What happened
On January 29, 2021, at 05:54 UTC, two Airbus A320 aircraft were involved in a serious airprox incident within the Mumbai Area North Control jurisdiction. The first aircraft, an Air Asia flight (VT-MLE) traveling from Ahmedabad to Chennai, was climbing through the flight level assigned to the second aircraft, an IndiGo flight (VT-IAL) traveling from Bangalore to Vadodara.
At the start of the encounter, the Air Asia crew requested a climb from FL320 to FL390, which the radar controller approved. Simultaneously, the IndiGo aircraft was cruising at FL380. As the two aircraft moved onto reciprocal tracks, the automation system issued a Predicted Conflict Warning (PCW) at 05:53:10 UTC. Despite this, the vertical separation continued to decrease. The Air Asia aircraft eventually climbed through FL380, triggering a TCAS RA (Resolution Advisory) for the crew of VT-MLE, while the IndiGo crew received only a TCAS TA (Traffic Advisory). The minimum vertical separation between the two aircraft was recorded at just 300 feet.
The investigation
The AAIB India investigation examined the radar controller's actions, the flight paths of both aircraft, and the automation system alerts. The investigation established that the controller was managing a larger surveillance area than usual, as the Lower Area Control (LAC) had been temporarily withdrawn due to pandemic-related staffing and traffic changes.
Investigators found that the controller failed to react to the initial automated warning. Furthermore, the investigation revealed that the controller's assessment of the situation was influenced by the unexpected routing of the Air Asia flight. While the controller expected the aircraft to follow a standard route, it was instead following a STAR (Standard Terminal Arrival Route) that the controller incorrectly assumed would involve a descent, leading to a failure to intervene before the conflict became critical.
Findings
- The primary cause of the incident was the loss of situational awareness by the air traffic controller.
- The controller evaluated the unfolding situation under the influence of a preconceived mind regarding standard flight routes.
- The controller did not react to the Predicted Conflict Warning (PCW) generated by the automation system.
- The controller's attempt to halt the climb of VT-MLE at FL370 occurred when the aircraft was already at FL371.
- A contributing factor was the increased workload and larger surveillance area required of the controller due to the temporary withdrawal of the Lower Area Control sector.
Safety action
- It is recommended that the involved radar controller receive corrective training focusing on situational awareness and the necessity of responding to automation system warnings.
- It is recommended that the Airports Authority of India reassess traffic volumes in sectors withdrawn during the pandemic and consider reinstating previous sector configurations to manage workload.