Britain now has an explanation for the day its airspace seized up. NATS, the company that runs air traffic control in the UK, published a preliminary report on Friday. It covers the September 8 failure that canceled more than 2,000 flights and disrupted travel for over 300,000 people.
It came down to a defect buried in a small piece of old code, and the window in which it could go wrong was about one millisecond.
Timeline
How Six Hours Grounded a Country
Cancellation figures from Cirium. Sequence as described in the NATS preliminary report.
Contrail
What Went Wrong
That defect sat inside the National Airspace System, the platform that manages flight data across the UK. One part of it assigns squawk codes, which are the four-digit identifiers a plane transmits so controllers can match it on radar to the right flight plan.
A controller made a perfectly valid manual request for one of those codes. While the system was working on it, a higher-priority message arrived and briefly paused the job. When the software went back to finish, a previously unknown defect corrupted the result.
Corrupted data then spread. It generated repeated loss-of-connection messages between London Area Control and the wider system, and controllers had to fall back on manual procedures. Handling traffic that way is slower and safer only if you reduce the number of aircraft, so restrictions went out across the country. NATS handled about 1,800 fewer flights than planned that day.
What It Wasn't
NATS ruled out a few explanations that had been circulating. Chief executive Martin Rolfe said there's no evidence of a cyberattack or any malicious activity, and he dismissed speculation about military interference. The report also found no incorrect actions by civil or military operators.
NATS says this was unrelated to its August 2023 flight-planning outage, the one that stranded travelers over a UK bank holiday weekend, and unrelated to a radar problem in July 2025. Three major failures in three years is still three major failures, which is why the resilience question hasn't gone away.
Who Got Hit Hardest
Ryanair canceled 260 flights affecting 48,000 passengers and British Airways canceled close to 200, according to aviation analytics firm Cirium. Heathrow, Gatwick, Manchester and Birmingham took the worst of it.
Transport Secretary Heidi Alexander, who requested this report, called the disruption completely unacceptable. Ryanair has criticized NATS over resilience and is seeking compensation for its costs. Rolfe apologized for the failure and rejected calls to resign.
What Happens Next
A mitigation is already in place, and NATS says a permanent fix is being safety tested before it goes live. The full investigation is due within 60 days of the incident. It'll look at root causes, at how resilient the system is, and at how NATS handled its response and communications. The Civil Aviation Authority is running an independent review alongside it.
If You Were Caught Up In It
Compensation for this kind of event is where travelers usually get a nasty surprise. Under UK rules, airlines owe cash compensation when a cancellation is their fault, but an air traffic control failure is generally treated as an extraordinary circumstance outside their control. That means most passengers won't get a compensation payment.
What airlines do still owe is care. If you were delayed long enough, that covers meals and refreshments, and a hotel plus transport when you were stuck overnight. If you paid for those yourself and kept receipts, you can still claim them back. You're also entitled to a rebooking or a full refund when your flight was canceled, and that right doesn't depend on whose fault it was.
If your airline sent you a blanket denial citing the outage, the compensation part is probably right and the care part may not be. Those are two different claims, and they're worth filing separately.
