2,000 UK flight cancellations linked to millisecond NATS defect

A NATS report has linked major disruption in UK airspace to a previously unknown software defect that corrupted a manually requested squawk code during processing.

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

The report says there was “nothing abnormal or invalid” about the request. A second, higher-priority activity temporarily paused its processing. When processing resumed, the generated squawk code was corrupted, and some later flight data suffered the same problem. The sequence occurred within a millisecond.

Squawk codes help controllers track aircraft during flight plans. With some flight data inaccurate, safety was prioritized. The defect affected flights within the London Area Control Centre, while restrictions were imposed across the UK for around six hours as the National Airspace System was safely restarted.

In total, 2,163 flights were affected and 300,000 passengers were impacted, including at Heathrow, Gatwick, Stansted and Manchester. The UK government, which owns just under half of NATS, has asked the CAA to conduct an independent review. Most affected passengers will not receive compensation, but the CAA says they could choose a rescheduled flight or a refund and were entitled to care and assistance.