Flight SIA 407, report your position.

Flight SIA 407, reporting position IBUDA at GMT 19.47, flight level 370 (37,000 feet).

The conversation between Calcutta airport’s air traffic control (ATC) and the pilot of a New Delhi-Singapore flight of Singapore Airlines was interrupted by a strange voice possibly speaking Chinese.

It happened at night a few days ago when the flight was at IBUDA — an aviation term to explain the point between Bhubaneswar and Khajuraho.

“The voice communication link with the pilot was disconnected for more than 10 minutes as the voice from another frequency mode, apparently speaking Chinese, continued to disrupt the system,” said an ATC official. “We had to communicate with the pilot over an emergency line but it took some time to change to that frequency.”

He said the frequency mode of a radio station could have been intercepted by the airport communication system.

This was not the only incident. “Strange voices in unknown languages” have kept disrupting communication between the ATC and aircraft over the past few months. The problem has occurred only in the southern sector of Calcutta’s control area, which routes flights to and from Southeast Asia, Europe and West Asia. More than 650 flights, including overflying aircraft, are handled by the ATC in this sector.

ATC officials blamed the very high frequency (VHF) system at the airport. Not only do FM radio channels interrupt communication, but phone conversations also block VHF at times. “Conversations between two persons on satellite phones can be heard instead of the pilot’s voice,” an official said.

Interruptions represent a threat to safety. “If an aircraft has to be redirected to a different route or the altitude needs to be changed on an emergency basis, this can become hazardous,” an airport official said.

Earlier this month, there was a problem with the approach VHF of the airport, causing delays in landing and take-off. Officials said the communication and navigational equipment were being upgraded as part of the modernisation drive.