Flight operations came to a halt at Calcutta airport for around 20 minutes on Thursday afternoon, as a key component of the Category II Instrumental Landing System (Cat II-ILS) and three radar went out of order following a thunderstorm.

Five flights had to hover over the airport and three others were stranded on the parking bay, unable to land or take off because of poor visibility.

The airport has two instrumental landing systems to guide aircraft to take off or touch down — Cat I and Cat II. For Cat I, the minimum visibility should be 550 metres, while the more advanced Cat II can function even when the visibility drops to as low as 350 metres.

On Thursday, the visibility at the airport started falling as soon as the thunderstorm struck at 12.42pm.

Soon after, the fuse of mid-runway visual range, a key component of the Cat II-ILS, blew off due to a short-circuit and three radar, scanning aircraft movement in the south, west and east, became defunct.

At 12.50pm, the visibility dipped to below 500 metres, making it impossible for flights to land or take off as the CAT-II system was not working. Visibility shot up to 1,500 metres after around 20 minutes, but the flight operations had been disrupted by then.

Five flights scheduled to land at Calcutta had to be kept on hold.

Two Jet Airways flights (from Bangalore and Guwahati) and three Air India flights (from Guwahati, Dimapur and Silchar) hovered over the airport for more than 15 minutes till the visibility improved.

On the ground, Air India’s Calcutta-Bagdogra flight and Air Deccan’s Calcutta-Agartala flights were delayed by more than an hour. Kingfisher’s Calcutta-Ahmedabad flight was also slightly delayed.

The mid-runway visual range equipment was restored in half an hour.

Wet spell alert: A section of airport officials is fearing a repeat of Thursday’s disruption, as the Met office has warned of heavy rain in Calcutta and the rest of south Bengal over the next 48 hours.

The director of the weather section at Regional Meteorological Centre, G.C. Debnath, said a low-pressure trough has formed over the north-west Bay, adjoining the Bengal coast.

There is also a cyclonic circulation (anti-clockwise movement of air) over the Bengal coast, resulting in inflow of moisture from the sea.

“Under the influence of the two systems, there is likely to be heavy rain in Calcutta and its adjacent districts,” said Debnath.