Nats software bug: millisecond data corruption caused six-hour outage and mass flight cancellations

National Air Traffic Services (Nats) says a previously unknown software defect in part of the National Airspace System (NAS) corrupted flight data in a millisecond and triggered a major outage that disrupted UK flights.

The fault first occurred at 10:00, engineers were notified at 10:02 and reported apparent recovery at 10:06 while continuing investigations. The link between national and London airspace dropped again at 12:32pm BST (11:32 UTC) and a major incident was declared; the system failed completely at 1:32pm BST (12:32 UTC). Nats resumed full operations at 7:30pm BST (18:30 UTC).

Nats’ preliminary report says the fault appeared when a manual request for an aircraft code was paused by a higher‑priority activity; when processing resumed the software defect prevented correct continuation and corrupted output, affecting subsequent flight data updates “in the space of a millisecond.”

Controllers carried out some tasks manually, Nats imposed flight restrictions from 12:45pm BST, airlines were advised at 1:40pm BST, and Nats rebooted the system between 2:50pm and 4:09pm BST. The disruption led to more than 2,000 cancelled flights and travel plans for hundreds of thousands of passengers were affected; recovery of the backlog took over two days.

Nats CEO Martin Rolfe said the issue is software‑related and that mitigation is in place while a permanent fix is safety tested. Transport Secretary Heidi Alexander has asked the Civil Aviation Authority (CAA) to conduct an independent review of Nats’ findings and investment plans; the CAA review will be published within six months. Airlines and industry groups have demanded compensation and a credible resilience plan.