What happened
On 10 November 2023, a serious airprox incident occurred near Indira Gandhi International Airport (IGI), New Delhi, involving two large commercial aircraft. An Ethiopian Airlines B787-800, registered ET-ATL, was departing from Runway 29R on a scheduled flight to Addis Ababa. Simultaneously, a Vistara A320-251N, registered VT-TQL, was on approach to Runway 29L.
During the approach, the Vistara flight encountered wind shear, forcing the crew to initiate a missed approach. As the aircraft climbed, its flight path intersected with the departure track of the Ethiopian Airlines jet. This conflict triggered the Traffic Alert and Collision Avoidance System (TCAS) on both aircraft, issuing Resolution Advisories (RA). A second TCAS RA occurred after air traffic control instructions failed to sufficiently de-conflict the two planes. The aircraft eventually reached a minimum separation of approximately 0.2 NM laterally and 400 feet vertically before the conflict was resolved.
The investigation
The Aircraft Accident Investigation Bureau (AAIB) India conducted an inquiry examining radar recordings, radio communications, and flight data from both aircraft. The investigation focused on the air traffic control (ATC) environment at IGI Airport, which was operating in a 'Segregated Dependent Westerly Mode.' This specific operational setup required departures from Runway 29R to be dependent on the position of arrivals on Runway 29L.
Investigators scrutinized the coordination between the Aerodrome Controller and the Approach Departure Controller. They also examined the technical status of the electronic flight strips and the impact of a previously cancelled flight plan for a different aircraft (a Learjet) which had caused a lack of correlation on the controller's situation display, potentially affecting situational awareness.