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

LONDON (AP) — Hundreds of flights to and from London's Heathrow Airport were canceled on Friday after a fire at a nearby substation knocked out power to Europe's busiest airport, disrupting travel plans for hundreds of thousands of people around the world.

After a day of chaos, Heathrow began reopening in the evening, with the first plane of the day landing at about 6 p.m. local time. That was just the start of what is expected to be a dayslong process of getting stranded passengers to their destinations and displaced aircraft into the proper locations.

Here's a look at what happened and its impact on air travel.

What happened?

A fire at an electrical substation in west London, about 3 kilometers (2 miles) from the airport, knocked out power to Heathrow Airport late on Thursday.

The “significant power outage" initially forced officials to announce that the airport would be closed until 11:59 p.m. on Friday “to maintain the safety of our passengers and colleagues." By late afternoon, the airport said its crews had “worked tirelessly” to restore service, allowing flight operations to restart on Friday evening.

The airport said the first priority would be to begin relocating aircraft so they were in the proper locations for regular service to resume.

The London Fire Brigade said that 10 fire engines and 70 firefighters responded to a fire at the substation that was reported at 11:23 p.m. Thursday. The blaze has been contained but firefighters would remain at the scene throughout Friday, the fire department said.

U.K. Energy Secretary Ed Miliband said the “catastrophic” fire appeared to have knocked out a backup generator as well as the electrical substation that supplies power to Heathrow Airport.

National Grid, which maintains energy infrastructure in Britain, said the blaze damaged equipment at the substation and crews were working to restore power supplies as quickly as possible. Power had been restored to the local community by early afternoon.

What caused the fire?

Officials said there was “no suggestion” of foul play, but the cause is still under investigation.

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

“It’s obviously an unprecedented event,” Miliband told Sky News. “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 added.

How was Heathrow affected?

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 have 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.

The impact on short-haul flights was delayed until Friday morning because flight operations at Heathrow are severely limited between 11 p.m. and 6 a.m. every day to minimize overnight noise in surrounding communities. Even so, thousands of people would be unable to travel to and from airports around Europe and the U.K. on Friday.

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

How long will the disruptions last?

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? Passenger and cargo."

In addition, she noted, “there's the issue of actually managing the human component of it. You have passengers that are impacted, crew are impacted and operations, so being able to remobilize everything.”

What's the bigger picture here?

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 think tank focused on security and democracy in Europe.

“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.”

Mendoza said that the goal should be to have backup systems that can be put in place quickly to mitigate the disruption caused by any so-called black swan — or unpredictable — events.

“The reality is there are always going to be issues that surprise you — black swans, as it were,” he said. “The key to it is not necessarily to have to predict black swans, because in many cases they are unpredictable, but it’s about having the fortitude and the sort of bend in the system to be able to get back up to working speed very quickly.''

How big is Heathrow?

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.

Does London have other airports?

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.