- Sources: primary
- Summary: The preliminary report names a software defect in the part of the National Airspace System that allocates aircraft codes on manual request. A higher priority message paused an in-flight code request, the resume path did not restore state correctly, and the corrupted output propagated into later flight data updates within a millisecond. Restrictions ran about six hours across the whole UK even though only the London Area Control centre was affected, because the system had to be restarted and flight data reloaded, and the passenger backlog took more than two days to clear.
- Why it matters: A preemption path that resumed without restoring state turned one request into a national restriction, and the blast radius came from the restart and reload rather than from the defect.
- Follow-up: Track the final report, which NATS has not yet dated.
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