UK air traffic control outage caused by software defect, says operator

By William James

LONDON, Sept 18 (Reuters) - The UK air traffic control outage that affected hundreds of thousands of passengers last week was caused by a software defect, the boss of system operator NATS said on Friday, as he defied airlines' calls for his resignation over the latest in a series of incidents.

The outage led to 2,000 flight cancellations and days of disruption, drawing a furious response from airlines that incurred millions of pounds in costs as they scrambled to get crews and aircraft back in place and refund passengers.

"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 CEO Martin Rolfe said in a statement accompanying a preliminary report on the incident.

"The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed."

He said the shutdown was not related to a problem in 2023, which also caused widespread disruption, nor was it linked to military intervention, as reported by some media outlets last week. Safety was not compromised during the incident, he added.

UK GOVERNMENT SAYS DISRUPTION UNACCEPTABLE

Transport minister Heidi Alexander, who had last week demanded NATS produce a report on the matter and ordered an independent review by the Civil Aviation Authority, said the disruption was "completely unacceptable".

"We need to urgently understand why this issue was not discovered and fixed before it caused chaos," she said in a statement. "It is crucial that our national infrastructure is fit for the future."

The previous shutdown in August 2023 cost airlines £100 million ($136 million), while a smaller NATS issue resulting from a radar fault also caused 150 cancellations in July 2025.

Ryanair, Europe's biggest airline by passengers flown, repeated its call for Rolfe to resign, something it has demanded since the 2023 failure.

"Martin Rolfe has run out of excuses. The 'software ate my homework' is not an acceptable explanation," said its Chief Operations Officer Neil McMahon.

"How many times must the UK ATC (Air Traffic Control) system collapse before Martin Rolfe accepts responsibility and resigns, or before Heidi Alexander takes effective action and replaces him with someone competent?"

ROOT CAUSE OF INCIDENT

The NATS report on the latest incident said it was triggered by a request for an identification code from an aircraft being followed by a request for a higher priority activity, pausing the first request.

"Had the higher-priority request arrived even one millisecond earlier or later, the update would have completed normally," the report said.

The error triggered backup protocols, which meant flight handovers had to be performed manually, resulting in reduced capacity that triggered knock-on delays through the system.

"Once the issue was understood, it was resolved under pre-defined, controlled conditions while continuing to deliver a limited air traffic service," the report added.

A NATS investigation into the 2023 outage found it was caused by a "one in 15 million" event in which a flight plan contained two identically named but separate waypoints, forcing both the system and its back-up into a "fail-safe" mode.

(Reporting by William James, additional reporting by Sam Tobin and Padraic Halpin; Editing by Sarah Young and Joe Bavier)