New Delhi, Jan 14 (IANS) The crash of the Air Traffic Control systems at Delhi's Indira Gandhi International (IGI) Airport, leading to chaos and disrupted flight services for over two hours Thursday evening, was primarily caused by overloading and a software glitch, sources said.

'The radar software in ATC failed due to overloading and some glitch. We have ruled out a virus because the software which is installed by American firm Raytheon is not connected to Internet. A team from Raytheon and our own Data Management System department have reinstalled the software now,' a highly placed source told IANS.

'It will take some time to get flight services back on track,' the official said, adding it was the first time that the system at the IGIA has crashed.

The official said before the year 2000, flights were handled without this radar software. 'It is called procedural control or in common sense blind controlling where the ATC officer contacts the flight captain and checks altitude, speed and other necessary parameters for landing of the aircraft.'

'We used it over two hours when the radars' software crashed around 5.45 p.m. The software was restored around 8 p.m. When we do blind controlling, the availibilty of airspace reduces to one-tenth, which means that instead of 10 aircraft, only one aircraft can operate in sky or ground at one time.'

'We stopped departures of all Delhi-bound flights from all airports across the globe. And international flights were directed not to fly over Delhi. Though it was not an alarming situation, we wish no such situation ever arises,' the official added.