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National Air Traffic Services (Nats), owners of the failed system at West Drayton near Heathrow, also revealed that last week's crash was caused by a known problem which had brought down the same Flight Data Processing System about two weeks earlier. A software fix to cure the problem was about to be installed when last week's crash occurred.
The West Drayton system generates the flight progress strips that are printed out at the £623m New En Route Centre at Swanwick in Hampshire, which finally became operational in January after lengthy delays.
A Nats spokesman said the two most recent system crashes happened when the mainframe system was trying to track a flight which temporarily left UK airspace.
Although the system was unable to keep track of the aircraft's movements outside of the UK's airspace, it expected to re-acquire the flight when the aircraft returned. But while the flight was outside of UK airspace an air traffic assistant made significant amendments to data stored in the system on that flight. So when the aircraft re-entered UK airspace the system failed to re-acquire the flight's details because it had no certain means of correlating the data.
The system tried up to six times to match the flight-plan data before shutting down automatically.
A software fix has now been installed which will stop the system trying repeatedly to read rogue data and then shutting itself down when it fails to do so.
The Nats spokesman said an investigation was continuing into whether air traffic assistants had entered data incorrectly or had changed flight data that should not have been altered.