An air traffic control issue which led to more than 2,000 flights being cancelled was caused by a defect in the software that allocates codes to individual aircraft, an investigation has found.
A preliminary report into the incident on September 8 was released by the National Air Traffic Services (Nats) on Friday, who said that the issue was not caused by any “incorrect actions”.
The fault caused flights both in and out of the UK to be axed, impacting hundreds of thousands of passengers.
The software which experienced issues usually allocates codes to individual aircraft automatically when manual requests are made, Nats said.
The codes are used to identify flights on radar while they are airborne.
Writing in the report, Nats chief executive Martin Rolfe said that at this stage he could confirm the incident was not related to a Nats outage in 2023 or a radar issue in July last year.
“I can also confirm that it was a software issue and not caused by any incorrect actions by either military or civil operators,” he said.
Mr Rolfe apologised again to travellers who were affected by the incident, as well for the challenges it caused to the aviation industry.
Responding to the report, Transport Secretary Heidi Alexander said the disruption last week was “completely unacceptable” and she knew how frustrating it was for passengers, airports and airlines.
She said: “I have now received Nats’ report and, while I am pleased to see that the safety of passengers was protected, it’s clear we need to urgently understand why this issue was not discovered and fixed before it caused chaos.
“I have therefore tasked the CAA (Civil Aviation Authority) with conducting an independent review to check Nats’ findings and investigate their investment plans to enhance resilience in the future, along with regulatory accountability.
“It is crucial that our national infrastructure is fit for the future, and this Government will make sure the aviation sector is as resilient as possible, so passengers can get to where they need to be.”
The CAA report is due to be published within six months.
Mr Rolfe has faced renewed calls for his resignation following the incident, which impacted airports including Heathrow, Gatwick, Manchester and Stansted.
Last July, a “radar-related issue” caused an air traffic control failure which disrupted thousands of passengers and forced the cancellation of more than 160 flights.
In August 2023, more than 700,000 passengers suffered disruption when flights were grounded at UK airports when Nats suffered a technical glitch while processing a flight plan.
