Expert comment: NATS air traffic control outages

News

Posted on Tuesday 22 September 2026

Following widespread disruption caused by recent outages in the UK's air traffic control system, run by NATS, questions are being raised about its resilience. ISA Co-Director Professor John McDermid, shares his perspective on what went wrong and what steps need to be taken to build future resilience. Professor McDermid is also Lloyd's Register Foundation Chair of Safety at the University of York and a Fellow of the Royal Academy of Engineering.

“The Preliminary Report on the NATS Outage on 8th September 2026 gives a little insight into the underlying cause of the problem – a fault in legacy code which was exposed by processing being interrupted by a higher priority task; this gave rise to data corruption, then repeated accesses to corrupted data led to the system being shut down, with the widely-reported impact on flights. The report says that the problem arose because of timing – if the interruption to the processing had occurred a millisecond earlier or later then the failure would not have occurred. While this might seem ‘unlucky’, techniques for managing interactions between code of different priorities are well-known and would be expected to avoid this form of data corruption.

“NATS plans to undertake a Major Incident Investigation, and the Civil Aviation Authority (CAA) has also said it will undertake a review. While it is important to consider the direct cause of the incident, and its rectification, a more systemic review is needed. NATS depends on legacy systems, in some cases decades old. Replacing such systems is costly, and not without risk – but the longer they remain in service, the harder they are to replace as more systems are built on top of them and access to developers of the original code becomes less practical. Thus, the reviews need to consider how NATS migrates to more modern and more easily manageable systems, including considering the challenges of updating and replacing software in a highly complex system that must operate continuously. However, this requires a depth of insight that was missing from the analysis of the 2023 NATS outage.

“There is currently considerable concern about the use of AI in critical national infrastructure (CNI), such as air traffic management – this incident, and the additional failure on Monday, makes clear that it is also important to address the role of more conventional software on which all CNI depends. It's also urgent, as the growing use of AI won’t remove the dependency on conventional software - indeed it might increase the risk of failure, as the conventional software will likely be “stressed” in ways that weren’t expected when it was initially developed. So, as well as looking at the NATS issues explicitly, more attention is needed on the resilience of software-intensive CNI. Some of the recent work of the National Preparedness Commission is relevant, but the recent incidents with NATS reinforce the need for action.”