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The IBM mainframe-based Flight Data Processing System (FDPS) based at West Drayton, near Heathrow, crashed twice in 2002, causing major disruption to air traffic and delays for thousands of passengers.
Nats told Computer Weekly in 2000 that the system was scheduled for replacement in 2007. But when the House of Commons Public Accounts Committee questioned him on Monday about the part privatisation of Nats, Richard Everitt, its chief executive, said the flight data processing system would continue to be dependent on the software until 2011 or 2012.
The system's IBM 390 hardware is modern but some of the software, based on the "Jovial" programming language, dates back to the 1970s. "[The software] does remain one of our vulnerabilities," Everitt told MPs.
He confirmed that the organisation was deeply in debt - owing hundreds of millions of pounds to banks. Money had needed to be "conserved", he said.
A Nats spokesman said earlier this week that there was no delay in the replacement of the West Drayton system. He added that replacing the system would start in 2007 with new software being installed "progressively" over several years. Nats would continue to be dependent on the old technology until 2011 or 2012, he confirmed.