Explained | How did a fire leave Heathrow Airport in darkness, with insufficient backup?

London: The British government has ordered an urgent investigation into how a fire at an electricity substation left Heathrow Airport in London in darkness on Friday, crippling one of the world's busiest airports.

"We are determined to properly understand what happened and what lessons need to be learned," British Energy Secretary Ed Miliband said in a statement late Saturday.

Friday's closure disrupted more than 1,000 flights, leaving planes and aviation crews out of position and stranding passengers -- some of whom may not reach their destinations for a few more days.

The fire, which authorities believe was likely accidental, raised questions about the resilience of Britain's key infrastructure and whether the country has invested enough to maintain it. But some experts said the blackout was probably unavoidable given the scale of the blaze at the substation.

Britain's government has faced pressure for years to maintain and modernize the country's transportation infrastructure, like roads and trains. But the country faces severe financial pressures, with public services such as health care underfunded. Any demands for additional major infrastructure spending would create political headaches for Prime Minister Keir Starmer while he also tries to increase military spending amid flatlining economic growth.

Within hours of the airport going dark, engineering experts were questioning whether Heathrow was supported by infrastructure befitting a major world hub.

Martin Kuball, a professor of physics at the University of Bristol, wrote in an online post that the fire was a warning sign about the nation's electrical systems.

"Unfortunately, there is no resilience built into the National Grid," Kuball, a Royal Academy of Engineering chair in emerging technologies, wrote at the Science Media Center. "In part, this is because we still rely on old technology in substations that use copper windings to distribute power rather than new technology, so-called solid state transformers."

The fire struck one of three substations that convert and distribute electricity to Heathrow. "We have other substations, but to switch them in takes time," Heathrow CEO Thomas Woldbye told the BBC.

That version of events was disputed Sunday by John Pettigrew, CEO of the National Grid, who told The Financial Times that the airport could have fallen back onto the other substations. He added it was a "question for Heathrow" as to why it took the action it did. "Losing a substation is a unique event -- but there were two others available," he said.

The airport has diesel generators and batteries to power crucial safety systems like runway lights. But airport officials have said that those emergency backups could not power the entire airport.

So the airport effectively went dark, and some experts said that any airport would have faced the same outcome given similar circumstances.

Simon Gallagher, managing director at U.K. Networks Services, which advises clients on the resilience of their electricity networks, said most airports do not have the backup capacity to run their entire operations after being disconnected from the grid.

He said that would require at least 20 diesel generators, each the size of a 40-foot shipping container and producing a megawatt of electricity. That setup would provide power for about six hours before needing to be refueled, he said.

A 2023 report by the U.S. Government Accountability Office found that 24 U.S. airports had experienced 321 unplanned outages longer than five minutes from 2015 to 2022.

A power outage in 2017 disrupted operations at Hartsfield-Jackson Atlanta International Airport, the world's busiest airport, causing the cancellation of more than 1,000 flights. A 2023 outage caused by an electrical panel failure shut down a terminal at Kennedy International Airport for a day.

British authorities said they expected preliminary results from the investigation in six weeks.

Heathrow said it would operate a full schedule of more than 1,300 flights Sunday as airlines tried to clear the backlog that has disrupted travel for tens of thousands of people. On Saturday, more than 250,000 passengers passed through the airport "with punctual departures," the airport said.

In a statement, Transport Secretary Heidi Alexander said that Heathrow "uses the energy of a small city, so it's imperative we identify how this power failure happened and learn from this to ensure a vital piece of national infrastructure remains strong."