An upgrade to a decades-old telecoms network in the US caused the longest nationwide grounding of flights since the 11 September 2001 attacks.
Friday's stoppage occurred when part of a key system that allows air traffic controllers and pilots to communicate was switched to fibre from copper wiring so old it has been discontinued, said US Transportation Secretary Sean Duffy on Saturday.
US planes were grounded for more than four hours, affecting thousands of flights and passengers.
Earlier Homeland Security Secretary Markwayne Mullin said there was no indication of a cyber-attack and that everybody was safe.
Here's what we know.
On Friday afternoon, a government contractor called L3Harris was continuing work on upgrading an aviation telecoms network.
As it swapped out copper lines for fibre at a remote location in the US, L3Harris installed a router - a piece of equipment which basically directs traffic across the network.
According to Duffy, that router started flooding others in the air traffic control system, sending 1,000 connection attempts every four to five seconds.
As a result, 17 out of 22 air route traffic control centres in the US experienced "instability" on voice and ground communications, said Bryan Bedford, administrator of the Federal Aviation Administration (FAA).
It took the aviation authority 45 minutes to identify where the problem was because maintenance is logged manually on paper.
This meant the FAA had to "pore through" the physical documents "to figure out who's working on what", Bedford admitted.
As a precaution as it tried to track down the cause, Bedford said the decision was made to ground all flights.
Aviation analyst Miles O'Brien described the US system as "a house that's about to be condemned".
He told the BBC: "I think most of us could look at this and go 'this is a system in the United States that we have allowed to decay'. It is worse than a fixer-upper."
Duffy said the system needed a complete overhaul, estimating a $50bn price tag.
In the meantime, he said authorities would work to determine whether it was "incompetence" or faulty equipment that led to the outage.
He also said there would be "re-evaluation" of allowing upgrades to be done during the day.
In a statement on Saturday afternoon, L3Harris confirmed it was working with the FAA to modernise the airspace communications infrastructure and said it was supporting the agency with its investigation.
"L3Harris takes the nationwide ground stop and its impact on travelers, airlines and air traffic operations seriously," the firm said.
Juliette Kayyem, US assistant secretary of Homeland Security under President Barack Obama, questioned why an upgrade was done on one of the week's busiest days for travel.
"You don't do it on a Friday when there's lots of air traffic controllers," she said
"You try to find ways to minimise its upgrade so you're not maybe doing it across the board in a single moment but you also have to have systems that are ready to step in if the upgrade in any way begins to threaten the system," Kayyem added.
In the US, UK and the EU, airlines may be liable for compensation if the reason for a delay is deemed within their control, such as a fault with the plane or pilot sickness.
As such, airlines do not have to provide compensation to passengers for air traffic control problems.
But passengers are still protected to a certain degree under US federal - and country-specific - laws.
Under the FAA Reauthorization Act of 2024, cancelled or "significantly delayed flights" - defined as three hours for domestic flights and six hours for international routes - must be offered alternative travel or a full refund of the fare if the passenger chooses not to fly.
It also requires all air carriers to issue refunds for tickets purchased with credit cards within seven business days of the request, and 20 days for tickets purchased with another form of payment. Passengers accepting a new flight cannot receive a refund.
In the US, there is no requirement for airlines to provide meal vouchers or accommodation, but some may offer complimentary support to customers.
However, if passengers are travelling from the UK to the US or flying with a UK carrier, you may have additional rights.
Under UK law, airlines have a duty to look after you when flights are delayed or cancelled for whatever reason.
This includes:
a reasonable amount of food and drink (often through vouchers)
a way for you to communicate (often by refunding the cost of calls)
free accommodation, if you have to stay overnight to fly the next day
transport to and from that accommodation
If your airline is unable to arrange assistance, you have the right to organise this yourself and claim back the cost later.
The Civil Aviation Authority advises people to keep receipts and not spend more than necessary.
