What happened
On February 7, 2018, at approximately 1500 UTC, a serious airprox incident occurred in the Mumbai Area-North airspace involving two commercial aircraft. The first, a Vistara A320 with registration VT-TNE, was operating a flight from New Delhi to Pune. The second, an Air India A319 with registration VT-SCV, was climbing from Mumbai toward Bhopal.
During the descent of the Vistara aircraft, the Area Radar Controller instructed the crew to descend from FL360 to FL290. Simultaneously, the Air India aircraft was instructed to climb to FL270. However, the Vistara crew entered a pre-assumed altitude of FL270 into the aircraft system and descended below the authorized FL290. This resulted in a conflict with the climbing Air India aircraft. A Predicted Conflict Warning (PCW) was generated, which subsequently escalated to a Current Conflict Warning (CCW). Both aircraft received Traffic Collision Avoidance System (TCAS) Resolution Advisies (RA), executing maneuvers to avoid a collision. The aircraft were reported clear of the conflict approximately one minute later.
The investigation
The AAIB India investigation focused on the actions of both the flight crew and the Air Traffic Controller (ATC). Investigators examined the automation system's performance, specifically the Indra Advance Air Control Automation System. The investigation noted that the system's level indicator had turned yellow, signaling a mismatch between the assigned altitude and the altitude entered by the crew, a safety feature that went unaddressed by the controller.
Furthermore, the investigation scrutinized the flight crew's monitoring of instructions and their use of the Navigation Display (ND). It was also noted that the Vistara crew did not provide the Cockpit Voice Recorder (CVR) for analysis, having obtained a verbal dispensation from the DGCA, which prevented investigators from cross-checking the crew's verbal statements regarding the descent clearance.
Findings
- The Air Traffic Controller failed to maintain adequate surveillance and did not identify the altitude mismatch indicated by the automation system.
- The controller failed to acknowledge the PCW and CCW alerts in a timely manner.
- The Vistara flight crew descended below the assigned altitude of FL290, having entered an incorrect level of FL270 into the aircraft's flight management system.
- The flight crew failed to properly monitor the controller's instructions and did not set the Navigation Display to a descent profile that would have revealed the conflicting traffic.