A previously unknown software defect that emerged during a manual aircraft identification-code request caused the UK air traffic control disruption of 8 September, according to a preliminary investigation by NATS.
The air navigation services provider said the fault corrupted flight data in the space of roughly a millisecond. Controllers were left with reduced information, prompting restrictions across UK airspace while the National Airspace System was restarted and flight data reloaded.
What NATS says went wrong
NATS said the problem occurred in a small part of the National Airspace System responsible for allocating identification codes to aircraft when a manual request is made. Such codes are normally assigned automatically and allow flights to be identified on radar.
While one manual request was being processed, the system received a message for a higher-priority activity. The original request was paused, but a previously unknown software defect meant processing did not resume correctly. The output became corrupted and affected some subsequent flight-data updates.
NATS said the sequence occurred in around one millisecond, illustrating how a very narrow software condition could have consequences across a highly interconnected air traffic system.
Why restrictions spread across the UK
The software problem directly affected flights handled by the London Area Control centre, primarily higher-level traffic operating above 24,500 feet. However, NATS said restrictions had to be imposed across the UK so the wider system could be safely restarted and flight data reloaded.
The restrictions remained in place for about six hours. NATS operations returned to normal that evening, but airlines and airports needed more than two days to clear the resulting passenger backlog.
More than 2,000 flights were delayed, cancelled or diverted during the disruption, according to reporting on the incident, affecting the travel plans of hundreds of thousands of passengers.
NATS says safety was not compromised
NATS chief executive Martin Rolfe said safety was never in question. The restrictions were introduced precisely because controllers had reduced information and traffic therefore needed to be limited while the system was restored.
The preliminary investigation also said the incident was unrelated to the major UK air traffic outage in August 2023. NATS said it had found no evidence that a cyberattack or military intervention caused the September failure.
Mitigation is already in place
NATS says it has identified the defect and introduced mitigation while a permanent software fix is safety-tested before deployment.
The company has described the document released on 18 September as a preliminary report, meaning further investigation will continue. The report was requested by the UK transport secretary after the scale of the disruption renewed scrutiny of the resilience of the country's air traffic infrastructure.
What the findings mean for passengers and airlines
The immediate technical problem has been contained, but the incident demonstrates how failures in core air traffic systems can quickly propagate into airline schedules. Aircraft and crews displaced by cancellations and diversions can continue to affect operations after the original system is restored.
For passengers, that is why a six-hour restriction can create disruption lasting days. Airlines must reposition aircraft and crews, airports have to absorb changed schedules and travellers whose flights were cancelled need to be rebooked.
The full investigation will be important for determining why the software condition had not previously been detected and what additional resilience measures may be required. For now, NATS says mitigation is operating while the permanent fix goes through safety testing.
