Visibility at Calcutta airport dropped to zero owing to heavy fog cover early on Wednesday, grounding flights for over four hours.

“The general visibility dropped to 0 metre at 5am, the first time this has happened in recent memory,” said an airport official.

Flight operations were stalled with no flights taking off or landing between 5am and 9am.

A British Airways flight was cancelled and rescheduled from 5.05am on Wednesday to 1am on Thursday. Its 271 passengers waited on board till 8am on Wednesday, when the flight was cancelled.

About 25 other flights, both domestic and international, were delayed by two to three hours. The Jeddah-Calcutta Air India Haj special was diverted to Nagpur. The flight, which was scheduled to land at Calcutta airport at 5am, finally landed around 11.30am.

“We started experiencing low-visibility problems because of the dense fog around 2am. The situation deteriorated steadily till visibility was zero around 5am,” an airport official said. “Visibility improved to 200 metres at 7am but dropped to 50 metres at 7.30am. At 8am, it was 100 metres.”

Officials said 27 domestic and international flights could not take off and 15 flights failed to land, till conditions improved around 9am.

The airport’s Category II instrument landing system (ILS) with it’s sophisticated runway visual range software was rendered redundant on Wednesday morning.

“Using the Category II ILS, pilots can land at a visibility of 350 metres and can decide whether or not to land from 30 metres, but when visibility is down to zero, nothing can be done,” an airport official said.

“No airport in India is equipped to continue flight operations when visibility is down to zero metres,” he added.

The fog after-effect was felt throughout the morning with runway congestion allowing only 10 flights to take off and 14 flights to land till 11.30am. Morning flights experienced a two-hour delay on an average.

Train services hit: Ten pairs of EMU local trains were cancelled and five pairs were delayed between 7.15am and 12.30pm on the Baruipur-Lakshmikantapur section of Eastern Railway’s Sealdah (South) division. “When our drivers were forced to slow down in the fog, some passengers protested by throwing banana leaves on overhead wires causing a short-circuit,” said an official.