What we know about the fire that brought London's Heathrow Airport to a standstill

A fire at an electrical substation in west London, about 2 miles (3.2 kilometers) from the airport, cut power to Heathrow Airport late on Thursday.

Residents reported an explosion and a fireball just before midnight. The London Fire Brigade said a transformer holding 25,000 liters (5,000 gallons) of cooling oil caught fire. It said 10 fire engines and 70 firefighters brought the blaze under control after seven hours, but isolated hotspots were still alight 24 hours after the fire started.

National Grid, which maintains energy infrastructure in Britain, said the blaze damaged equipment at the substation and cut power to 67,000 properties, including Heathrow. It said power was restored to all of them by Saturday morning.

The “significant power outage” initially forced Heathrow officials to announce that the airport would be closed until 11:59 p.m. on Friday, but some flights began to resume on Friday evening.

Officials said there was no suggestion of foul play, but the cause is still under investigation. The fire brigade said its investigation would focus on the electrical distribution equipment at the substation.

Even so, London's Metropolitan Police Service said counterterrorism detectives were leading the investigation because of the fire's impact on critical national infrastructure.

Energy Secretary Ed Miliband said it was "an unprecedented event.”

“Obviously, with any incident like this we will want to understand why it happened and what if any lessons it has for our infrastructure,” he told Sky News.

The closure disrupted the travel plans of around 200,000 people who were expected to travel through Heathrow on Friday. Heathrow advised passengers not to travel to the airport and to contact their airlines to rebook flights.

With all takeoffs and landings canceled, the first impact was on dozens of long-haul flights from North America and Asia that were in the air when the airport was shut down. Some were forced to turn around, while others were diverted to airports around the United Kingdom and Europe.

Heathrow-bound aircraft landed at Schiphol Airport in Amsterdam; Shannon Airport in western Ireland; Glasgow, Scotland; Manchester, England; Charles de Gaulle in Paris; Lyon, France; and Frankfurt, Germany, among others.

About 4,000 tons of cargo were also stranded by the closure, according to Anita Mendiratta, an aviation and leadership consultant.

The disruptions are expected to last for days as airlines move stranded aircraft and flight crews back into position and work to accommodate passengers whose flights were canceled. Mendiratta estimated that it would take two to four days to clear all the backlogs.

“This is an extreme situation where the entire aviation ecosystem is impacted,” she said.

“There will be two things that will be happening as a priority No. 1. First is airport operations and understanding, from an electrical system point of view, what has been impacted, if anything,” she said. “Did anything short out, for instance? What needs to be reactivated? And then how do you literally turn the airport back on again?"

Heathrow was Europe’s busiest airport last year, with 83.6 million passengers traveling through it. Its closure will have far-reaching impacts because it's a major hub for connecting flights to cities throughout the U.K. and around the world, as well as for travel to London.

Yes. Five other air hubs in southeastern England identify themselves as London airports, but they are much smaller than Heathrow. London Gatwick, Britain’s second-biggest airport, handled 43.2 million passengers last year. It's located in the town of Crawley, 28 miles (45 kilometers) south of London.

The fire raises concerns about the U.K.’s ability to withstand attacks or natural disasters that damage critical infrastructure such as communications and power networks, analysts said.

The incident is particularly worrisome given recent comments by Britain’s security services that Russia is conducting a reckless campaign of sabotage across Europe, said Alan Mendoza, the executive director of the Henry Jackson Society, a London-based security think tank.

“The U.K.’s critical national infrastructure is not sufficiently hardened for anywhere near the level it would need to be at to give us confidence this won’t happen again,” he said. “I mean, if one fire can shut down Heathrow’s primary systems ... it tells you something’s badly wrong with our system of management of such disasters.”

Robin Potter, an expert on resilience at London-based think tank Chatham House, said that successive governments have been slow to respond to repeated recommendations from the National Infrastructure Commission to strengthen the ability of Britain’s power, communications, transport and water networks to withstand major shocks.

“We still have yet to see a kind of clear response from the government to those recommendations,” he said. “And we hope that maybe in the government’s upcoming resilience review, which we expect will be published at some point this year, it might seek to address some of those questions.”

Heathrow defended its response. Its CEO Thomas Woldbye said “the same would happen in other airports” faced with a similar fire.