A software defect corrupted flight data “in the space of a millisecond” and triggered the six-hour outage that cancelled more than 2,000 flights in total across the United Kingdom on Tuesday 8 September, National Air Traffic Services said in a preliminary report published Friday.
NATS, which operates the UK air traffic control system, traced the failure to a legacy and previously unknown software defect in the National Airspace System, the part of the network that allocates codes so controllers can identify individual aircraft on radar. The company said the fault stemmed from “a combination of very specific events occurring simultaneously” rather than any human error.
According to the report, the error first occurred at 10am, when engineers were notified at 10.02am that the link between the National Airspace System and the London area control system had dropped. By 10.06am the system appeared to have recovered with no operational impact, and engineers continued investigating.
The fault presented when a manual request for an aircraft code was paused for a higher-priority activity. When processing resumed, the software did not resume correctly, corrupting the output and affecting some subsequent flight data updates.
The link dropped again at 12.32pm and a major incident was declared, forcing controllers to perform some tasks manually. NATS began restricting flights at 12.45pm before the system failed completely at 1.32pm. Airlines were advised of restrictions at 1.40pm. The system was rebooted between 2.50pm and 4.09pm, with full operations resuming at 7.30pm. Clearing the passenger backlog took more than two days.
NATS chief executive Martin Rolfe apologised to those affected and said the incident was unrelated to a previous outage in 2023 or a radar issue in July last year. “The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed,” he said, adding that safety was never in question.
Transport Secretary Heidi Alexander said the disruption was “completely unacceptable”. “It’s clear we need to urgently understand why this issue was not discovered and fixed before it caused chaos,” she said. She has tasked the Civil Aviation Authority with an independent review to check the NATS findings and examine its investment plans.
The failure was the third major air traffic control failure in the UK since summer 2023. Ryanair has demanded Rolfe’s dismissal, while Airlines UK members are writing to NATS to demand compensation and a resilience plan.
The UK government owns 49 per cent of NATS, the largest shareholding in the public-private partnership, with a veto over key decisions. The CAA review will be published within six months.