Two passenger aircraft passing through airspace monitored by Calcutta airport came within less than 1,000 feet of each other on Monday after one of the pilots mistakenly acted on a “descent clearance” from air traffic control to a third plane, sources said.
Built-in alarm systems designed to go off when the vertical distance between aircraft falls below the threshold of 1,000ft alerted the cockpits to the possibility of a collision in the nick of time.
The two aircraft involved in the mid-air scare belonged to Bangladesh airline United Airways and Saudi Arabia carrier Saudia.
The alerts sent through the traffic collision avoidance system prompted the Riyadh-bound Saudia plane to make a sharp descent and the Dhaka-bound United Airways flight to increase its altitude. Both were then flying in opposite directions over Jamshedpur, which is managed by ATC Calcutta.
The confusion, sources said, was possibly caused by the pilot of the Bangladesh airline picking up an instruction from ATC to another aircraft flying on the Dubai-Dhaka route.
The descent instruction was meant for an Emirates flight that had then reached Rajshahi, about 240km from Dhaka, while the United Airways flight was still more than 600km from the Bangladesh capital, the sources said.
“The port of departure and destination was the same for the Emirates and United Airways flights. What may have added to the confusion was the similarity in the flight numbers,” a senior official at Calcutta airport said.
The instruction to descend was meant for the Emirates flight EK-584 but the pilot of the United Airways flight 4H-582 thought it was for him.
As the United Airways flight descended, it breached the 1,000ft vertical distance that two aircraft are supposed to maintain as a safety protocol. As soon as the vertical separation between the Bangladeshi and Saudi flights shrunk, both pilots received a resolution advisory through the traffic collision avoidance system.
In accordance with standard operating procedure, the captains followed the advisory and moved in vertically opposite directions to regain the 1,000ft distance between them.
The ATC tower at Calcutta airport received an alert as well.
If two aircraft are on a collision course, the traffic collision avoidance system sends various alerts, depending on their proximity.
“If two aircraft are less than 40 seconds away from each other, the system gives a resolution advisory asking the aircraft either to climb or descend to move away from the other plane,” a pilot said.
In modern aircraft, a voice usually asks the pilot to either descend or ascend to avoid a collision. The altitude and position of the flight are simultaneously displayed on the screen in front of the pilot.
Sources in the directorate general of civil aviation (DGCA) said a separation of five nautical miles or 9.26km was mandatory between two flights moving at the same level with radar coverage. In places where there is no radar coverage, the recommended distance is 10 nautical miles or 18.52km.
“The vertical separation between two flights is usually 1,000 feet or more,” an official said.
On July 11, two passenger flights were seconds away from colliding at 30,000ft over Bagdogra when alerts from the traffic collision avoidance system forced the pilots to change course. IndiGo’s Bagdogra-Delhi flight with 131 passengers on board made a sharp descent while Air India’s Delhi-Bagdogra flight turned right to avoid colliding.
Airport officials said the DGCA had instituted an inquiry into Monday’s incident.