The Airports Authority of India (AAI) is probing the circumstances that led three Calcutta-bound flights to hover as the airport’s primary runway was unavailable for landing on July 28.

“We are trying to find out how many flights had to hover and the reason for the unavailability of the primary runway,” said an official.

The primary runway was closed for maintenance on July 28 and flights were operating from the secondary runway. The minimum visibility on the secondary runway, which does not have the Instrumental Landing System, has to be 3,600 metre.

Around 4pm, the air traffic control (ATC) told the ground control that visibility had dropped below 3,600 metre because of clouds and that the main runway had to be made available immediately.

“But it takes at least half an hour for a runway under repair to be made available for flight operations. The basic safety norms have to be adhered to,” said an official.

“After the machines and tools for repairs are removed, the runway has to be thoroughly inspected before it can be declared fit for operation,” the official pointed out.

While attempts were being made to open the main runway, ATC sources said, three aircraft bound for Calcutta were asked to “go around” for 15-20 minutes on an average.

The runway was opened after 20 minutes, but the weather had improved by then.

“The ground control said the ATC should have warned them in advance that it might requisition the main runway in an emergency. The ATC, on the other hand, blamed the ground control for its failure to vacate the runway quickly,” an official said.

Airport sources pointed out that flight operations would suffer unless the departments coordinate with each other. “The problem such as the one on July 28 will not crop up once the instrumental landing system is installed on the extended part of the secondary runway,” the official added.