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Hundreds of thousands of passenger flights were delayed or cancelled after a problem with UK air traffic control on Monday. But what caused it?

What was the ‘technical failure’?

The automatic flight planning system used by Nats, the company that provides national air traffic control services in the UK, was out of action for several hours on Monday. Airspace was not closed, but the number of planes in the sky was severely restricted while the automated system was down.

What caused it to fail?

Nats says that the failure was triggered by a single piece of data in a flight plan that was wrongly input to its system by an unnamed airline.

What are flight plans and how are they filed?

Flight plans in effect are a plane’s timing and chosen route, and for most regular short-haul flights they are completely standardised. An airline’s flight operations team files them electronically to air traffic controllers well before takeoff, and pilots are responsible for checking them and updating them if necessary.


Does Nats not have a backup system?

Yes it does. According to its chief executive, Martin Rolfe, “several layers of backup” exist, but apparently the dodgy data caused the secondary automatic processing system to be suspended “to ensure that no incorrect safety-related information could be presented to an air traffic controller or impact the rest of the air traffic system”.

How could just one faulty flight plan cause this?

Nats has not really explained this yet. Willie Walsh, the director general of the global airlines body, Iata, said it was “staggering” that inputting a single flight plan incorrectly could knock the whole system over. Pilots say plans can frequently be rejected. Rolfe said the whole system did not fail and that airspace was slowed down rather closed down, but that the computers were “designed to fail safely” to isolate the problem.


And on a bank holiday. Definitely not a cyber-attack?

“No indication” according to Nats. Despite the timing, which has increased speculation after similar airline woes occurred at peak holiday seasons in the past, aviation experts said it was unlikely, even before the transport secretary, Mark Harper, said the government had ruled it out.

Has this happened often?

Not quite at this scale and duration. The only comparable incident at Nats was in December 2014 when flights were held for about an hour during a glitch.


What happens next?

Nats is investigating internally and has to report to the regulator, the Civil Aviation Authority (CAA), which has promised to get an initial report to Harper by Monday. Rolfe told broadcasters on Wednesday that this particular error should not recur: “We understand the way the system didn’t handle the data … the way it failed, if you like,” he said.

Who pays for the fallout?

At the moment, airlines and their passengers. Many people face additional bills and lost holidays, possibly alleviated by travel insurance if they have a decent policy, and airlines have incurred more costs and lost revenue. Airlines have the legal duty to accommodate, feed and eventually fly passengers whose flights have been cancelled or severely disrupted, but compensation is not due because the fault was beyond their control. Walsh estimates that the lost revenue, extra work and additional flights have cost airlines £100m.


Will Nats have to pay compensation?

At the moment, no. Walsh told the BBC: “I think there is a great opportunity post-Brexit to look at the way passenger compensation is dealt with to ensure that the people who are responsible for the delays and cancellations ultimately bear the costs.” Given airline and passenger anger, it is not impossible that some kind of review of compensation rules could be ordered.

Who owns Nats?

Nats is a public-private company, 49% owned by the state, which has a golden share. It is licensed by the CAA to run national airspace from its HQ in Swanwick, Hampshire, and funded via hundreds of millions of pounds in overflight fees paid by airlines.

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