Some flights began to land at London's Heathrow Airport late on Friday after a fire at a power station grounded planes at Europe's busiest air hub, causing travel chaos for thousands of people around the world.
The electricity substation fire shut down the airport for most of the day, leading to the cancellation or diversion of hundreds of flights and raising questions about the infrastructure's vulnerability.
An AFP journalist saw planes landing on the tarmac at Heathrow late on Friday, while British Airways said it had received clearance to depart eight long-haul flights from 1900 GMT to cities including Johannesburg, Singapore and Riyadh.
Earlier, an airport spokesperson said "some flights" could begin taking off and that the first would be "repatriation flights and relocating aircraft".
Restrictions on overnight flights have also been temporarily lifted to help ease congestion, according to the UK's transport department.
Heathrow Airport's chief executive Thomas Woldbye said "tomorrow, we expect to be back in full operation".
He apologised and said the decision to close the airport came after a backup transformer failed, meaning the power supply had to be restructured.
"We have lost power equal to that of a mid-sized city," he said, calling it an "incident of major severity".
Planes from Heathrow serve around 80 countries, and around 1,350 flights had been due to land or take off from the airport's five terminals on Friday, according to the Flightradar24 tracking website.
Around 230,000 passengers use Heathrow every day -- 83 million a year -- making it one of the world's busiest airports.
Late on Friday the London Fire Brigade said the fire was "believed to be non-suspicious" and that an investigation would "focus on the electrical distribution equipment".
It came after London's Metropolitan Police said the force's Counter Terrorism Command was leading the investigation into the fire given its impact.
"While there is currently no indication of foul play we retain an open mind at this time," a spokesman said earlier.
Firefighters were called to the blaze shortly after 2320 GMT on Thursday and had brought it under control by 0800 GMT on Friday.
The outage left 100,000 homes without power overnight. Electricity distribution network National Grid said that at around 1400 GMT power had been partially restored to Heathrow as well as to local residents affected.
"We need to understand what caused an incident of this magnitude at an electricity substation that is very close to a critical piece of national infrastructure," said Transport Secretary Heidi Alexander.
- Disruption -
The airport shutdown left many frustrated passengers struggling to make alternative arrangements.
Talia Fokaides, 42, was supposed to leave London on Friday morning for Athens but rushed to Gatwick to get a new ticket when she heard the news.
Her mother was having open heart surgery later in the day.
"I just need to be there," she told AFP, her voice shaking with emotion.
Around 120 Heathrow-bound planes were in the air when the closure was announced, according to Flightradar24.
The UK's second-busiest airport, Gatwick, accepted some flights from Heathrow, while others were diverted to European airports including Shannon in southwestern Ireland, Frankfurt in Germany and Paris Charles de Gaulle.
London Fire Brigade deputy commissioner Jonathan Smith said firefighters had worked "tirelessly in challenging and very hazardous conditions".
He told reporters the fire had involved a "transformer comprising of 25,000 litres of cooling oil fully alight".
One resident recounted hearing a "massive explosion" as the power cut out just before midnight, while another reported seeing a "bright flash of white".
Around 150 people were evacuated from nearby properties because of the fire.
- Questions -
As the scale of the disruption began to emerge, Energy Secretary Ed Miliband faced questions about how one fire could shut down an entire airport.
He said National Grid had told him it had not seen "anything like the scale" of what happened.
"But it makes Heathrow look quite vulnerable and therefore we've got to learn lessons... not just Heathrow but how we protect our major infrastructure," he told ITV news.
Willie Walsh, director general of the airline industry's trade association IATA, said the closure was "yet another case of Heathrow letting down both travellers and airlines".
"How is it that critical infrastructure -- of national and global importance -- is totally dependent on a single power source without an alternative," he wrote on X.
Flights from all over the world were affected by the closure, and aviation consultant Philip Butterworth-Hayes told AFP it would cost the airport and airlines "more than £50 million ($64.7 million)".
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