Air traffic computer systems shut down after receiving duplicate data, a report found, forcing plans to be processed manually.
The technical failure that led to hundreds of flight cancellations and severe disruptions for thousands of people traveling in and out of Britain last week resulted from a “one in 15 million chance,” the country’s air traffic control service said on Wednesday.
“We have processed 15 million flight plans with this system,” Martin Rolfe, the chief executive of Britain’s National Air Traffic Service, told the BBC’s “Today” program. And the service, he said, had “never seen this before.”
On Wednesday, the service published a report based on an internal investigation of the event, detailing what Mr. Rolfe described as “an incredibly rare set of circumstances.”
According to the report, the air traffic control system encountered two separate pieces of navigational data in one aircraft’s flight plan that had the same name. As a result, the system’s primary and backup computer systems both shut down to avoid passing incorrect information to the controllers.
The service then reverted to manual air traffic control, meaning that fewer flights could be processed.
“Keeping the sky safe is what guides every action we take, and that was our priority during last week’s incident,” Mr. Rolfe said in a statement.
The problem was fixed several hours later, but 799 outbound and 786 inbound flights were canceled on Aug. 28, according to Cirium, an aviation analytics company. The disruption continued into Aug. 29, when more than 300 flights were canceled.
Mr. Rolfe apologized again to the affected passengers, many of whom were stranded in airports or on tarmacs for hours or had to wait several days for alternative flights. He said that if the issue were to happen again, the National Air Traffic Service would be able to deal with it.
“Action has been taken to ensure such an incident does not recur,” Mark Harper, Britain’s secretary of state for transportation, wrote on social media on Wednesday.
The Civil Aviation Authority, which oversees aviation safety in Britain, said on Wednesday that it had begun an independent review of the issue and the response to assess whether the National Air Traffic Service had breached its obligations. The results would be published by the end of the month, the authority said.
Wednesday’s report came as a New York Times investigation has found an alarming pattern of safety lapses and near misses in the United States’ skies and on airport runways.