A “vulnerability window” lasting just one millisecond was responsible for the grounding of more than 2,000 flights across the UK earlier this month, according to a preliminary report by National Air Traffic Services (NATS).
The technical failure, which occurred on 8 September 2026, resulted in more than 2,000 flight cancellations. Gatwick Airport, the primary international hub for travellers from Chichester and West Sussex, was among the worst-affected sites during the six-hour system outage.
The millisecond defect
The report, published on Friday 18 September, identifies a specific defect within the National Airspace System (NAS). This software handles manual “squawk code” requests—the four-digit identifiers used by air traffic controllers to track individual aircraft.
Engineers found that the system corrupted flight data in the space of a millisecond when a manual request for a squawk code was interrupted by a higher-priority system message. This rare conflict triggered a safety shutdown of the automated processing system to prevent incorrect data from being displayed to controllers.
NATS has confirmed that while the outage caused significant travel disruption, at no point was the safety of aircraft in UK airspace compromised. The report also clarified that this incident was unrelated to the August 2023 NATS failure, which involved a different sub-system known as FPRSA-R.
Compensation and passenger rights
For Chichester residents seeking compensation for the 8 September disruption, the outlook remains challenging. The Civil Aviation Authority (CAA) has issued guidance stating that the incident is likely to be classified as “extraordinary circumstances.”
Under UK air passenger rights (UK261), airlines are not required to pay out fixed compensation for delays or cancellations caused by factors outside their control, such as air traffic control failures.
According to a statement from the UK Civil Aviation Authority, while statutory compensation for the delay itself may not be applicable, airlines still maintain a “duty of care.” This means passengers were still entitled to food, drink, and overnight accommodation where necessary, regardless of the cause of the delay.
Travellers who incurred out-of-pocket expenses at Gatwick or other hubs during the 8 and 9 September peak are advised to keep all receipts and submit claims directly to their airline’s customer service department. If these claims are rejected, passengers may need to rely on their private travel insurance, though many policies also exclude air traffic control industrial action or technical failure.
Independent review ordered
Transport Secretary Heidi Alexander described the level of disruption as “completely unacceptable” following a meeting with NATS CEO Martin Rolfe. The government has now ordered the CAA to conduct a full independent review into the resilience of NATS and its software testing protocols.
The review will examine the speed of the manual recovery process, which took six hours to restore full service to UK airspace, leading to a backlog of cancellations that lasted into the following day.
