Train service to and from the airport was also down. Pubs and hotels nearby quickly filled with passengers and their luggage, media here reported.

Heathrow is the busiest airport in Europe, handling a record 83.9 million passengers last year, and the fifth-busiest in the world. It operates international flights to 240 destinations in about 90 countries, more than any other European city, according to the British government.

Guy Gratton, an associate professor of aviation and the environment at Britain’s Cranfield University, said the closure was “going to be very significant” and would probably have “knock-on effects all around Northern Europe.”

The major challenge to air traffic and safety, he said, comes from the planes already in the air. Heathrow, by far the busiest of London’s six airports, is a point of departure, a destination and a connection for long-distance flights. “They’ve all got to go somewhere,” Gratton said. “The question is where the capacity is.”

At least 120 Heathrow-bound planes were in flight when the closure was announced early Friday. Many were on long-haul routes from Asia and the Americas, according to the tracking service Flightradar24. Some turned back to the airports they departed from; others were diverted to runways in Britain, Germany, France, Belgium, Spain and other countries. Some travelers made the final leg of their journeys to London by bus.

Qantas Flight 9, a 17-hour flight from Perth, Australia, to London, was over Germany when the crew was redirected to Charles de Gaulle Airport in Paris.

“Everyone seemed fine and understanding about the diversion, but obviously very tired,” passenger Richard Crease told The Washington Post by text message.

The approximately 300 passengers aboard the Boeing 787 Dreamliner waited about four hours for the airline to arrange five coaches to drive the passengers from Paris through the Channel Tunnel to London, Crease said. With police blocking access to Heathrow, he said, it was unclear where passengers would be dropped off.

Firefighters responding to the substation early Friday found a transformer fully alight and sending flames into the night sky, Jonathan Smith, deputy commissioner of the London Fire Brigade, told reporters.

The conflagration cut power to Heathrow Terminals 2 and 4, almost 5,000 homes, and six schools. About 150 neighbors were evacuated, Smith said. By midday, the fire was mostly contained.

Prime Minister Keir Starmer said he was “in close contact with partners on the ground” and “receiving regular updates.”

“I know the situation in Heathrow is causing distress and disruption, especially for those traveling or without power in their homes,” he wrote on social media.

Heathrow officials said they did not know when power would be restored or normal operations resumed. Passengers should expect “significant disruption” to continue in the coming days, the airport said.

Some in the industry said the complete and sudden shutdown raised questions about planning and resilience.

“How is it that critical infrastructure - of national and global importance - is totally dependent on a single power source without an alternative?” Willie Walsh, head of the International Air Transport Association, said in a statement. “If that is the case - as it seems - then it is a clear planning failure by the airport.”

Heathrow has multiple energy sources, including backup generators, which worked as expected during the outage, airport officials said. But they’re not intended to power a “full operation.”

“Our back up systems are safety systems which allow us to land aircraft and evacuate passengers safely,” they said in a statement. “As the busiest airport in Europe, Heathrow uses as much energy as a small city, therefore it’s not possible to have back up for all of the energy we need to run our operation safely.”

Tens of thousands of travelers, including many who were setting off for spring break, scrambled to salvage plans. They endured long waits to reach airline personnel and watched plane seats fill within minutes. Fares in some cases more than doubled in the time between selecting a flight and submitting payment details.

Mike Prior, a director of Scott’s Travel in London, tried to rebook his daughter’s flight from Dublin to London. An experienced travel agent, he found her a seat on EasyJet for about $160. Ten minutes later, the price of a similar seat had increased to more than $350.

Prior has also been helping clients rebook. He said that flights in and out of other London airports were almost completely sold out, and that Saturday flights were filling fast.

“We’ve been in the queue for British Airways for more than an hour, and that’s on a priority line,” he said.

Georgie Bitcon was somewhere over the Atlantic when she woke to see the airplane icon pointed the wrong way on the flight map. A cabin neighbor told her the captain had said they were turning back to New York because of a power outage at Heathrow. At John F. Kennedy International Airport, gate agents told anyone who lived in New York to head home, and the rest to go to the terminal to be booked into a hotel.

Bitcon, 32, a tech sales rep, had been traveling to London to celebrate her partner’s birthday. American Airlines told her all the Friday flights were canceled, she said, and all Saturday flights were fully booked. Those still available to other European cities cost thousands of dollars.

“Monday morning was the best they could offer me,” she said. “Arriving at the start of the workweek for a birthday weekend is not ideal."

SUNDAY TRIBUNE