What aircraft was involved and where did it happen?

The accident on null involved a aircraft, registration VT-WGE, at IN.

What was the probable cause of the accident?

The incident was caused by a failure in ATC coordination between the North and South radar sectors, compounded by the flight crew of VT-WGE performing a maneuver that contradicted the TCAS Resolution Advisory.

TCAS Resolution Advisory Conflict Near Varanasi

Two Airbus A320 aircraft narrowly avoided a collision near Varanasi after a series of ATC coordination failures and improper flight crew responses to TCAS warnings.

What happened

On April 23, 2018, an airprox incident occurred in the Varanasi airspace involving two Airbus A320 aircraft. The first aircraft, VT-WGE (operating flight GOW685 from Mumbai to Patna), was descending from FL370 to FL330 when it encountered the second aircraft, VT-IGK (operating flight IGO6488 from Kolkata to New Delhi), which was cruising at FL360.

During the encounter, the flight crew of VT-WGE received a TCAS Resolution Advisory (RA) indicating a need to climb. However, the aircraft's descent rate actually increased to 3,900 feet per minute following manual inputs. Simultaneously, the crew of VT-IGK followed a TCAS RA to descend. The vertical separation between the two aircraft dropped to approximately 500 feet during the conflict. Both crews eventually reported being clear of the conflict to Air Traffic Control (ATC).

The investigation

The AAIB India investigation focused on the coordination between the Radar (South) and Radar (North) controllers, as well as the flight crew's response to automated warnings.

Investigators found that the Radar (South) controller was managing VT-WGE despite the aircraft being within the jurisdiction of the Radar (North) controller. The South controller issued descent and heading instructions without coordinating with the North controller. Furthermore, the North controller failed to identify the jurisdictional mismatch or recognize the impending conflict between the two aircraft.

Regarding the flight crew of VT-WGE, the investigation examined the aircraft's flight data recorder (DFDR). While the crew reported sluggish engine response during a climb attempt, the data showed normal engine performance. The investigation also noted that the crew did not follow standard procedures for altitude selection, which impacted their situational awareness regarding nearby traffic.

Findings

The Radar (South) controller maintained control of VT-WGE beyond the designated transfer point and issued descent instructions without coordinating with the North sector.
The Radar (North) controller failed to maintain adequate surveillance, failed to acknowledge conflict warnings, and did not prioritize transmissions effectively.
The flight crew of VT-WGE failed to follow standard procedures for altitude selection, reducing their awareness of surrounding traffic.
The flight crew of VT-WGE applied incorrect procedures when responding to the TCAS RA, maneuvering the aircraft in direct contradiction to the RA command.

Safety action

Enhanced surveillance and approval protocols for simulator training must be implemented.
Training programs for controllers must be updated to include improved coordination and handover procedures.