Britain's civil aviation regulator said on Wednesday that it will conduct an independent review into the air traffic control failure that brought chaos to UK airports last week.
The CAA said this would also seek to establish whether National Air Traffic Services, the country's air traffic control provider, had breached its statutory and licensing obligations.
This came as the Nats chief claimed the technical glitch which caused widespread disruption to flights on August 28 was a “one in 15 million” occurrence.
This led to more than 1,500 flights being cancelled and left thousands of passengers stranded.
Many had to bed down in airport terminal buildings as no hotels were available. Passengers said that food was scarce at some airports.
Further details of the review will be published by the end of September and the inquiry is expected to take around three months, said CAA officials.
“If there is evidence to suggest Nats may have breached its statutory and licensing obligations we will consider whether any further action is necessary,” Rob Bishton, CAA joint-interim chief executive, said.
The regulator said Nats had provided its preliminary report which blamed an anomaly that forced the system to stop processing flight plans.
Air traffic controllers then closed the system to maintain safety and switched to manual operation to continue service.
The switch meant the average number of plans it could handle dropped from around 400 an hour to as few as 60, leading to restrictions on flights to and from UK airports.
More than a quarter of flights were cancelled that day – affecting around 250,000 people.
Cancellations continued for the following two days as aircraft and crews were out of position.
Nats chief executive Martin Rolfe said this type of problem would not be repeated.
He said one of Nats' systems failed after it “didn’t process a flight plan properly”.
The plan submitted by the airline – which has not been named – was “not faulty”, he added.
This problem led to Nats being unable to process flight plans automatically for several hours.
The CAA said it had shared this analysis with the government on Monday and outlined its next steps.
“The CAA agrees with the Nats statement that at no stage did this incident represent a safety concern,” the regulator said in a letter to Transport Minister Mark Harper.
The regulator said the event was now understood and, if it happened again, should be fixed quickly with no effect to the aviation system.
Asked what the odds of this happening were, Mr Rolfe said: “We know it’s at least one in 15 million, because we’ve had 15 million flight plans through this system and we can be absolutely certain that we’ve never seen this set of circumstances before.”
In a preliminary report shared with Mr Harper, Nats did not identify the route of the flight plan which led to the chaos but stated the aircraft was scheduled to enter UK airspace during an 11-hour journey.
Airlines’ flight plans feature way points – locations identified by a combination of letters and numbers.
The flight plan which caused last week’s disruption was submitted to Eurocontrol – which oversees air traffic control across Europe – before being passed on to Nats.
The process led to the plan featuring two way points around 7400km apart but with identical names.
Nats’ software was unable to extract a valid UK portion of the flight plan and reacted by shutting down, said officials.
A back-up system followed the same steps and also stopped working.
Nats said an “operating instruction” has been put in place to allow the “prompt recovery” of the system if there is a repeat of these circumstances.
A “permanent software change” to prevent it shutting down in such an event is expected to be implemented in the coming days.
