NATS, the UK’s major provider of air traffic services, has today published a preliminary report into the events which led to widespread disruption to travellers earlier this month.

This initial report was requested by Heidi Alexander, Secretary of State for Transport. It describes what caused the problem and its resolution, with a full report to follow.

Martin Rolfe, Chief Executive Officer, confirmed that this incident was unrelated to the major system outage in August 2023. He also dismissed speculation that it was caused by military intervention.

“This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding,” he said. “The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed.”

The preliminary report reveals that the incident on Tuesday 8 September was caused by a software defect in a small part of the National Airspace System (NAS), which underpins the management of UK airspace. The software in question allocates codes to individual aircraft when manual requests are made; these are usually allocated automatically. These codes are used to identify flights on radar when they are airborne.

While a manual request for an aircraft code was being processed, the system received a message for a higher priority activity which resulted in the aircraft code request being paused. When processing of the aircraft code request resumed, the software defect meant it did not resume correctly and the resulting output was corrupted, and affected some subsequent flight data updates. This happened in the space of a millisecond.

With reduced information available to controllers, restrictions were put in place to limit air traffic to maintain safety. Although the software issue only affected flights in the London Area Control centre – higher level flights operating mainly above 24,500ft – the restrictions had to be placed across the UK so that the NAS could be restarted and flight data reloaded.

Restrictions were in place for some six hours and while NATS operations returned to normal the same evening, it took more than two days for the backlog of passenger disruption to be cleared, with hundreds of thousands of passengers’ travel plans disrupted.

Mr Rolfe said: “I would like to apologise again, very sincerely, to everyone who was affected last week. It’s our job to get people where they want to go, quickly and without delay and we are devastated when that goes wrong. However, our primary role is to keep our skies safe, and everyone who flies through them. At no point last week was safety in question.”

Over the past 10 years, NATS has invested well over one billion pounds in systems and technology, and we will soon submit plans to the Civil Aviation Authority to invest a further billion pounds by the end of 2033.