The technical problem that brought widespread chaos at UK airports last week was a “one in 15 million” occurrence, an air traffic control boss has said.
National Air Traffic Services (Nats) chief executive Martin Rolfe confirmed the failure occurred because one of its systems failed to process a flight properly.
In a preliminary report released on Wednesday, Nats said the plan submitted by an unnamed airline was not faulty.
Mr Rolfe said Nats was implementing a permanent software change that “will prevent this incident from happening ever again”.
As a result of the failure, Nats was unable to process flight plans automatically for several hours on bank holiday Monday, leading to more that a quarter of flights being cancelled and impacting roughly 250,000 people.
The switch to manual processing meant the average number of plans Nats could process dropped from 400 per hour to 60.
Some families were still stranded for days after the disruption, while some airlines were criticised for abandoning passengers or failing to offer refunds.
Asked what the odds were of such an incident occurring, 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.”
According to Nats, the flight plan that caused the chaos last week was for a plan that was scheduled to enter UK airspace during an 11-hour journey.
The plan was submitted to European air traffic control before being passed on to Nats. This process resulted in the plan featuring two waypoints around 4,000 nautical miles with identical names.
Nats software was unable to process this and reacted by shutting down, while 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 this happens again, while a “permanent software change” is expected in the coming days.
The Civil Aviation Authority has launched an independent review of the incident, which is expected to take around three months.