Two Indian domestic flights averted a potential mid-air clash on Wednesday in Bangladesh airspace, thanks to a Calcutta air traffic control (ATC) alert in the nick of time, airport officials said.

A Guwahati-Calcutta IndiGo flight came “very close” to a Chennai-Guwahati flight of the same airline when both were in the Dhaka flight information region above Rajshahi, an official said.

A flight information region is a zone monitored by a particular ATC.

The flights were about 120 nautical miles or 222km from Calcutta at 5pm on Wednesday when a possible mid-air clash was averted.

The Guwahati-Calcutta flight was at 36,000ft while the Chennai-Guwahati was at 35,000ft, the airport official said.

“We heard the Dhaka ATC asking the Guwahati-Calcutta flight to descend 1000ft, which would have been dangerous as the Chennai-Guwahati flight was at that altitude,” he said.

The Calcutta ATC immediately contacted the Chennai-Guwahati flight’s pilot. “Every second matters in aviation safety. We realised time would be lost if we tried contacting the Dhaka ATC... so, we contacted the pilot,” the official said.

“We asked him to change the plane’s direction and he promptly did so. This was essential to move away from the descending flight’s path,” the official said.

By changing the Chennai-Guwahati flight’s direction, a lateral separation of 10 nautical miles or 18.52km between the two flights was achieved, he said.

“This was a safe distance between the two flights.”

The Calcutta airport authorities later alerted the Dhaka ATC about the matter.

Flights have a traffic collision avoidance system (TCAS) that alerts pilots if two planes come dangerously close to each other, a pilot said.

“After the TCAS alerts pilots, planes either change direction or altitude. Usually, the flight that is at a lower altitude descends further because an abrupt descent is easier than an abrupt ascent,” the pilot said.

The Directorate General of Civil Aviation cannot inquire into the matter because it happened in Bangladesh airspace.