Heathrow blackout: Airport took seven hours to reopen after restoring power to terminals

Power was restored to Heathrow Airport’s terminals around seven hours before flights resumed on the day it was closed by a substation fire, an investigation has found.

More than 270,000 air passenger journeys were disrupted by the closure on March 21, which led to questions over the airport's energy resilience.

An interim report by the National Energy System Operator (NESO) has found the flow of electricity to all four of the West London airport’s passenger terminals was restarted by 10.56am that day.

Flights did not resume until approximately 6pm.

The root cause of the fire that led to the airport being closed for almost an entire day is still unknown, the report said. The Metropolitan Police confirmed on March 25 that it had “found no evidence to suggest that the incident was suspicious in nature”.

Heathrow Chief executive Thomas Woldbye previously acknowledged to MPs that the airport may have been able to reopen slightly sooner, but said "the fact that the lights were on at Terminal 5, which is entirely correct, doesn’t mean the terminal was operational.

“We didn’t have all CCTV, we didn’t have fire surveillance. The fire systems would work … but the fire surveillance systems of the airport were down, so we didn’t know where the systems were up and safe. All that had to be secured before we started operation.”

NESO was commissioned by the Department for Energy Security and Net Zero and Ofgem to conduct a review into the power cut, with a final report due by the end of June, including recommendations and lessons for the future.

Disruption

The fire sparked huge disruption to global air travel with flights cancelled and passengers stranded. More than 1,000 flights were affected, scrambling the travel plans of tens of thousands of people including passengers in the UAE.

The closure paralysed one of the world's biggest travel hubs, with about 120 flights already in the air either turned back or landing in other countries such as France, Ireland and the Netherlands.

At the time, the government said it would “look harder” into how airports can withstand shocks. Heathrow said that as an energy consumer on the scale of a small city it could not possibly have backup power for all its operations.

Heathrow is the largest airport in Europe and connects to more than 230 destinations. It serves more than 82 million passengers a year, and over 26 per cent of the UK’s exports by value.

Sequence of events

The report noted that when the fire broke out, power was lost to some airport terminals and some shared systems required for overall airport operations.

The airfield ring generators automatically started to maintain supplies to the runways and essential safety systems, maintaining the ability to land aircraft safely.

Three independent electricity points each supply different areas and systems across the airport. These were reconfigured to restore the power, but this takes significant network switching, and before the event was understood to take a number of hours to enact.

The decision to close the airport was taken at 1.11am. Engineers began to restore supply to some terminals by 2am. By 6.25am low voltage was restored across the airport network and by 10:56am power was restored to all terminals. That was followed by “a period of safety checking” to ensure “safety critical systems were fully operational before passengers arriving at the airport”. Re-energisation of the wider network was completed by 2.23pm.

The report set out the sequence of events at the nearby North Hyde electricity substation. It said that on Thursday, 20 March at 11.21pm, one of three supergrid transformers tripped, followed by an associated circuit connecting it to the wider transmission system. It was later confirmed to have caught fire. Another transformer then tripped, resulting in loss of connection to the third transformer. The consequence was the loss of all electricity supplies, impacting thousands of customers including Heathrow Airport.

Resilience

Energy Secretary Ed Miliband welcomed the interim report on Thursday, pointing out the summary of the incident ruled out the possibility of suspicious activity.

“We now await the full report to understand what happened and learn lessons to strengthen UK energy resilience and protect our critical national infrastructure,” he said.

Heathrow said the report raises “important questions” for the National Grid – which owns the substation that caught fire – and Scottish and Southern Electricity Networks (SSEN), which is responsible for power distribution in the area.

A spokesperson said: “Heathrow welcomes the NESO review’s initial report, which raises important questions for National Grid and SSEN that we hope the final report will provide answers to, including the cause of the fire.

“Further clarity on how the fire started and why two transformers were subsequently impacted can help ensure greater resilience for the UK’s energy grid moving forward.”

NESO anticipates that the final report will make findings and recommendations on the resilience of energy infrastructure; the response and restoration of energy infrastructure; and the resilience of critical national infrastructure to energy disruption.

Fintan Slye, NESO's chief executive, said the power cut affected around 67,000 customers, a number of commercial customers and Heathrow Airport. “It is important that the right lessons are learnt from this incident to prevent future instances where possible and to manage them effectively when they do occur,” he said.