A software defect in part of the UK’s air traffic control system corrupted flight data “in the space of a millisecond”, leading to a six-hour outage and mass airline cancellations and delays across the UK last week, National Air Traffic Services has said. But ministers said the Nats report into the incident still left questions unanswered, saying they needed “to urgently understand why this issue was not discovered and fixed before it caused chaos”.
According to the report, the error first occurred at 10am and was reported to engineers who investigated – but the system appeared to be working again, until 12.30pm when it happened again and a major incident was declared.
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The transport secretary, Heidi Alexander, said the industry regulator the Civil Aviation Authority (CAA) would now “check their findings” and investigate Nats’ plans.
Hundreds of thousands of passengers had their travel disrupted after the Nats failure on Tuesday 8 September, which initially only stopped flights departing in the London area but eventually spread to disrupt travel across the UK and beyond by the evening.
Airlines have called for compensation and a “credible plan”, with Ryanair demanding the dismissal of the Nats chief executive officer, Martin Rolfe, after the third major air traffic control failure in the UK since summer 2023.
However, Rolfe said the incident was unrelated to the previous failures and dismissed speculation that it was caused by military intervention.
The preliminary report said that the failure was a software defect in part of the National Airspace System, which allocates codes to individual aircraft to identify flights on radar.
Nats said the software error occurred at 10am, with engineers first notified at 10.02am that the link between the national airspace system and the London area control system had been dropped. The engineers reported at 10.06am that the system had apparently recovered with no operational impact, but continued to investigate.
The report said the fault occurred when a manual request for an aircraft code was being processed and another request paused it. It said: “When processing of the aircraft code request resumed, the software defect meant it did not resume correctly and the resulting output was corrupted, and affected some subsequent flight data updates. This happened in the space of a millisecond.”
However, at 12.32pm the link between the national and London airspace management dropped again and a major incident was declared, with controllers having to conduct some tasks manually. By 12.45pm, Nats was restricting flights as the system continued to glitch, before failing completely at 1.32pm.
At 1.40pm airlines were advised of restrictions, and between 2.50pm and 4.09pm Nats rebooted the system, eventually resuming full operations at 7.30pm – but leaving mass congestion, displaced aircraft and hundreds of thousands of delayed passengers in its wake.
It took more than two days for the backlog of passenger disruption to be cleared, with hundreds of thousands of people’s travel plans disrupted.
Rolfe said: “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.”
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The transport secretary said: “The disruption we saw last week was completely unacceptable aI know how frustrating it was for passengers, airlines, and airports.”
Alexander said she had received Nats’ report and added: “While I am pleased that passenger safety was protected, it is clear we urgently need to understand why this issue was not found and fixed before it caused chaos.
“I have therefore asked the CAA to carry out an independent review to check Nats’ findings, investigate their investment plans for improving resilience in the future, and examine regulatory accountability.
“It is vital 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 where they need to be.”
The CAA’s review will be published within six months.
Tim Alderslade, chief executive of Airlines UK, a lobby group representing the industry, said: “Once again, it is passengers who have suffered and airlines who have picked up the pieces – and the bill – of an ATC system failure, while Nats itself faces no real consequences.
“Passengers don’t need more hollow apologies or empty promises that lessons will be learned. They deserve commitment and proof that the system is being fixed, so the CAA’s review must require real investment in Nats’ resilience, so a single fault can never again cause nationwide disruption.”
He said his members were writing to Nats to “demand immediate compensation for the costs incurred and a credible plan to improve resilience.”
Frequently Asked Questions
FAQs Software Error Causes Flight Chaos for Hundreds of Thousands
Basics
What happened
A software error caused major flight disruptions affecting hundreds of thousands of passengers in a matter of milliseconds
How can a problem happen in a millisecond
Computers process data incredibly fast A single bad instruction or faulty line of code can cascade through systems almost instantlyfar faster than any human could react
What kind of software error was it
Reports point to a faulty update or configuration error in a critical system One small mistake in code or settings can trigger widespread failures
Which flights and airlines were affected
The disruption hit multiple airlines and airports that rely on the same shared software system which is why the impact spread so widely
How many people were affected
Hundreds of thousands of passengers faced cancellations delays and rebooking problems
Is this a cyberattack
In most cases like this no Its usually an internal errora bug or bad updatenot a hack
Common Problems
Why did one error affect so many flights
Many airlines depend on the same centralized systems for scheduling checkin and dispatch When that shared system fails everyone using it fails at once
Why did it take so long to fix if the error happened instantly
Detecting diagnosing and safely reversing a bad update takes time Systems must be rolled back carefully to avoid making things worse
Why couldnt airlines just switch to backup systems
Backups exist but switching over isnt instant Staff need to be trained data must sync and manual processes are slow
Why were passengers stranded instead of rebooked quickly
When booking systems go down agents cant see availability or issue tickets Everything has to be done manually which creates massive bottlenecks
Examples RealWorld Context
Have similar incidents happened before
Yes Outages at major airlines and IT providers have caused mass cancellations in the pastshowing how fragile interconnected systems can be
Whats a real example of a small error causing big problems
A single mistyped configuration line or a failed update has previously grounded entire fleets or shut down checkin systems worldwide
Practical Tips
What should I do if my flight is affected by an outage
Check your airlines app and website for updates
Contact the airline directlydont
