A previously unknown flaw buried in the UK's air traffic control software caused the glitch that grounded and delayed more than 2,000 flights earlier this month, according to Nats' preliminary investigation report.
The failure hit on Tuesday 8 September, when Nats — the company managing UK airspace — flagged a technical issue. Around 2,163 flights were affected, and aviation analytics firm Cirium estimated that at least 300,000 passengers saw plans disrupted, given a roughly 30 per cent cancellation rate. Chaos rolled on into the following day at Heathrow, Gatwick, Stansted and Manchester before schedules stabilised by Thursday evening.
So what actually went wrong? The report points to a "legacy and previously unknown defect" in a software module inside the National Airspace System, the setup that handles squawk codes — the identification signals aircraft transmit so controllers can match each plane's altitude, speed and direction to its flight plan. A perfectly routine, valid request for a squawk code came in from a controller. Midway through processing it, the system was told to handle a higher-priority task, which is a normal everyday function. But when it switched back to the original request, the faulty code corrupted the output. Nats says nothing abnormal or invalid about the flight plan or the request triggered the failure — the bug had simply never surfaced before.
Nats chief executive Martin Rolfe traced the problem to a small subsection of coding and said a mitigation is in place while a permanent fix undergoes safety testing. He confirmed the incident was unrelated to the far bigger August 2023 outage, which disrupted around 700,000 passengers, and ruled out speculation of military involvement — something the Ministry of Defence also denied after weekend reports suggested a British military aircraft's flight plan was to blame.
The political fallout has been swift. The government owns 49 per cent of Nats, and Transport Secretary Heidi Alexander summoned Rolfe to account for the mess, saying the disruption was "completely unacceptable" and that she did not believe the issue was unavoidable. She has ordered the Civil Aviation Authority to run an independent review to verify Nats' findings and scrutinise the company's investment plans for future resilience. Ryanair boss Michael O'Leary has gone further, calling for Rolfe to be sacked. A cyber-attack has been ruled out.
For travellers, the takeaway is that safety was never compromised — this was a data-processing failure, not a loss of control over aircraft. But it is the second major Nats outage in just over two years, and until the CAA review reports back, a resilience question hangs over UK airspace. If you are flying out of a major UK airport, the usual advice applies with extra weight: sign up for your airline's delay notifications, know your rights under UK261 for compensation on cancellations and long delays, and consider travel insurance that covers disruption. Allow buffer time if you have a connection or a same-day commitment like a cruise departure or wedding.
Nats has apologised to everyone affected. The story is still developing, with the CAA probe now the next milestone to watch.