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Software Defect Caused Air Traffic Outage, NATS Says
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A paused aircraft code request that failed to resume correctly triggered restrictions across UK airspace on 8 September, according to the company's preliminary findings.
A software defect caused the air traffic control failure that grounded thousands of flights earlier this month, according to preliminary findings published by NATS on Friday.
The UK's main provider of air traffic services said a defect in its flight data system produced corrupted information, which then affected subsequent flight data updates. Restrictions were imposed to limit air traffic and maintain safety.
According to the preliminary report, a manual request for an aircraft code was being processed when the National Airspace System received a message relating to a higher priority activity, pausing the request. When processing resumed, the software defect meant it did not resume correctly, setting off the chain of events that led to the grounding. NATS said the sequence occurred "in the space of a millisecond".
The initial error happened at 10am on 8 September, but it was two and a half hours before it became apparent that a major incident was under way. Restrictions were in place for six hours and operations returned to normal that evening, though it took more than two days to clear the resulting backlog. Around 2,000 flights were cancelled and hundreds of thousands of journeys disrupted.
Around 2,000 flights were cancelled and hundreds of thousands of journeys disrupted.
Although the fault affected only the London Area Control centre, restrictions were applied across the UK so that the National Airspace System could be restarted.
Cyber attack ruled out
NATS said its investigation had ruled out a cyber attack by a hostile actor, and had also ruled out sabotage. Martin Rolfe, the company's chief executive, had publicly discounted a cyber attack on 9 September, the day after the incident and before the investigation concluded.
Mr Rolfe said on Friday: "This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding. The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed."
He added that the shutdown was not related to the 2023 failure, and said that while he apologised to those affected, his primary responsibility was safety. "At no point last week was safety in question," he said.
The findings published on Friday are NATS's own. Heidi Alexander, the Transport Secretary, who requested the preliminary report, has also ordered an independent review by the Civil Aviation Authority. That review has not yet reported. Ms Alexander described the disruption as "completely unacceptable".
Previous failures
The incident is the second major air traffic control failure in three years. A shutdown in August 2023 caused widespread disruption and cost airlines an estimated £100 million.
The CAA's independent review of that failure found more than 700,000 passengers were affected, and made 34 recommendations. It identified shortcomings in NATS's contingency planning, its management of engineering resources and its arrangements for notifying airlines and airports during an incident. The full report is published as CAP2993.
A smaller technical issue in July 2025 caused around 150 cancellations. Ryanair, Europe's largest airline, has repeatedly called for Mr Rolfe to resign since the 2023 failure, and some have renewed those calls following this month's disruption.
NATS said it had invested more than £1 billion in systems and technology over the past decade, and intends to submit plans to the CAA to invest a further £1 billion before the end of 2033.
