Close shave...

The traffic collision avoidance system (TCAS), which monitors the airspace around a plane for other aircraft equipped with corresponding active transponders and gives pilots warnings of possible collision risks, came to the aid of the Indigo flights on Tuesday.

The system also instructs pilots about the avoidance action (resolution advisory) to maintain the ideal distance between two aircraft.

“The Tcas-resolution Advisory system was triggered on two of our aircraft operating Coimbatore-hyderabad and Bangalore-cochin routes respectively on July 10. Following normal procedure this has been reported to the regulator,” Indigo said.

The Hyderabad-bound flight 6E-779 had 162 passengers and the one flying to Kochi, 6E 6505, was carrying 166 passengers.

“The planes came face to face when 6E-779 was at 27,300 ft and 6E-6505 was at 27,500 ft, reducing the distance to just 200 ft. The aircraft were hardly four miles from each other,” a DGCA official said on condition of anonymity.

Two aircraft have to be at least 10 nautical miles (lateral separation) and 1,000 feet (vertical separation) apart from each other. DGCA has initiated an enquiry and will check the flight data before questioning the pilots, said an official who asked not to be named.

In Thursday’s incident, Air Arabia flight G9-521 (Sharjah to Chittagong) and Indigo flight 6E-732 (Hyderabad to Lucknow) came close to each other and were just 6 nautical miles apart east of Jabalpur.

“If the distance reduces to less than 10 nautical miles it is called breach of separation and pilot gets an alert. The pilot acts accordingly and remains in touch with ATC,” said an ATC officer on the condition of anonymity.

According to the Airports Authority of India (AAI), the pilots did not issue any resolution advisory and the required distance was maintained after that.

“Our pilot operating 6E-732 flight from Hyderabad to Lucknow followed all instructions given by the Nagpur ATC. We rely upon ATC’S traffic advisory and comply with their instructions. The aircraft made a normal landing and at no point was safety compromised,” said an Indigo spokesperson.

An email sent to Air Arabia on Thursday evening did not elicit a response.

“Institutional correction and better technology is required for safer skies,” said Kapil Kaul, CEO and director, Centre for Asia Pacific Aviation (CAPA), an aviation research body.

“Due to large flow of flights, such breach of threshold do happen. As long as TCAS is activated and reasons are probed, we learn from it.”

TWO AIRCRAFT HAVE TO BE AT LEAST 10 NAUTICAL MILES (LATERAL SEPARATION) AND 1,000 FEET (VERTICAL SEPARATION) APART FROM EACH OTHER.