NATS, the UK’s air-traffic-control operator, said on Friday that last week’s outage was a software defect in part of its flight-data system. Reuters reports about 2,000 cancellations and days of disruption, costing airlines millions of pounds.

Chief executive Martin Rolfe rejected two lines of speculation: a link to the major 2023 incident and military intervention. Aircraft safety, he said, was not compromised. The defect sat in a “subsection” of code; mitigations are in place while a permanent fix goes through safety tests before deployment.

The operator’s report describes a race: a request for an aircraft identification code, then a higher-priority activity that paused the first update. Had the higher-priority request arrived a millisecond earlier or later, the update would have completed normally. Finding that mechanism does not close the investigation.

Transport Secretary Heidi Alexander last week demanded a NATS report and an independent Civil Aviation Authority review. She called the disruption “completely unacceptable” and asked why the problem had not been found before it caused chaos.

The August 2023 precedent cost airlines about £100 million. A smaller NATS issue in July 2025 led to 150 cancellations. Ryanair has repeatedly called for Rolfe to resign after the 2023 failure. That history explains the political pressure; it does not automatically cause this defect.

Questions remain about testing, redundancy and modernising the infrastructure. A software cause is a starting point, not a certificate that the system is “fit for the future,” as the secretary put it.

Image: Heathrow control tower / Wikimedia Commons. A UK airport tower, not the NATS operations room at Swanwick. Cropped to 16:9.

Source consulted: UK air traffic control outage caused by software defect | Reuters; NATS news.