A legacy, previously unknown software defect corrupted flight data in a millisecond and caused a six-hour failure in part of the UK’s air traffic control system on Tuesday 8 September, National Air Traffic Services said. The outage delayed or cancelled flights across the UK and beyond, disrupting hundreds of thousands of passengers and contributing to more than 2,000 cancellations.
How the failure unfolded
The defect affected the National Airspace System, which allocates identification codes to aircraft shown on radar. The report said the fault appeared when a manual aircraft-code request was paused for another higher-priority activity. When processing resumed, the software produced corrupted output that affected subsequent flight-data updates.
Engineers were notified at 10.02am after the link between the National Airspace System and the London area control system dropped. They reported an apparent recovery at 10.06am, but the link dropped again at 12.32pm and a major incident was declared. Flights were restricted from 12.45pm, the system failed completely at 1.32pm and full operations resumed at 7.30pm after a reboot. Read the context: UK air traffic control disruption enters a second day.
Regulator to examine Nats response
Transport secretary Heidi Alexander said the report did not explain why the defect had not been discovered and fixed earlier. She has tasked the Civil Aviation Authority with independently checking Nats’ findings, examining its investment plans for future resilience and investigating regulatory accountability.
The CAA review is due to be published within six months. Airlines UK said carriers were seeking compensation for costs incurred and a credible resilience plan. EasyJet said passengers needed firm action, while Ryanair called for Nats chief executive Martin Rolfe to accept responsibility and resign.
Rolfe said the incident was unrelated to earlier failures and was not caused by incorrect actions by military or civil operators. He said a mitigation was in place while a permanent fix was safety-tested and deployed, and said safety had not been in question during the incident.
What happens next
The Civil Aviation Authority will conduct its independent review and publish it within six months. Nats said a permanent software fix would be safety-tested and deployed, while airlines are seeking compensation and a resilience plan.
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