2,000 UK flight cancellations caused by ‘previously unknown defect’ that happened in the ‘space of a millisecond’ — travel chaos took days to fix and left hundreds of thousands stranded

• September 18, 2026

• Nats report reveals a software defect caused days of flight disruption
• The defect occurred in the ‘space of a millisecond’
• Severity of the incident was not identified until 2.5 hours later

The UK airspace came to a grinding halt earlier this month after the UK’s National Air Traffic Services (Nats) announced that there was a glitch in its flight processing system.

Following an investigation, NATS has determined that the fault was caused by a manual request for a squawk code that became corrupted, forcing the National Airspace System (NAS) that covers the UK to be restarted, grounding and diverting thousands of aircraft.

The disruption began in the morning of 8 September, and while the initial glitch was fixed within six hours, the subsequent disruption across the UK lasted over two days leaving hundreds of thousands of passengers stranded.

Squawk code corrupted in the ‘space of a millisecond’

Squawk codes are used to track and identify aircraft during flight plans, allowing air traffic controllers to monitor altitude, speed, direction, and flight progress.

The glitch occurred when a flight manually requested a squawk code, with the report noting that there was “nothing abnormal or invalid” about the request. During the processing of this request, a second “higher priority activity” came through to NAS, temporarily pausing the processing of the manually requested squawk.

When the process resumed the generated squawk was corrupted, causing some subsequent flight data to suffer the same fate. All of this occurred within the “space of a millisecond”, the report states.

With flight data now inaccurate for some flights, safety had to be prioritized due to the limited information available at the time. Only flights within the London Area Control Centre were affected by the software defect, but in order to safely restart the NAS, flight restrictions were put in place for around six hours across the UK.

But the report also notes that the severity of the issue was only discovered two and a half hours

Due to the number of cancelled and delayed flights, disruption continued for two days as stranded passengers waited to catch rescheduled flights.

All in all, 2,163 flights were affected by the restrictions and 300,000 passengers were impacted across airports of all sizes, including major international transport hubs such as Heathrow, Gatwick, Stansted and Manchester.

Scrutiny and criticism directed at Nats

The UK government, which owns just under half of Nats, has placed additional pressure on the service, with Transport Secretary Heidi Alexander labelling the disruption as “completely unacceptable”.

“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,” the transport secretary said. “I have therefore tasked the CAA 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.”

“I would like to apologise again, very sincerely, to everyone who was affected last week,” Martin Rolfe, Chief Executive of NATS, said in a statement. “It’s our job to get people where they want to go, quickly and without delay and we are devastated when that goes wrong. However, our primary role is to keep our skies safe, and everyone who flies through them. At no point last week was safety in question.”

Affected passengers have also raised questions about the level of support they received. Due to the extraordinary circumstances involved in the disruption, almost all of those affected will not receive compensation. According to the Civil Aviation Authority, passengers had the option of choosing between a rescheduled flight or a refund, and were entitled to care and assistance from airlines including food, drink, accommodation, and transfers.

Via BBC

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Nats report reveals a software defect caused days of flight disruption The defect occurred in the ‘space of a millisecond’ Severity of the incident was not identified until 2.5 hours later The UK airspace came to a grinding halt earlier this month after the UK’s National Air Traffic Services (Nats)…