Collision of 2 IndiGo planes averted in November; AAIB probing incident

The two planes came dangerously close shortly after departing from Delhi airport

Two A321neo planes belonging to IndiGo, one en route to Raipur and the other to Hyderabad, came dangerously close to each other in the airspace shortly after departing from Delhi airport on November 17 last year, prompting the Aircraft Accident Investigation Bureau (AAIB) to investigate the "serious incident".

"At the time of the closest vertical separation (400 feet), lateral separation was 1.2 nautical miles (between the two planes). At the time of the closest lateral separation (0.2 nautical miles), vertical separation was 800 feet," stated the AAIB's preliminary report reviewed by Business Standard.

As per the regulations, aircraft in the air must maintain a vertical separation of at least 1,000 feet and lateral separation of a minimum of five nautical miles between them.

IndiGo did not respond to Business Standard's queries on whether action had been taken against the pilots involved in the incident. The Airports Authority of India (AAI), responsible for air traffic control services in India, also did not respond to queries from the newspaper on whether any action had been taken against the air traffic controller concerned.

The AAIB's report detailed the incident. An IndiGo A321neo plane (registration number VT-IUO), which was heading to Hyderabad, departed from runway 27 of Delhi airport at 12.31 PM on November 17, the report noted. The air traffic controller gave this aircraft's pilots a specific route to follow and asked them to climb to 8,000 feet. However, the pilots turned left towards the takeoff path of another runway (named 29R), instead of following the specific route given by the air traffic controller.

At the same time, the pilots of the second IndiGo A321neo aircraft (registration number VT-ISO), bound for Raipur, received clearance from the air traffic controller to depart from runway 29R and follow a specific route (different from the one given to the first plane's pilots) while climbing to an altitude of 4,000 feet, the report stated.

Consequently, a "breach of separation" occurred between the two aircraft, activating the Traffic Alert and Collision Avoidance Systems (TCAS) in both planes, which sent alerts to the pilots, the report noted. 

TCAS is a safety feature in aircraft that helps prevent mid-air collisions by providing advisories to pilots regarding the presence of other nearby aircraft. The entire incident on November 17 happened within less than one minute. 

IndiGo is India's largest airline and carries about 60 per cent of the country's domestic air traffic. It is not the first time that two IndiGo planes have come too close in airspace above an Indian airport. On January 7, 2022, two IndiGo aircraft - one heading to Kolkata and the other to Bhubaneswar - came too close in the Bengaluru airspace after their simultaneous take-offs.

The Directorate General of Civil Aviation in May 2022 suspended the air traffic controller for three months for approving these simultaneous departures.