What happened
On November 2, 2016, at approximately 18:27 UTC, a serious air proximity incident occurred within the Delhi Air Traffic Control (ACC) airspace involving two commercial aircraft. The first, an IndiGo Airbus A320 (registration VT-IFO) flying from Delhi to Bengaluru, was climbing through flight level 341. The second, a KLM Royal Dutch Airlines Boeing 787-900 (registration PH-BHG) traveling from Amsterdam to Kuala Lumpur, was cruising at flight level 350.
As the Airbus A320 climbed, the flight crew observed the KLM aircraft at a 2 o'clock position. Due to heavy radio frequency congestion, the crew was unable to establish contact with Air Traffic Control to report the traffic. Simultaneously, the Traffic Alert and Collision Avoidance System (TCAS) on both aircraft triggered a Traffic Advisory (TA), followed quickly by a Resolution Advisory (RA). The crew of VT-IFO executed an immediate descent as instructed by the TCAS. During the maneuver, the lateral separation between the two aircraft had decreased to 3.6 NM, while the vertical separation was reduced to a critical 200 feet.
The investigation
The AAIB investigation focused on the coordination between various Delhi ACC sectors during a period of airspace consolidation. The inquiry examined the handover processes between the South East, East, and West sectors. Investigators reviewed radar snapshots, ATC tape recordings, and flight progress strips to determine how the conflict developed.
Key elements examined included the automation system's performance, specifically the generation of Predicted Conflict Warnings (PCW) and Current Conflict Warnings (CCW). The investigation also scrutinized the communication between the radar controller and the flight crews, noting that the controller had inadvertently used an incorrect call-sign, "Vistara 977," when attempting to issue a descent instruction to the IndiGo crew.