A one-millisecond glitch: What caused the NATS meltdown that grounded UK flights

NATS has released its report on last week’s air traffic control meltdown that disrupted hundreds of flights and left the travel plans of thousands of passengers in tatters.

While rumours of cyberattacks and a potential issue with a military flight have circulated, NATS has narrowed the problem down to a previously unidentified software defect that was exposed during a one-millisecond window.

The bizarre sequence of events on the 8th September forced NATS to restrict the number of flights entering UK controlled airspace while its engineers worked to rectify the fault. Although the system was restored around six hours after the initial fault occurred, disruption continued into the following days, with more than 300,000 passengers estimated to have been affected.

NATS identifies squawk code request as triggering software glitch

In its report today, NATS says it traced the failure to the part of its flight data software that assigns aircraft identification codes.

Every aircraft operating in controlled airspace transmits a four-digit transponder code, also known as a ‘squawk’. Air traffic controllers use these codes to identify and track individual aircraft on radar, with codes usually being assigned automatically.

On 8 September, an aircraft requested a squawk code manually – a perfectly normal and valid action. NATS said that this request was made correctly and that there was nothing abnormal about the flight plan.

However, while this request was being processed, the National Airspace System received a request for a ‘higher priority activity’ to be undertaken. This led to the squawk code request being paused while the system processed the higher priority action.

Once complete, the system attempted to restart the squawk allocation request, but a software defect meant it didn’t resume correctly. The resulting output produced corrupted data, leading to a widespread flight data processing failure.

NATS estimated that the vulnerable window lasted approximately one millisecond. The higher-priority request had to arrive during that exact moment, while the software was partway through updating a value.

Had the second request arrived one millisecond earlier or later, NATS said the update would have been completed normally.

“This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding,” NATS chief executive Martin Rolfe said. “The issue has been identified, and mitigation is in place while a permanent fix is safety tested and deployed.”

CAA to independently investigate the NATS failure

While NATS has identified what went wrong last week, the UK government is still concerned with the lack of resilience in air traffic control.

Transport Secretary Heidi Alexander said last week’s disruption was ‘completely unacceptable,’ and while she praised NATS for protecting the safety of passengers, she also made it clear that the fallout is not over yet.

“It’s clear we need to urgently understand why this issue was not discovered and fixed before it caused chaos,” Alexander said. “I have therefore tasked the CAA with conducting an independent review to check NATS’ findings and investigate their investment plans to enhance resilience in the future, along with regulatory accountability.”

The CAA will verify NATS’ findings while also examining its investment plans, operational resilience and regulatory accountability. Alexander noted that a cyber attack can now be ruled out, something that was a concern after Manchester Airports Group (MAG) was attacked in late August. However, she said that she does not believe this issue was unavoidable.

“It is crucial that our national infrastructure is fit for the future,” she added, “and this government will make sure the aviation sector is as resilient as possible, so passengers can get to where they need to be.”