Thousands of passengers remained in limbo on Saturday, and airlines warned that severe disruption would last for days as they scrambled to relocate planes and crews and get travellers to their destinations.
Friday's travel mayhem raised concerns about Britain's ability to withstand disasters or attacks on critical infrastructure.
Inconvenienced passengers, angry airlines and concerned politicians all want answers about how one seemingly accidental fire could shut down Europe's busiest air hub.
"This is a huge embarrassment for Heathrow airport. It's a huge embarrassment for the country that a fire in one electricity substation can have such a devastating effect," said Toby Harris, a Labour Party politician who heads the National Preparedness Commission, a group that campaigns to improve resilience.
Heathrow said it had "hundreds of additional colleagues on hand in our terminals and we have added flights to today's schedule to facilitate an extra 10,000 passengers".
It advised passengers to check with their airline before going to the airport.
British Airways, Heathrow's biggest airline, said it expected to operate about 85 per cent of its 600 scheduled flights at the airport on Saturday. It said that "to recover an operation of our size after such a significant incident is extremely complex."
While many passengers managed to resume stalled journeys, others remained in limbo.
Laura Fritschie from Kansas City was on vacation with her family in Ireland when she learned that her father had died. On Saturday she was stranded at Heathrow after her BA flight to Chicago was cancelled at the last minute.
"I'm very frustrated," she said.
"This was my first big vacation with my kids since my husband died, and ... now this. So I just want to go home."
More than 1300 flights were cancelled and some 200,000 people were stranded on Friday after an overnight fire at a substation 3.2 kilometres away from the airport cut power to Heathrow, and to more than 60,000 properties.
Residents in west London described hearing a large explosion and then seeing a fireball and clouds of smoke when the blaze ripped through the substation.
The fire was brought under control after seven hours, but the airport was shut for almost 18.
A handful of flights took off and landed late Friday.
Police said they do not consider the fire suspicious, and the London Fire Brigade said its investigation would focus on the electrical distribution equipment at the substation.
Still, the huge impact of the fire left authorities facing questions about Britain's infrastructure.
The government acknowledged that authorities had questions to answer and said a rigourous investigation was needed to make sure "this scale of disruption does not happen again".
Harris, from the preparedness commission, said the airport shutdown points to a broader problem with Britain's economy and infrastructure.
"The last 40, 50 years we've tried to make services more efficient," he said.
"We've stripped out redundancy, we've simplified processes. We've moved towards a sort of 'just in time' economy. There is an element where you have to make sure you're available for 'just in case.' You have to plan for things going wrong."
Heathrow chief executive Thomas Woldbye said he was "proud" of the way airport and airline staff had responded.
"Remember, the situation was not created at Heathrow Airport," he told the BBC.
"The airport didn't shut for days. We shut for hours."
He said Heathrow's backup power supply, designed for emergencies, worked as expected, but it wasn't enough to run the whole airport, which uses as much energy as a small city.
"That's how most airports operate," said Woldbye, who insisted "the same would happen in other airports" faced with a similar blaze.
Heathrow is one of the world's busiest airports for international travel and saw 83.9 million passengers last year.
Friday's disruption was one of the most serious since the 2010 eruption of Iceland's Eyjafjallajokull volcano, which shut Europe's airspace for days.
Passengers on about 120 flights were in the air when Friday's closure was announced and found themselves landing in different cities, and even different countries.
Mark Doherty and his wife were halfway across the Atlantic when the in-flight map showed their flight from New York's John F. Kennedy Airport to Heathrow was turning around.
"I was like, you're joking," Doherty said before the pilot told passengers they were heading back to New York.
Doherty called the situation "typical England — got no back-up plan for something happens like this. There's no contingency plan."
