A “previously unknown software defect” caused the air traffic control meltdown last week, which saw more than 2,000 flights affected, tearing up the travel plans of hundreds of thousands of passengers.
The preliminary report on National Air Traffic Services’ (Nats) investigation into the incident, which occurred on Tuesday, 8 September, determined it was caused by a “legacy and previously unknown defect” in a software module within the National Airspace System (NAS), which manages UK airspace and processes requests for the allocation or reassignment of squawk codes.
A squawk code is an identification code transmitted by aircraft during flight, enabling air traffic control systems to match the aircraft’s altitude, speed and direction with its intended flight plan so that flight progress can be monitored.
The incident was triggered by a manual request for a squawk code. Nats, the UK’s air traffic control provider, said this manual request was made correctly and there was “nothing abnormal or invalid” about the flight plan.
While this request was being processed, the NAS received a message for a “higher priority activity” to be undertaken, which resulted in the squawk code allocation being paused while the system processed the higher priority message.
While switching between activities is a normal function for the system to undertake, when the processing of the squawk allocation request resumed, the software defect meant it did not resume correctly and the resulting output was corrupted. This affected some subsequent flight data updates.
This all happened in the space of a millisecond, the report said.
With reduced information available to controllers, restrictions were put in place to limit air traffic to maintain safety. While the software issue only affected flights in the London Area Control Centre, which controls higher-level flights operating mainly above 24,500ft, the restrictions had to be placed across the UK so that the NAS could be restarted and flight data reloaded.
Restrictions lasted for around six hours and while Nats operations returned to normal the same evening, it took more than two days for the backlog of passenger disruption to be cleared, with hundreds of thousands of passengers’ travel plans disrupted.
Around 2,163 flights were affected, with delayed and cancelled flights at major airports including Heathrow, Gatwick, Stansted and Manchester.
At least 300,000 passengers would have been impacted, based on the 30 per cent cancellation rate of flights, aviation analytics firm Cirium predicted.
By Thursday evening, flight schedules stabilised, but Nats has since faced scrutiny from the government, which owns 49 per cent of the provider, and key travel industry players, including Ryanair’s CEO Michael O’Leary, who called for the chief executive officer of Nats, Martin Rolfe, to be sacked.
Mr Rolfe said on Friday, 18 September: “This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding.”
“The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed.”
He added: “I would like to apologise again, very sincerely, to everyone who was affected last week. It’s our job to get people where they want to go, quickly and without delay and we are devastated when that goes wrong. However, our primary role is to keep our skies safe, and everyone who flies through them. At no point last week was safety in question.”
Transport Secretary Heidi Alexander said on the release of the preliminary report: “The disruption we saw last week was completely unacceptable and I know how frustrating it was for passengers, airlines and airports.
“I have now received Nats' report and, while I am pleased to see that the safety of passengers was protected, it’s clear we need to urgently understand why this issue was not discovered and fixed before it caused chaos. 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.
“It is crucial that our national infrastructure is fit for the future, and this government will make sure the aviation sector is as resilient as possible, so passengers can get to where they need to be.”
Mr Rolfe was summoned by Ms Alexander last Wednesday to account for the disruption and was given a week to report back on Nats’ investigation into what caused the glitch on its system.
Mr Rolfe confirmed that this incident was unrelated to the major system outage in August 2023. He also dismissed speculation last week that it was caused by military intervention.
The Ministry of Defence (MoD) denied the report over the weekend that said a military aircraft was to blame for the air traffic control system failure, the Financial Times reported, citing sources familiar with the incident.
The sources said a flight plan submitted by a British military aircraft caused the outage, yet an MoD spokesperson said it did not believe there was “any indication of an error on the part of the military”.
A Nats spokesperson also confirmed to The Independent last week that it was not the same issue as that which occurred in 2023, when more than 700,000 passengers saw their flight plans disrupted after Nats suffered a technical glitch while processing a flight plan.
The report also found no evidence at this stage to suggest that this incident was caused by a malicious actor or cyber-related activity.
Nats will release a full report in two months. Ms Alexander has also ordered an independent probe by the UK’s Civil Aviation Authority.
Read more: MoD denies ‘military jet’ error caused air traffic control outage
