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

A software defect caused widespread flight cancellations across the UK, impacting 2,163 flights and leaving 300,000 passengers stranded for two days. The glitch occurred in the National Air Traffic Services (NATS) flight processing system when a flight manually requested a squawk code, which was temporarily paused due to a higher priority activity.

The corrupted squawk code led to inaccurate flight data and resulted in the restart of the National Airspace System (NAS), grounding thousands of aircraft. The incident took only a "space of a millisecond" to cause the widespread disruption. NATS reported that only flights within the London Area Control Centre were directly affected, but the entire UK airspace experienced flight restrictions for six hours as safety became the top priority.

The UK government, which owns nearly half of NATS, criticized the incident as "completely unacceptable" and ordered an independent review to investigate the cause and improve future resilience. Transport Secretary Heidi Alexander tasked the Civil Aviation Authority (CAA) with conducting the review and assessing NATS' investment plans.

Although safety was never compromised during the disruption, passengers had limited options for compensation, with most choosing between rescheduled flights or refunds, along with basic care and assistance from airlines.