National Air Traffic Services (NATS) said on Friday 18 September that a software defect in its flight data system caused the nationwide air traffic control failure on 8 September that disrupted flights at Bristol Airport, according to a preliminary report published today.
NATS said the fault occurred in "the space of a millisecond" within a small part of its National Airspace System (NAS), which manages UK airspace. The software allocates codes to aircraft when manual requests are made. While one such request was being processed, the system received a message for a higher-priority task, which paused the request. When processing resumed, the report says, the software did not resume correctly, corrupting its output and affecting subsequent flight data updates.
Although the fault itself only affected flights in the London Area Control Centre, covering aircraft flying above roughly 24,500ft, NATS applied restrictions across the whole of the UK to maintain safety while the system was restarted. Restrictions lasted around six hours. Bristol Airport was among the airports hit, with dozens of arrivals and departures cancelled on the day. NATS said operations returned to normal that evening, but it took more than two days to clear the backlog nationally, with hundreds of thousands of passengers' journeys disrupted.
NATS chief executive Martin Rolfe said the fault has been identified and a mitigation is in place while a permanent fix is safety-tested. He confirmed the incident was unrelated to the August 2023 system outage and dismissed earlier speculation, including reports that