Heathrow airport closed : What we know so far
EPAFlights around the world have been disrupted after Heathrow Airport closed due to a fire at a nearby electrical substation.
Heathrow, the UK's busiest airport, said on Friday afternoon that they will "restart" some flights later after the blaze at North Hyde substation in Hayes, west London. The airport said it hoped "to run a full operation" on Saturday,
More than 1,300 flights have been affected, tracking website Flightradar24 said, and passengers have been told not to travel to the airport unless their airline has advised them to.
The National Grid said earlier that an "interim solution" has been found to allow power to be restored to customers including Heathrow Airport, saying that the network has been "reconfigured to restore all customers impacted".
Here's what we know so far.
Why is Heathrow closed?
A fire at an electrical substation in west London, which supplies Heathrow, caused a major power outage at the airport, prompting its closure.
It is not yet known what caused the fire at the substation, but emergency services were first called to Nestles Avenues in Hayes, west London at 23:23 GMT on Thursday.
Jonathan Smith, London Fire Brigade (LFB) deputy commissioner, said the fire at the electrical substation involved a transformer containing 25,000 litres of cooling oil.
The fire was "very visible and significant," he said.
LFB said the fire was under control by 06:28.
The substation is only about a mile and a half away from the airport.
Energy Secretary Ed Miliband told BBC Radio 4's Today programme the substation's back-up generator "appears to have been knocked out".
Videos shared on social media showed tall flames and plumes of smoke billowing from the substation overnight.
People living nearby have been advised to keep doors and windows closed to avoid smoke inhalation.
How did the fire lead to so much disruption?
With Heathrow being such a busy airport for passenger journeys and global trade, questions have been raised over whether it has back-up systems in the event of power cuts.
The BBC understands that Heathrow does have back-up power for its key systems, but kickstarting these alternative power supplies for the whole airport takes time.
A source said it was not possible to switch the power back on immediately.
A Heathrow source also told the BBC that they have "multiple sources" of energy at the airport – with diesel generators and "uninterruptable power supplies" in place.
They added when the power outage happened the back up systems "all operated as expected".
The systems, however, are not enough to run the whole airport – hence the decision to close it down.
The source said the airport is in the process of redirecting power to the affected parts of the operation – but that it "takes time".
And even once the power is back on, there are countless systems which need to be rebooted and checked to ensure they are working properly and are stable.
It is unclear why Heathrow's own back-up systems were not adequate to keep the airport running when one critical component of its energy supply was knocked out, and it is also unclear why the National Grid distribution network was not set up to supply sufficient electricity.
A Heathrow source said these questions would be investigated.
When will Heathrow Airport be re-opened?
PA MediaThe airport said on Friday afternoon that it will "begin some flights" later prioritising repatriation and relocation of aircraft, but urged passengers not to travel to the airport unless their airline has advised them to do so.
The travel disruption affected many other airports, as airlines cancelled and diverted flights.
Heathrow earlier said it does "not have clarity on when power may be reliably restored".
The airport has apologised for the disruption and has advised passengers to contact their airlines for further information.
Its helpline number is 020 8757 2700.
Who has been affected?
At least 1,351 flights to and from Heathrow will be affected on Friday, Flightradar24 said, with some 120 affected aircraft already in the air when the closure was announced.
The Foreign Office is advising UK citizens who are abroad and require urgent assistance to contact their teams via an online query form.
The Heathrow Express railway service said it was running a reduced half-hourly service to help passengers leave the terminals.
It said it was not accepting passengers at Paddington station.
All Virgin Atlantic flights arriving and departing flights from Heathrow have been cancelled until midday on 21 March, the airline said.
US-based carrier United Airlines told the BBC that "following the closure, seven United flights returned to their origin or to other airports", adding that additional flights to Heathrow have been cancelled.
The airline said it was working with its customers to offer alternative travel options.
Gatwick Airport told the BBC it was aware of the situation at Heathrow Airport and stands "ready to support as required".
Several of Australia's Qantas airline planes have been diverted from London to Paris, with other flights likely to be affected, it said.
British Airways has cancelled all its short-haul flights due to operate to and from the airport on Friday.
Some long-haul flights - including to Cape Town, Johannesburg, Singapore and Rio de Janeiro - were later given clearance to depart from Heathrow from 1900 GMT and the airline said it was reviewing the fire's implications for Saturday's schedule and beyond.
Cathay Pacific, Hong Kong's main airline cancelled all its flights to London on Friday.
How have locals been affected?
In addition to passengers expecting to fly, disruption has been caused to thousands of homes in west London, which have been left without power.
About 150 people had to be evacuated from surrounding properties.
More than 16,300 homes have lost power in a large-scale outage caused by the fire, energy supplier Scottish and Southern Electricity Networks (SSEN) said.
"We're aware of a widespread power cut affecting many of our customers around the Hayes, Hounslow and surrounding areas," it added.
The National Grid said on Friday afternoon the network had been "reconfigured to restore all customers impacted, including the ability to resupply the parts of Heathrow airport that are connected to North Hyde".
Meanwhile, two nurseries and four schools in Hillingdon - the London borough Heathrow Airport is located in - are shut today.
Hillingdon Council are assisting 12 people who were evacuated from their homes by the emergency services with hotel accommodation.
Bin collections will also be impacted on Friday, the council warned.
Bus routes in the Hillingdon area have been affected and the M4 is closed between junction three and four, while the Terminal Four spur roads are also closed.
No injuries from the fire have been reported.
What happens now?
Counter-terrorism officers from the Metropolitan Police are now leading the investigation into the fire because of the location of the substation and the "impact on critical national infrastructure".
It added there was currently no indication of foul play but it is keeping "an open mind".
London Fire Brigade said it was working closely with the Metropolitan Police.
In the meantime, customers have been told to stay away from the airport until further notice and to contact their airlines directly.