National Air Traffic Services (NATS) said on Friday 18 September that a software defect in its flight data system caused the air traffic control failure on 8 September that disrupted flights at Heathrow and across the UK, according to a preliminary report published today.
NATS said the fault occurred in "the space of a millisecond" within a small part of its National Airspace System (NAS), the system that manages UK airspace. The software allocates codes to aircraft when manual requests are made. While one such request was being processed, the system received a message for a higher-priority task, pausing the request. When processing resumed, the report says, the software did not resume correctly, corrupting the output and affecting subsequent flight data updates.
With reduced information available to air traffic controllers, NATS put restrictions in place across the whole of the UK to maintain safety, even though the fault itself only affected flights in the London Area Control Centre, covering aircraft flying above roughly 24,500ft. Restrictions lasted around six hours. Although NATS said operations returned to normal the same evening, it took more than two days to clear the backlog, with hundreds of thousands of passengers' journeys disrupted, including many travelling through Heathrow.
NATS chief executive Martin Rolfe said the fault has been identified and a mitigation is in place while a permanent fix is safety-tested. He confirmed the incident was unrelated to the August 2023 system outage and rejected earlier speculation, including reports that a military aircraft was to blame. "At no point last week was safety in question," he said, apologising to passengers affected.
The report was commissioned by transport secretary Heidi Alexander. According to the Guardian, ministers said the report leaves questions unanswered over why the defect was not discovered and fixed sooner. The BBC reported that the incident led to more than 2,000 flight cancellations nationwide.
Passengers are unlikely to receive compensation for the disruption, as regulators have judged the incident to fall under extraordinary circumstances exempting airlines from payouts.
A full report is expected to follow. NATS said it has invested more than 1 billion pounds in its systems over the past decade and plans to submit proposals to the Civil Aviation Authority for a further 1 billion pounds of investment by the end of 2033.
What this means for your journey: with the root cause now identified but a permanent fix still being tested, isolated delays remain possible at Heathrow in the coming weeks. Check your flight status before you travel, and if you are driving to the airport, see our Heathrow parking guide for up to date advice on avoiding queues.