London — NATS Blames Millisecond Software Flaw for Flight Chaos

A software defect that lasted a millisecond corrupted flight data and triggered the UK air traffic control failure that grounded thousands of flights earlier this month, national air traffic service Nats said in a report released Sept. 18, according to the BBC.

The defect struck at 10:00 a.m. on Sept. 8 inside the system that underpins UK airspace management. Nats said the corrupted data left air traffic controllers with reduced information, forcing the agency to restrict air traffic across the country to keep flights safe.

What caused the outage on Sept. 8?

Nats chief executive Martin Rolfe told the BBC the flaw was "very, very obscure," occurring "in the space of a millisecond." The agency runs a national network that lets planes fly "to almost anywhere from almost any airport at almost any time," Rolfe said. "If that stops, even for a moment, then you get huge disruption."

Rolfe said Nats had tested the system extensively before the failure. "We spent so much time making sure they work that anything that does happen of this nature is, by its very nature, going to be incredibly complicated and probably difficult to understand at the time," he said.

Why didn't Nats catch the problem sooner?

According to the BBC report on the Nats findings, the defect did not become apparent as a serious problem until two and a half hours after it occurred. The system restart did not begin until just after 3:15 p.m., and restrictions on UK airspace stayed in place until 7:30 p.m. — more than nine hours after the initial fault.

Transport Secretary Heidi Alexander said the delay demands scrutiny. "It's clear we need to urgently understand why this issue was not discovered and fixed before it caused chaos," she said, adding that the Civil Aviation Authority has a review underway.

How many flights and passengers were affected?

By the numbers:

• More than 2,000 flights were cancelled, per Nats and the BBC.
• Hundreds of thousands of passengers were disrupted, with some sleeping on airport floors or stranded overseas.
• Major UK airports hit included Heathrow, Gatwick, Luton and Glasgow.
• The outage began at 10:00 a.m. on Sept. 8 and airspace restrictions were not lifted until 7:30 p.m. that day.

The disruption took days to fully clear because so many aircraft and crews ended up out of position, the BBC reported.

Will Nats chief executive Martin Rolfe resign?

Rolfe told the BBC he will not step down. Some airlines have questioned his position following the failure, according to the BBC, and passengers have voiced frustration over the level of support they received from carriers during the disruption.

Rolfe apologized to those affected, saying Nats was "devastated" by the incident. "However, our primary role is to keep our skies safe, and everyone who flies through them," he said. "At no point last week was safety in question."

What is the government and airlines' next move?

Alexander called the disruption "completely unacceptable" and "frustrating" for passengers, airlines and airports, the BBC reported. The Civil Aviation Authority review will examine why the defect went undetected before it caused widespread cancellations.

Industry group Airlines UK is writing to Nats to demand immediate compensation for costs airlines incurred during the shutdown, according to the BBC. Separately, many passengers are unlikely to receive compensation directly because the regulator has judged the event to fall under "extraordinary circumstances," a classification that limits airline payout obligations under existing rules.

Has this happened before?

This marks the third national air traffic incident in the UK in three years, the BBC reported. In August 2023, a computer shutdown over the bank holiday weekend caused similar disruption across the country's air network.

The repeat pattern is likely to fuel questions from lawmakers and airlines about whether Nats' systems are resilient enough to prevent another single point of failure from grounding flights nationwide. The Civil Aviation Authority's review, along with Nats' own internal findings, is expected to shape any additional oversight measures going forward.

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