With the fog disrupting flights in the northern region early this winter, the Airports Authority of India on Friday said it was in the process of upgrading its Instrument Landing System from CAT (category) III-A to III-B "very shortly".

However, the problem is that, barring the Indian Airlines and some foreign carriers, pilots are not yet trained to use the system to take-off or land in zero or near-zero visibility.

Industry sources said it was very costly to train the pilots in these systems. The pilots, they said, were to be taken out of operations and sent abroad to undergo training for long periods.

AAI sources said the Runway Visibility Range used both for landing and take-off, under CAT-III A is 200 metres and the decision height meant for landing is 15 metres.

Other CAT-III A requirements are lighting of the runway and taxiway, approach way indicators, besides various other procedures meant for low-visibility operations, which are already operational.

When fog descended over Delhi unexpectedly on Wednesday morning, the first aircraft to take off at 0830 hours from IGI Airport was that of the Indian Airlines as its pilot was trained to operate the CAT III-A system.