Aircraft flying over Calcutta hit a rough patch late on Thursday — not because of Cyclone Sidr, which veered off at the last moment, but due to the malfunctioning of a key link tool.

A direct speech circuit (DCS) at the Calcutta air traffic control, which provides hotline communication with Yangon airport, has not been functioning for three months.

As a result, around 300 aircraft that flew over Calcutta from 12.30am to 3.30am — the period of high route congestion because of the cyclone alert — stood the risk of coming dangerously close to each other or flying at levels lower than what is allowed.

“It’s a problem communicating with Yangon on a normal day. On a night when routes are jammed, it’s a nightmare,” said an airport official.

The Sidr alert prompted many planes flying over Calcutta to change their route. “The flights take either the Area East or Area South route. After the warning, the flights that were assigned Area East opted for Area South to avoid any danger,” the official recalled.

“There was a high level of congestion in Area South and to ensure the required separation between the planes, we needed to be in direct touch with Yangon airport,” the official added. But direct contact could not be established in the absence of the DSC.

“With the link down, we depend on standby modes like ISD and teleprinter. While the ISD lines are often engaged, communicating via the teleprinter is time-consuming,” another official explained.

“Pilots were seeking to change levels because of the congestion, but we could not give them prompt permission. Without Yangon’s consent, we could not allow any change in the pre-determined flight height,” the official said.

Some planes were forced to fly at the “minimum possible height” to ensure a safe vertical distance with other aircraft. The Boeing 747 and Airbus 330, which usually maintain a height of 30,000 ft, flew at 24,000 ft on Thursday night.

“A delay of a few seconds could lead to a disaster in aviation. We are lucky that no mishap occurred on Thursday night,” said an officer at Calcutta’s air traffic control.

An Airports Authority of India official said the problem in the DSC was at the Yangon end. “We have asked Yangon to repair the defect,” the official claimed.