NATS, the UK’s major provider of air traffic services, has published a preliminary report into the widespread disruption that occurred earlier this month. The incident on Tuesday, 8 September, was caused by a software defect in a small part of the National Airspace System, specifically within the flight data system that allocates aircraft codes. According to CEO Martin Rolfe, the issue stemmed from a manual code request being paused when a higher priority message arrived.

When processing resumed, a software bug prevented correct execution, resulting in corrupted output within a millisecond. This triggered safety restrictions across the UK, affecting the London Area Control centre and grounding higher-level flights above 24,500ft. Restrictions lasted approximately six hours, with normal operations resuming that evening.

However, clearing the backlog of passenger disruption took over two days, impacting hundreds of thousands of travel plans. Rolfe emphasized the incident was unrelated to the August 2023 system outage and dismissed military speculation. Over the past decade, NATS has invested over one billion pounds in technology, with plans to invest a further billion by the end of 2033.

A full investigation is pending.