- NATS said a software defect corrupted information in part of its flight data system.
- The initial error happened on September 8 but was not recognised as a major incident until two and a half hours later.
- Around 2,000 flights were cancelled and the passenger backlog took more than two days to clear.
The UK’s biggest recent air traffic disruption was traced not to a cyber attack or military operation, but to a software defect that unfolded in a matter of milliseconds.
National Air Traffic Services (NATS), which provides air traffic control services across the UK, said its initial investigation found that a small subsection of coding in its flight data system caused information to become corrupted.
The problem began at around 10am on September 8. Yet it was not immediately clear that a major incident was developing. NATS said it took around two and a half hours for the scale of the problem to become apparent.
By then, restrictions had to be introduced to limit the number of aircraft entering affected airspace and maintain safety.
The original fault was confined to NATS’s London Area Control Centre, but its impact spread much further. Restrictions were imposed across the UK because the National Airspace System, which underpins the management of UK airspace, had to be restarted.
The result was thousands of cancelled flights and hundreds of thousands of disrupted journeys.
How a tiny error became a major outage
The chain of events began with a manual request for an aircraft code being processed by the system.
According to NATS, processing of that request was paused when the National Airspace System received a message for a higher-priority activity. When the aircraft code request resumed, the software defect meant it did not restart correctly.
That corrupted information used in subsequent flight data updates and triggered the wider failure.
NATS said the events occurred “in the space of a millisecond”.
But the consequences lasted far longer.
Although flight restrictions were in place for about six hours and NATS operations returned to normal that evening, it took more than two days to clear the backlog of passengers affected by the disruption.
The initial investigation also ruled out a cyber attack by a hostile actor or deliberate sabotage.
NATS chief executive Martin Rolfe said the problem had been identified and that mitigation was already in place while a permanent fix was being safety tested and prepared for deployment.
“This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding,” Rolfe said.
He also said the incident was not connected to a separate NATS failure in 2023, which caused widespread disruption.
Why the outage is raising bigger questions
The latest failure has renewed questions about the resilience of the systems responsible for managing Britain's increasingly busy airspace.
NATS said it has invested more than £1bn in systems and technology over the past decade and plans to submit proposals to the UK Civil Aviation Authority for another £1bn of investment through to the end of 2033.
The incident has also put pressure on Rolfe, with Ryanair, Europe's biggest airline, again calling for his resignation after previously demanding his departure following the 2023 disruption.
Rolfe apologised to passengers affected by the latest outage but defended NATS’s handling of the safety issue.
“I would like to apologise again, very sincerely, to everyone who was affected last week,” he said. “It’s our job to get people where they want to go, quickly and without delay and we are devastated when that goes wrong.”
He added that his primary responsibility was to keep UK skies safe and said that “at no point last week was safety in question”.
The government is not relying solely on NATS’s preliminary findings. Transport Secretary Heidi Alexander has ordered an independent review by the UK Civil Aviation Authority into the disruption, describing it as “completely unacceptable”.
The latest outage follows another major NATS failure in August 2023, which caused widespread travel chaos and cost airlines an estimated £100m. A smaller technical problem in July 2025 also resulted in around 150 cancellations.
For passengers, the latest incident offers a reminder of how dependent modern air travel is on highly complex digital systems. A fault lasting milliseconds can be enough to trigger restrictions across an entire network when safety requires the system to be brought back under control.
The immediate software problem has been identified and a permanent fix is being tested. The bigger question for the independent review will be whether the systems, safeguards and contingency arrangements around it are strong enough to prevent another tiny fault from creating such a large disruption.
