours after the grounding order was issued.
However, he warned that flights could be "slow" to take off again.
"If you are watching, if you are at an airport, check with your airline, it depends on [if they] have crews available," Mr Duffy said in his latest press conference.
"If not, we are going to start tomorrow morning with our airline partners to move those aircraft but it is going to take us some time to unwind what has happened this afternoon."
What's the advice for Australian travellers?
Australians travelling to the US should expect to hear from their airlines or travel agents as the disruption continues, the Australian Travel Industry Association said.
"It is a developing situation, and there will be knock-on impacts for travellers," the industry body said in a statement.
