A software defect in part of the UK’s air traffic control system corrupted flight data “in the space of a millisecond”, leading to a six-hour outage and mass airline cancellations and delays across the UK last week, National Air Traffic Services has said.
But ministers said the Nats report into the incident still left questions unanswered, saying they needed “to urgently understand why this issue was not discovered and fixed before it caused chaos”.
According to the report, the error first occurred at 10am and was notified to engineers who investigated – but the system appeared to be working again, until 12.30pm when a repeat occurred and a major incident was declared.
The transport secretary, Heidi Alexander, said the Civil Aviation Authority (CAA) industry regulator would now “check their findings” and investigate Nats more widely.
Hundreds of thousands of passengers had their travel disrupted after the Nats failure on Tuesday 8 September, which ended up delaying and cancelling flights across the UK and beyond late into the next day.
Airlines have called for compensation and a “credible plan”, with Ryanair demanding the dismissal of the Nats chief executive officer, Martin Rolfe, after the third major air traffic control failure in the UK since summer 2023.
A government source said the CAA review would be “a chance to fundamentally investigate” if Nats was fit for purpose.
Rolfe said the incident was unrelated to the previous failures. He said he could “confirm that it was a software issue and not caused by any incorrect actions by either military or civil operators”.
The preliminary report said the failure was a “legacy and previously unknown software defect and a combination of very specific events occurring simultaneously” in part of the National Airspace System (NAS), which allocates codes to individual aircraft to identify flights on radar.
Nats said the software error occurred at 10am, with engineers first notified at 10.02am that the link between the NAS and London area control system had dropped. The engineers reported at 10.06am that the system had apparently recovered with no operational impact, but continued to investigate.
The report said the fault presented when a manual request for an aircraft code was being processed but paused for another, unspecified “higher priority activity” request. It said: “When processing of the aircraft code request resumed, the software defect meant it did not resume correctly and the resulting output was corrupted, and affected some subsequent flight data updates. This happened in the space of a millisecond.”
At 12.32pm the link between the national and London airspace management dropped again and a major incident was declared, with controllers having to conduct some tasks manually. By 12.45pm, Nats was restricting flights as the system continued to glitch, before failing completely at 1.32pm.
At 1.40pm airlines were advised of restrictions, and between 2.50pm and 4.09pm Nats rebooted the system, eventually resuming full operations at 7.30pm – but leaving mass congestion, displaced aircraft and hundreds of thousands of delayed passengers in its wake.
It took more than two days for the backlog of passenger disruption to be cleared, with more than 2,000 flights cancelled in total and hundreds of thousands of people’s travel plans disrupted.
Rolfe said: “The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed.”
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He added: “I would like to apologise again, very sincerely, to everyone who was affected last week. It’s our job to get people where they want to go, quickly and without delay and we are devastated when that goes wrong. However, our primary role is to keep our skies safe, and everyone who flies through them. At no point last week was safety in question.”
The transport secretary said: “The disruption we saw last week was completely unacceptable and I know how frustrating it was for passengers, airlines and airports.”
Alexander said she had received Nats’ report, adding: “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 with conducting an independent review to check Nats’ findings and investigate their investment plans to enhance resilience in the future, along with regulatory accountability.”
The CAA’s review will be published within six months.
Tim Alderslade, the chief executive of Airlines UK, a lobby group representing the industry, said: “The CAA’s review must mandate real investment in Nats’ resilience so a single fault can never again descend into nationwide disruption.”
He said his members were writing to Nats to “demand immediate compensation for the costs incurred and a credible plan to improve resilience”.
The EasyJet chief executive, Kenton Jarvis, said: “Passengers deserve more than just another promise that lessons will be learned. Firm actions must be taken … and there must be a fair solution to ensure that airlines are not left to pick up the bill.”
The Ryanair chief operations officer, Neil McMahon, said: “‘The software ate my homework’ is not an acceptable explanation … How many times must the UK ATC system collapse before Martin Rolfe accepts responsibility and resigns.”
