Martin Rolfe, chief executive of the air-traffic control service, revealed for the first time that the fault was initially identified at 8.30am on Monday – almost three hours before the automatic system went offline, leaving controllers to handle aircraft manually.
The system is designed for caution when confronted with anomalous data. Rather than risk air-traffic controllers being presented with false information, the system went into its back-up mode – which stores up to four hours of data.
“We were working on a timeline of restoring the system before 12.30 [pm],” Mr Rolfe said.
“But obviously, as you will appreciate, we’re not getting new flight plans in at that stage – or only those flight plans we can manually process. So effectively you erode the store of data.
“We knew that if we hadn’t solved it by that point, we would be in the situation where we needed to significantly reduce the flow of aircraft to make sure we didn’t overwhelm the system and more particularly the controllers.
“Probably about halfway through that, we concluded that there was a reasonable chance that we might not be able to recover it in the timeframe. The problem was significantly different from anything we had seen before, and of course that always raises questions as to what the cause was.
“It is only in circumstances where things are so outside of what we expect to see that we would revert to manual operations.
“At the time we had no idea. We have a better idea now, but at the time we didn’t. So we started to invoke contingency procedures: talking to airlines, talking to the Dft [Department for Transport], talking to the CAA [Civil Aviation Authority] to make it clear that there was a potential for a significant problem.
“Being a bank holiday, we knew that was pretty likely given the volume of traffic.
“The engineers managed to figure out what the problem was to the point of being able to say ‘we know how to fix it’ – which is not the same, obviously, as the root cause.
“By 1.30[pm] there was news from the engineers that they thought they knew how to effectively recover the system, despite not knowing the root cause.”
Within a hour the system was working again, he said – but “you don’t go from 10 per cent to 100 per cent in a moment”.
As a result of the system failure, almost 1,600 flights were cancelled on Monday – grounding around 250,000 travellers.
On Tuesday, around 300 departures were cancelled as airline struggled with aircraft and crew being out of position.
Mr Rolfe said that in normal times the UK’s air-traffic control service is “the envy of most of the world” because of the amount of traffic it can handle.
“At this time of year we would expect a handle between 7,500 and 8,000 flights a day. The majority of them will experience no delay attributable to us. That works out to just over 2 million flights a year.
“We handle about 25 per cent of European air traffic and we contribute about 2 per cent of the delay.”
The Nats CEO also said there was never any danger to travellers.
“We have an absolutely enviable safety record which we never take for granted,” he said.
“The reason for the disruption was precisely because we are absolutely wanting to guarantee everybody’s safety.”
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