London's Heathrow Airport has partially resumed operations Friday evening after an electrical substation fire forced Europe's busiest airport to close for approximately 18 hours, causing widespread disruption affecting over 200,000 passengers [1].

The first British Airways flight landed just before sunset on Friday after Heathrow lifted its closure order. Further arrivals followed, including a short flight from Manchester [1]. The British government has temporarily lifted restrictions on overnight flights to help clear the mounting backlog [1].

The fire, which started around 11:23 PM Thursday at a North Hyde electrical substation about 2 miles from the airport, took firefighters seven hours to bring under control [2]. The blaze involved 25,000 liters of cooling oil and affected both main and backup power systems [2,3].

"This has been an incident of major severity. It's not a small fire," said Heathrow's chief executive Thomas Woldbye [2]. The Metropolitan Police's Counter Terrorism Command is leading the investigation, though authorities have found no evidence of foul play [7].

The closure affected approximately 1,350 flights and stranded roughly 200,000 passengers [1]. Aviation experts warn the disruption could last several days as airlines work to reposition aircraft and crews [2].

"We're talking about several days worth of disruption to get the planes recovered and start using them again to move planned and disrupted passengers," said aviation consultant John Strickland, who likened the scale of disruption to a "contained version of 9/11" [8].

British Airways, which operates about 51% of Heathrow flights, said it would initially focus on repatriating passengers who were diverted to other European airports [2]. The airline's CEO Sean Doyle warned of "huge impact on all of our customers flying with us over the coming days" [2].

The incident has raised questions about infrastructure resilience at one of the world's crucial aviation hubs. "The U.K.'s critical national infrastructure is not sufficiently hardened," said Alan Mendoza, executive director of the Henry Jackson Society security think tank [1].