Heathrow power outage: How airport chaos unfolded as cause remains ‘unknown’

The collapse of power supplies in Spain and Portugal in April led to the cancellation of around 500 flights. Yet when London Heathrow suffered a partial loss of power on Friday 21 March, nearly three times as many departures and arrivals were grounded.

The airport’s chief executive, Thomas Woldbye, insisted the shutdown was the only possible response because of safety concerns. He told MPs on the Transport Select Committee: “On the day of the incident, our focus was very clear and immediate: to protect the safety of passengers and airport staff, to protect the security of the airport and the integrity of our borders, and to minimise disruption and resume operations as soon as possible.”

Losses to airlines could be as much as £100m, according to Nigel Wicking, chief executive of the Heathrow Airline Operators’ Committee.

“There were 1,300 cancellations on the day, and nearly 300,000 passengers were disrupted,” Mr Wicking says. More than 100 further flights were cancelled over the next couple of days.

One of two reviews into the shutdown has now been published. The National Energy System Operator (NESO) has published the North Hyde Review Interim Report, which sets out a timeline from the power perspective – though the cause of the fire remains unknown.

The Independent has combined new information from the report with on-the-day events and other sources to piece together the definitive sequence of events behind a shutdown with devastating emotional and economic consequences.

Heathrow’s own investigation, the Kelly Review, is expected to be handed to the airport’s board later this month.

Thursday 20 March

11.21pm: One of three “supergrid transformers” at the North Hyde substation in Hayes, northeast of Heathrow, goes out of service. One minute after SGT3 fails, SGT2 automatically switches into service. It had been “running on hot standby,” according to NESO.

“This operated as designed and restored a two-transformer supply into North Hyde 66kV substation, with no interruption of supply to customers,” the report says.

North Hyde usually feeds one of the three Heathrow substations. The airport has two other substations, B and C, and two other supply sources – from Iver and Laleham.

11.22pm: London Fire Brigade receives the first call about a blaze at the substation.

Heathrow CEO Thomas Woldbye later tells MPs that the fire and subsequent power failure is "a very low-probability event”.

NESO later says: “When necessary, for example in the event of a fault, it is possible to reconfigure supplies to be fed from a different point. This takes significant network switching, and prior to the event was understood to take a number of hours to enact.”

11.28pm: First fire appliance arrives on site. Nine more would join it during the night.

11.49pm: All power is lost from North Hyde when SGT1 and its circuit, which was also feeding SGT2, trips. “The sudden and extreme heat of the fire meant that all three transformers on site tripped out of service,” says Alice Delahunty, president, UK Electricity Transmission, for National Grid.

The outage hits 66,919 customers, including Heathrow Substation A.

“Power was lost to some airport terminals and some shared systems required for overall airport operations,” says NESO.

Heathrow’s airfield ring generators automatically start. They provide power to the runways and essential safety systems, maintaining the ability to land aircraft safely.

11.57pm: Within 50 minutes, 42,000 customers have had power restored. But Heathrow still has problems. Airport boss Thomas Woldbye says later that the power cut hit Terminal2, parts of Terminal 4 and the access tunnel to Terminals 2 and 3 – “critical parts of the infrastructure”.

He adds that power was also lost to “buildings that house systems that are airport-wide, such as CCTV, some of the fuelling safety systems, and security locking systems for doors”. They were lost “at least momentarily”.

Friday 21 March

12.15am: First response meeting convened by National Grid.

12.30am-1am: Thomas Woldbye later testifies: “We realised we were losing power to the airport. In our operations centre you would have seen all the red lights go, that all the systems were powering down. We had no indication as to why. We then at a slightly later stage got a call from the fire department that the substation was on fire.

“We were in a very uncertain situation for many, many hours, until 6 o’clock in the morning, because we could not get information on how long this could last, but we could see that one-third of the airport was powering down.

“This is first and foremost a safety situation. We need to make sure, when a crisis happens, that people are safe.”

12.42am: Met Police declare a “major incident”.

1.11am: Decision is made to close Heathrow all day on Friday. Soon afterwards, some flights return to their starting points while others seek diversion airports.

2.14am: Heathrow issues its first media statement, confirming closure until 11.59pm on Friday – wiping out the entire day’s operations. The statement says “We do not have clarity on when power may be reliably restored. To maintain the safety of our passengers and colleagues, we have no choice but to close Heathrow until 23h59 on 21 March 2025.

“We expect significant disruption over the coming days and passengers should not travel to the airport under any circumstances until the airport reopens.”

2.22am: London Fire Brigade drone takes photo of the height of the blaze at the SGT3 transformer.

3.30am: BBC Radio 5 Live tells listeners the news, shortly followed by LBC Radio. At the same time, Heathrow notifies the Department for Transport about the closure.

4am: By now, power has been restored to 92 per cent of customers – but not to the Heathrow power intake.

5am: Some of the first flights due to land at Heathrow on Friday morning touch down at Paris CDG. Qantas passengers from QF9 from Perth, and QF1 from Sydney and Singapore, are put on buses to London.

The chief executive of the Australian airline, Vanessa Hudson, later tells reporters: "London’s airport infrastructure was paralysed, and there was no way to continue travel by plane. A train transfer would have been logistically difficult, as hundreds of seats weren't available at short notice. Buses were the fastest available option."

5.30am: Airlines join a “community call” with Nats, the air-traffic control organisation. Nigel Wicking of the Heathrow Airline Operators’ Committee later says: “They had run out of space for aircraft to divert to in the UK. Aircraft were then going to Europe, and some were even halfway across Europe and were going back to base in India.

“It was quite a level of disruption for those passengers, let alone all of the cancellations.”

Mr Wicking’s organisation reminds airlines of their duty of care – that they provide meals and hotels where required, as well as finding alternative flights or paying refunds swiftly.

6am: “The fire was still blazing,” Heathrow boss Thomas Woldbye later says. He asserts the choice to close down was correct: “I am satisfied that the decision that we took to power up, based on our two substations, was the right one. That then implies that we would have to shut down the airport in order to power down, rewire and power up the systems. That requires closing the airport.”

The airport CEO adds the process takes about 10 hours. “We have to power down maybe 1,000 systems before we do that switch, and then we have to switch it back on and power up 1,000 systems.

“We would be able to land aircraft, but we would then be leaving them on the runways with passengers in them, not being able to process them because we did not have the safety. That is not an acceptable situation for passengers.”

At the same time, BBC Breakfast, Sky News Breakfast, GB News Breakfast and ITV’s Good Morning Britain go on air, all leading their programmes with the news from Heathrow. This is the first that some passengers due to travel on Friday 21 March get to hear about what is expected to be a complete shutdown.

6.25am: Heathrow’s control centre re-energises all primary substations by reconfiguring its private network.

9.49am Power is restored to Heathrow’s substation and all remaining customers.

10am: SSEN, the local energy provider, tells Heathrow it can draw all the electricity it needed. This is the point from which Terminal 5, according to airlines’ representative Nigel Wicking, “was fine to operate”.

But Heathrow’s Thomas Woldbye says that at 10am “fuel lines were still a question”.

10.56am: Power is restored to all the airport terminals.

12 noon: Ryanair is halfway through a very profitable Friday, with stranded passengers spending an estimated €200,000 (£170,000) on flights with Europe’s biggest budget airline.

“We took about 10,000 extra bookings,” chief executive Michael O'Leary later says. “Just another example of a Great British f***-up.” The Ryanair boss clarifies the expression, saying: “That’s an aviation term.”

Eurostar, easyJet and Jet2 also see last-minute bookings surge as people seek alternatives to Heathrow.

At about the same time, passengers on BA176 from New York are finally allowed to leave the aircraft from New York which has been on the ground in Manchester for five hours. Phil Dean told The Independent: “The pilot told us the diversion was due to a fire that had closed Heathrow, he said Manchester wasn’t prepared for the diversions and there was a queue as the airport would have to bring stairs and buses. He said we were third in the queue and anticipated we might have to wait around an hour and a half.

“At midday, we were eventually allowed to disembark. The staff seemed surprised to see us, laughing as they told us they thought all the diverted passengers had long gone. “

1.30pm: Airport teams begin safety checks to reopen for some repositioning and repatriation flights to Heathrow.

2.23pm: Heathrow’s entire power network is restored. Once the lights were back on, says NESO: “There was a period of safety checking to allow all parties operating the airport to access their systems and to ensure safety critical systems were fully operational prior to passengers arriving at the airport.”

4pm: “The airport was 100 per cent confident that all systems were safely operating across the whole network,” says the transport secretary, Heidi Alexander.

At the same time, a Microsoft Teams meeting between the airport and interested parties “maxes out” with 1,000 participants.

6.01pm: The first of 10 “repatriation flights” – bringing back some of the many diverted planes – lands at Heathrow.

8.42pm: British Airways flight BA259 becomes the first departure of the day, taking off for Riyadh. Seven more flights follow, mainly on BA. “Sean Doyle [British Airways CEO] and I were instrumental in getting some critical BA flights with passengers out on the same evening.” Mr Woldbye later says.

Saturday 22 March

2.22am: North Hyde’s SGT2 transformer, which is 100 feet from the badly damaged SGT1 and 3, is put “on load”.

4am: Heathrow opens fully – though there are 100 further cancellations as airlines struggle to get their schedules back on track.

Around a dozen further flights are grounded on Sunday due to the closure two days earlier.

Wednesday 26 March

5.13pm: Only now does London Fire Brigade confirm “cessation of firefighting” at North Hyde – five days and 18 hours after it first arrived on site to tackle the blaze.

Thursday 8 May

7am: NESO publishes the North Hyde Review Interim Report (the final report will be made by the end of June).

7.18am: A Heathrow spokesperson sends a response, saying the report “raises important questions for National Grid and SSEN that we hope the final report will provide answers to, including the cause of the fire”.

The airport statement adds: “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.”

A National Grid spokesperson said: “We now look forward to the full findings and will continue to work closely with the National Energy System Operator as its independent review process continues."

A spokesperson for SSEN Distribution welcomed “the timeline detailing the significant operational response, of which SSEN played its part”.

10.05am: Heathrow Reimagined, a lobbying group representing airlines and a leading property owner at the airport, says: “For too long, inefficient spending has been enabled by a flawed regulatory framework and resulted in a lack of basic operational resilience.”

Simon Gallagher, managing director of the energy consultancy UK Network Services, posted on LinkedIn: “It took Heathrow six hours to do the HV [high voltage] switching to get their primary network live, and a further five hours to get the full network live – this is very long time for this.

“Heathrow needs to answer questions on why it took so long to do the HV switching, and then why it took so long to open the airport.”