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

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”.