(Bloomberg) -- The UK Civil Aviation Authority’s review into a technical issue that saw hundreds of flights delayed and canceled in late August will take “months rather than weeks,” Chairman Stephen Hillier said.

“We are currently just finishing our consultation on the terms of reference and in the process of appointing the members of that independent review,” said Hillier, who was in Singapore Wednesday for the Global Urban & Advanced Air Summit. 

“We haven’t fixed a timeline for when it’s going to report. This is going to be, I imagine, months rather than weeks. I want it to be thorough, I want it to be timely as well.”

NATS, the country’s air-traffic management provider, said earlier this month that the extensive disruption, which left hundreds of passengers stranded, was the result of an issue with a flight plan filed in its system that included “two identically named, but separate way point markers outside of UK airspace.” It occurred on one of the busiest travel days of the year.

Read more: UK Authorities Race to Fix Air-Traffic System After Outage

Air traffic control has emerged as a persistent pinch-point for the aviation industry amid a surge in demand for travel, with air-traffic control strikes in France and staffing shortages from the US to the UK.

Hillier said the incident responsible for last month’s chaos was “fortunately extremely rare.”

“The last time we had a failing of this magnitude was in 2014, a different issue,” he said. “So they are rare but that’s no comfort if you are a passenger and you find yourself stuck on the ground for days.”

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