Fog-delay relief is in sight at Calcutta airport with the visibility requirement for landing and take off being lowered.

The earlier guideline for the Category (CAT) II Instrument Landing System, installed at the airport in 2007, prohibited flight operations if the runway visibility dropped below 350m.

But a recent circular from the Directorate General of Civil Aviation, based on a revised international flight protocol, fixed the take-off visibility requirement for a CAT II airport at 150, 175 and 200 metres for various categories of aircraft. The landing criterion has been fixed at 300m.

“The revised guideline was implemented recently following a nod from the airport meteorological department, which was asked to check whether the visibility requirement could be reduced to 150 metres with the existing equipment,” Malay Dutta, the joint general manager (standards and procedure) of the Airports Authority of India (AAI), said on Thursday.

Airport sources said all aircraft using Calcutta airport have been categorised under four heads.

Category C aircraft — such as Boeing 737, and Airbus A319 and A320 — have been allowed to take off with a minimum runway visibility of 150m.

Wide-bodied aircraft like the jumbo jets (Boeing 747) fall under Category D and have been assigned the visibility requirement of 175m.

All other aircraft, fit to take off with a minimum visibility of 200m, have been tagged A or B.

Officials and pilots said the new guidelines would minimise fog delays at Calcutta airport. During the winter months, visibility often drops to 50m after midnight because of fog. The situation starts improving in the morning when the visibility rises to 150m.

But it takes at least two-three hours for the visibility to touch the 350m mark, the minimum requirement under the previous system for flights to operate.

“With the take-off criteria being significantly reduced, it will now be possible to operate flights in the early morning hours during winter,” said an official.