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Monday, June 01, 2020

How aircraft face, avoid collision

Two planes came perilously close over Bengaluru. How frequent are such scares, how do pilots deal with them?

Written by Pranav Mukul | New Delhi | Updated: July 13, 2018 10:01:08 am
How aircraft face, avoid collision, Indigo, indigo sale, indigo offer, Indigo said the incident has been reported to DGCA. (file photo)

On Tuesday night, two IndiGo aircraft in Bengaluru airspace, with over 300 passengers between them, came close to each other during flight, triggering an inbuilt alarm so that the pilots could react to prevent a collision. This “near-miss” was not the first this year. In February, two Qatar Airways aircraft with more than 500 passengers came dangerously close to each other in Chennai airspace; before that, an Air India and a Vistara aircraft with more than 250 passengers narrowly avoided colliding. As per the latest available information from the Directorate General of Civil Aviation (DGCA), 135 “aircraft proximity” incidents took place between 2013 and 2016 in India.

What happened

Flights 6E779 (Coimbatore to Hyderabad) and 6E6505 (Bengaluru to Kochi) came close enough vertically to trigger the planes’ collision avoidance alarm. While a senior DGCA official said the vertical separation was 950 feet, industry sources told The Indian Express that the separation was between 200 and 475 feet at their closest points. According to one source, 6E779 was cleared by air traffic control to climb to 36,000 ft, and 6E6505 to climb to 28,000 ft. They came face to face around 6 km apart, at around 27,000 ft height. According to data sourced from flight tracking website Flightradar24, both were flying at around 400 knots at that point (740 kph). Before they got close enough for the threat to be unavoidable, a mandatory instrument on both aircraft rang warning bells, allowing the pilots to take corrective measures. This instrument is the traffic collision avoidance system (TCAS).

Click image to enlarge.

Avoidance system

TCAS, which is independent of ATC, monitors airspace around an aircraft for other aircraft equipped with similar equipment. The system is mandated by the International Civil Aviation Organization (ICAO), and is to be equipped on all aircraft with a maximum takeoff mass over 5,700 kg or authorised to carry more than 19 passengers. The TCAS has two levels of alarms — traffic advisory and resolution advisory. Pilots get the traffic advisory when another aircraft is in the caution area, or 20-48 seconds away from an imminent collision, and the resolution advisory when a threat is 15-35 seconds away. The system calculates the threat based on various parameters such as speed, altitude and heading. The second level also advises the pilots to make evasive manoeuvres. The norms stipulate that airlines develop policies, procedures and training for avoiding collisions and include them in their operations manual.

Levels of risk

Based on rules prescribed by the ICAO, the DGCA classifies aircraft proximity according to their relative positions and speeds. Category A, the most severe, is classified as a critical incident and involves a high risk of collision with the aircraft passing within 250 feet vertically and 500 feet laterally. Category B is classified as a serious incident and occurs if no action was taken by either the flight crew or the air traffic controller when the aircraft are vertically more than 250 ft away but at less than half the prescribed minimum separation; and laterally more than 500 ft apart but at less than half the approved minimum separation. An incident could also be classified as Category B if no avoiding action was taken but the parameters of the two aircraft were such that safety may have been compromised. Category C is no-hazard and involves the breakdown of prescribed separation standards where direction and altitude would have made a mid-air collision improbable, regardless of evasive action. Category D covers certain incidents in which insufficient information was available to determine the risk involved.

Global aviation norms stipulate that planes in one direction have to fly at altitudes that are odd multiples of 1,000 ft (3,000, 5,000, 7,000… 33,000, 35,000) above mean sea level, while aircraft in the opposite direction are to fly at even multiples. This essentially puts two aircraft at a minimum vertical separation of 1,000 feet.


India has witnessed an over 20% growth in air passenger traffic over the last year, and aircraft movement has nearly doubled to 2.18 lakh in May 2018 from 1.11 lakh in December 2008. While air navigation systems and policies have also been improved over time, DGCA data show that out of the 135 aircraft proximity incidents between 2013 and 2016, 82 were attributable to air traffic control officers. Measures taken by the government include implementation of flexible airspace that reduces traffic congestion, and asking airlines to avoid using similar or confusing call signs. The Airports Authority of India (AAI) has installed additional radars and air surveillance systems for improved tracking. Over the last three years, AAI has hired more than 1,000 air traffic control personnel to meet the additional demand.

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