NATS Major Incident Preliminary Investigation Report [pdf]

NATS Major Incident Preliminary Investigation Report NAS Incident 08 September 2026 Report date: 16 September 2026 NATS Major Incident Preliminary Investigation Report NATS Public Report date: 16 September 2026 Page 2 of 18 1. Foreword 3 2. Executive Summary 4 3. Scope of this Report 5 4. Definitions and Technical Context 6 5. High Level Timeline 8 Tuesday 08 September 2026 8 Wednesday 09 September 2026 11 Thursday 10 September 2026 11 Friday 11 September 2026 11 6. Technical Causes 12 7. ATC Response 13 Operational preparedness 13 8. Incident Management and Stakeholder Communications 14 9. Resolution and Short Term Mitigations 15 Resolution 15 Short term mitigation measures 15 10. Next Steps 16 Appendix A: Traffic Relative to Previous Week 17 Appendix B: Glossary 18 NATS Major Incident Preliminary Investigation Report NATS Public Report date: 16 September 2026 Page 3 of 18 The failure of our flight data system on Tuesday, 8 September 2026 caused significant disruption to airlines, airports and the travelling public. I would like to apologise for the challenges it presented to the wider aviation system, to those whose travel plans were affected and to those who have worked tirelessly to recover normal operations across the industry. Our full investigation into the circumstances is under way and our major incident report will be completed within 60 days. At this early stage, however, I can confirm that this incident is not related to either the 2023 NATS FPRSA system failure or the radar issue in July last year. I can also confirm that it was a software issue and not caused by any incorrect actions by either military or civil operators. The NATS near-term traffic forecast from August 2026 indicated that NATS expected to handle around 8,000 flights on 8 September. Latest EUROCONTROL records show that we handled 6,094 illustrating that the incident led to disruption involving more than 2,000 flights being delayed, cancelled or diverted. Our primary focus, and the primary accountability under our licence, is the safety of all air traffic using UK airspace. We will always do what is necessary to preserve that. That can mean placing restrictions to limit air traffic, as we did on 8 September; we do not do that lightly, and we deeply regret the inconvenience it caused to so many people. This preliminary report for the Secretary of State for Transport and CAA sets out what caused the failure on 8 September and the measures we have put in place to rectify it. Our subsequent Major Investigation report due within 60 days will examine root causes, our response, and identify where lessons can be learned so that we can continually improve the critical service we provide to UK aviation. Martin Rolfe Chief Executive Officer 1. Foreword NATS Major Incident Preliminary Investigation Report NATS Public Report date: 16 September 2026 Page 4 of 18 NATS (En Route) PLC (NATS) operates under licence from the Civil Aviation Authority which includes obligations to prioritise and preserve safety during routine and exceptional operations. In a typical year, we handle more than 2.5 million flights travelling to, from and over the UK, and across the North Atlantic. Standard operating procedures along with controller and engineering training ensured that the incident on Tuesday 8 September was managed in a way that maintained the safety of all aircraft flying within UK airspace. This preliminary report covers the actions and investigations undertaken in the 72 hours from the start of the incident to the last of 15 coordination calls with aviation stakeholders on Friday 11 September. Full investigation is ongoing and the Major Investigation report will expand on the themes identified here. On 8 September 2026 at 13:25 the NATS CEO advised DfT and CAA senior officials that a major incident was underway at the London Area Control (LAC) centre which controls flights primarily operating above 24,500ft over England and Wales. Under NATS incident management arrangements the first aviation industry coordination call was held at 13:40 to brief airlines, airports, and other aviation stakeholders. Air traffic controllers in the LAC centre had lost access to some live flight data because of a technical issue within the National Airspace System (NAS) and had invoked practised fallback procedures. This included coordinating the handover of aircraft between neighbouring national and international air traffic control units manually, a task that usually happens automatically. As this increased controller workload, the number of aircraft permitted to fly in the airspace was reduced to maintain safety margins. Restrictions were applied to reduce the number of aircraft that could enter each controller’s sector, including limiting or stopping departures from UK and overseas airports. With the number of arriving aircraft exceeding the numbers departing, eventually increased ground congestion at airports led to invoking established procedures for the diversion of inbound aircraft already airborne. At all times controllers could speak with aircraft and monitor them on radar. Engineers identified that the connection between the LAC system and the NAS had been dropped as a result of a significant error in processing flight data. This inbuilt functionality is one of many layers of resilience in the overall air traffic control system designed to localise significant issues when detected. Once the issue was understood, it was resolved under pre-defined, controlled conditions while continuing to deliver a limited air traffic service. Analysis has identified that flight data within the NAS had corrupted because of a legacy and previously unknown software defect and a combination of very specific events occurring simultaneously. There is no evidence at this stage to suggest that this incident was caused by a malicious actor or cyber related activity. A mitigation plan is currently in place while the permanent fix of the underlying software, which has already been developed and delivered by our supplier, is tested, safety assured and deployed. The full scale of disruption will take time to confirm; but at this stage more than two thousand flights were delayed, cancelled or diverted. NATS handled some 1,800 fewer flights than forecast over the course of the day but we appreciate that does not reflect the full scale of the disruption experienced. 2. Executive Summary NATS Major Incident Preliminary Investigation Report NATS Public Report date: 16 September 2026 Page 5 of 18 This preliminary report details the actions and activities undertaken from the initial incident up to the final situational call with customers on Friday 11 September. The scope of this report is to: 1) Establish the cause of the system failure. 2) Detail the chronology of the incident from initial occurrence through detection, escalation, investigation, closure and post-event actions. 3) Evaluate the impact on Air Traffic Control (ATC) and engineering operations, including the scale and duration of operational disruption. 4) Review the resilience and recovery measures. Due to the impact and disruption caused, the events of 8 September have been categorised as a Major Incident and are being investigated under our Major Incident Process. The scope of the Major Incident Investigation is contained in section 10. 3. Scope of this Report NATS Major Incident Preliminary Investigation Report NATS Public Report date: 16 September 2026 Page 6 of 18 These terms are defined here as they are used throughout the remainder of the report. London Area Control (LAC) London Area Control (LAC) ensures the safe and efficient routing of aircraft primarily operating above 24,500 ft, over England and Wales and handles around 6,500 flights on a typical September day. National Airspace System (NAS) The National Airspace System (NAS) is the core flight data system that underpins many of the air traffic control systems used in NATS domestic control centres, including LAC. Data in the NAS also supports systems at several UK airports and external ATC centres. It maintains flight plans and flight data, processes amendments and controller inputs, allocates and manages discrete aircraft identification codes (squawks), and associates flight plans with radar tracks. LAC System The hardware and software platform used to manage air traffic within London Area Control. The system receives data from the NAS and provides tools that enable air traffic controllers to make timely and informed decisions in order to safely and efficiently maintain aircraft operations within UK airspace. Airspace sector A sector is a subdivision of controlled airspace within which a controller is responsible for providing air traffic services to aircraft operating in that volume of airspace. Airspace restrictions Airspace restrictions take a variety of different forms but are intended to ensure that the number of aircraft passing through a sector, or arriving at an airport, do not exceed the number that can be safely handled at one time. Squawk code A code transmitted (squawked) by an aircraft that enables it to be identified by air traffic control systems. Bronze Command team Part of incident management response, the Bronze Command team is primarily composed of engineers that focus on resolving the technical issue. 4. Definitions and Technical Context NATS Major Incident Preliminary Investigation Report NATS Public Report date: 16 September 2026 Page 7 of 18 Silver Command team Part of incident management response, the Silver Command team is primarily composed of operational staff that focus on safely managing the ATC operation during the loss of any part of the underlying technical system. Their priority is to run a safe operation as close as practicable to normal service. Major Incident Manager The Major Incident Manager (MIM) focuses on resolution of the incident. They ensure the right people are engaged, keep senior commanders informed, manage escalation and