NATS report reveals software defect behind major UK air traffic control outage

NATS has published its preliminary investigation into the major UK air traffic control disruption on 8 September, revealing that a previously unknown software defect caused the outage.

The disruption affected more than 2,000 flights across the UK, with airports and airlines experiencing widespread delays and cancellations as restrictions were placed on UK airspace.

According to NATS, the problem occurred within the flight data processing system used by air traffic controllers.

The incident began when a valid manual request for a squawk code was being processed. The process was then interrupted by a separate higher-priority activity.

When the original process resumed, a previously unknown software defect caused the system to produce corrupted data.

NATS said the sequence happened within milliseconds.

With the system no longer able to reliably process flight information, restrictions were introduced across UK airspace while engineers worked to restore the system.

The restrictions lasted for around six hours, but the effects continued throughout the day as airlines and airports dealt with the resulting disruption.

Major UK airports, including Heathrow, Gatwick and Manchester, were among those affected, with passengers facing cancellations, delays and changes to their travel plans.

NATS has since introduced mitigation measures to reduce the risk of the issue happening again.

A permanent software fix is now being developed and will undergo safety testing before being introduced.

NATS also confirmed that the incident was not caused by incorrect actions by the personnel operating the system and was separate from the major air traffic control outage experienced in August 2023.

The organisation said safety was maintained throughout the disruption, with traffic restrictions used while the system was recovered.

The Civil Aviation Authority is also carrying out an independent review of the incident and NATS’ findings.

The NATS investigation provides the first detailed explanation of what caused the disruption, with further findings expected as the independent review continues.

Source: NATS preliminary investigation into the 8 September 2026 incident.