A catastrophic failure in the UK's air traffic control system, which led to widespread travel chaos earlier this month, was triggered by a software defect that occurred in the space of a single millisecond, the national air traffic service Nats has confirmed.

The incident, which took place on September 8, caused the system to produce corrupted data, forcing air traffic controllers to impose significant restrictions to ensure safety. This resulted in over 2,000 flights being cancelled and left hundreds of thousands of passengers stranded, with some experiencing overnight stays at airports or being stuck abroad.

Nats chief executive Martin Rolfe stated that the software defect was "very, very obscure" and defended his position, asserting that his role is to minimize the occurrence of such events. He acknowledged the immense disruption caused, noting that any cessation of air traffic, even briefly, can lead to substantial chaos, but emphasized that passenger safety was never compromised during the outage.

Transport Secretary Heidi Alexander described the disruption as "completely unacceptable" and initiated a review by the Civil Aviation Authority to determine why the issue went undiscovered. Industry body Airlines UK is also preparing to seek compensation from Nats for the financial costs incurred by the airlines.

The initial report from Nats detailed that the software defect, occurring at 10:00 AM, meant controllers had access to reduced information. The full impact of the problem was not realized for another two and a half hours, with the system restart not commencing until after 3:15 PM. Restrictions on UK airspace were not fully lifted until 7:30 PM, and the knock-on effects, including misplaced aircraft and crew, meant the disruption took several days to fully resolve.

Mr. Rolfe expressed Nats' regret, stating the organization was "devastated" for those affected. He reiterated that the primary mission of Nats is to maintain sky safety, a responsibility he believes was upheld throughout the incident, despite the technical failure. He highlighted the extensive testing of Nats' systems, suggesting that such rare events are inherently complex and difficult to anticipate.

Reactions from airlines and passengers have been strong, with questions raised about the resilience of Nats' systems and the leadership of Mr. Rolfe. Many passengers have voiced frustration over the level of support received from airlines and the likelihood of receiving compensation, as the situation is likely to be classified as "extraordinary circumstances."

This incident marks the third significant national air traffic control issue in the past three years, raising further concerns about the reliability of critical infrastructure. The previous incidents, including a computer shutdown during the August bank holiday week in 2023, underscore a pattern of vulnerability within the air traffic management system.