An air traffic control issue which led to more than 2,000 flights being cancelled was caused by a software defect and not by “any incorrect actions”, an investigation has found.
A preliminary report into the incident on September 8 was published by the National Air Traffic Services (Nats) on Friday, who said it was the result of a fault in the software that allocates codes to individual aircraft.
The fault, which happened in “the space of a millisecond”, caused flights both in and out of the UK to be axed, impacting on hundreds of thousands of passengers with disruption continuing into the following day.
The software which experienced issues usually automatically allocates codes, which are used to identify individual aircraft on radar while they are airborne.
Nats said the problem had occurred “in the space of a millisecond”.
Writing in the 18-page report, Nats chief executive Martin Rolfe said that at this stage he could confirm the incident was not related to a Nats outage in 2023 or a radar issue in July last year.
“I can also confirm that it was a software issue and not caused by any incorrect actions by either military or civil operators,” he said.
Mr Rolfe apologised again to travellers who were affected by the incident, as well for the “challenges it presented to the wider aviation system.”
The report read: “Air traffic controllers in the LAC (London Area Control) centre had lost access to some live flight data because of a technical issue within the National Airspace System (NAS) and had invoked practised fallback procedures.
“This included co-ordinating the handover of aircraft between neighbouring national and international air traffic control units manually, a task that usually happens automatically.
“As this increased controller workload, the number of aircraft permitted to fly in the airspace was reduced to maintain safety margins.”
Responding to the report, Transport Secretary Heidi Alexander said the disruption last week was “completely unacceptable” and she knew “how frustrating it was for passengers, airlines and airports”.
She said: “I have now received Nats’ report and, while I am pleased to see that the safety of passengers was protected, it’s clear we need to urgently understand why this issue was not discovered and fixed before it caused chaos.
“I have therefore tasked the CAA (Civil Aviation Authority) with conducting an independent review to check Nats’ findings and investigate their investment plans to enhance resilience in the future, along with regulatory accountability.
“It is crucial that our national infrastructure is fit for the future, and this government will make sure the aviation sector is as resilient as possible, so passengers can get to where they need to be.”
Ms Alexander previously told MPs that she did not believe the air traffic control issue was “unavoidable”.
Nats reiterated in its report that there was no evidence at this stage that the incident was caused by “a malicious actor or cyber-related activity.”
Mr Rolfe has faced renewed calls for his resignation following the travel disruption, which affected airports including Heathrow, Gatwick, Manchester and Stansted.
The CAA previously said passengers were “unlikely to be entitled to compensation for delays or cancellations that were directly caused by the incident”.
The aviation regulator’s review is due to be published within six months, while Nats said its own full report will be completed within 60 days.
Tim Alderslade, chief executive of industry body Airlines UK, said of Friday’s report: “Passengers don’t need more hollow apologies or empty promises that lessons will be learned.
“They deserve commitment and proof that the system is being fixed and so the CAA’s review must mandate real investment in Nats’ resilience, so a single fault can never again descend into nationwide disruption.
“Airlines UK is writing to Nats to demand immediate compensation for the costs incurred and a credible plan to improve resilience.”
EasyJet chief executive Kenton Jarvis said: “Repeated failures from Nats have caused unacceptable disruption for our customers leaving airlines, who worked tirelessly to support their customers, to serve as insurer of last resort yet again.
“Passengers deserve more than just another promise that lessons will be learned.
“Firm actions must be taken to ensure the UK’s air traffic control systems are modernised and reliable, and there must be a fair solution to ensure that airlines are not left to pick up the bill for Nats failures.”
Ryanair’s chief operations officer Neal McMahon repeated his call for Mr Rolfe’s resignation on Friday.
He said: “The ‘software ate my homework’ is not an acceptable explanation for the hundreds of thousands of passengers whose flights were cancelled and/or inordinately delayed on 8th Sept last.”
Last July, a “radar-related issue” caused an air traffic control failure which disrupted thousands of passengers and forced the cancellation of more than 160 flights.
In August 2023, more than 700,000 passengers suffered disruption when flights were grounded at UK airports when Nats experienced a technical glitch while processing a flight plan.
An inquiry into the 2023 meltdown, which occurred on a bank holiday, found resolving the air traffic control issue was made more difficult because of delays in verifying the password of an engineer allowed to work remotely.
