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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 that the 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”.
The transport secretary, Heidi Alexander, said the industry regulator the Civil Aviation Authority (CAA) would now “check their findings” and investigate Nats’ plans.
Hundreds of thousands of passengers had their travel disrupted after the Nats failure on Tuesday 8 September, which initially only stopped flights departing in the London area but eventually spread to disrupt travel across the UK and beyond by the evening.
Airlines called for the dismissal of the Nats chief executive officer, Martin Rolfe, after the third major air traffic control failure in the UK since summer 2023.
However, Rolfe said the incident was unrelated to the previous failures and dismissed speculation that it was caused by military intervention.
The preliminary report said that the failure was a software defect in part of the National Airspace System, which allocates codes to individual aircraft to identify flights on radar.
The report said that while a manual request for an aircraft code was being processed, another request paused it, adding: “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.”
Rolfe said: “The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed.”
Nats said restrictions were put in place across the UK so that the system could be restarted. The restrictions were in place for six hours and it took more than two days for the backlog of passenger disruption to be cleared, with hundreds of thousands of passengers’ travel plans disrupted.
Rolfe 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.”
However, Alexander said: “The disruption we saw last week was completely unacceptable and I know how frustrating it was for passengers, airlines and airports.”
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She said she had received Nats’ report, adding: “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 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.”
The CAA’s review will be published within six months.
