Millisecond software fault caused UK flight chaos, NATS says

A previously unknown software defect that took about one millisecond to trigger caused the disruption that hit UK air traffic on 8 September, NATS, the UK’s air traffic control provider, has said.

Martin Rolfe, the NATS chief executive, said the fault was “a software issue in a specific part of our flight data system” that engineers had traced to a small subsection of coding.

Rolfe confirmed the incident was unrelated to a major system outage in August 2023, and dismissed speculation that it had been caused by military intervention.

In a preliminary report published on 18 September, NATS said it had expected to handle around 8,000 flights that day but handled 6,094, and that the incident led to more than 2,000 flights being delayed, cancelled or diverted.

Restrictions on UK airspace were in place for around six hours, with departures from UK airports stopped for a total of about four and a half hours. It then took NATS more than two days to clear the backlog of disruption that followed, with hundreds of thousands of passengers’ travel plans affected.

The report said the defect lay in the part of the system that allocates identification codes to aircraft, and was triggered when a higher-priority request arrived during the exact millisecond a code was being processed.

The fault has been identified and a mitigation is in place while a permanent fix is safety-tested and deployed.

Rolfe apologised “very sincerely, to everyone who was affected last week”, saying it was NATS’s “job to get people where they want to go, quickly and without delay”.

“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,” Rolfe added.

The preliminary report was requested by Transport Secretary Heidi Alexander, NATS said, and a full Major Incident Investigation would be completed within 60 days of the incident.