What happened
On October 15, 2018, two Airbus A320 aircraft were operating on reciprocal tracks along ATS route W69 near Kolkata. The first, an Air Asia flight VT-DEL traveling from Bagdogra to Kolkata, was cruising at FL310. The second, an IndiGo flight VT-IHJ traveling from Kolkata to Bagdogra, was climbing toward FL320.
As the Air Asia flight requested a descent, the radar controller instructed the IndiGo aircraft to halt its climb at FL290. Simultaneously, the controller cleared the Air Asia aircraft to descend to that same level, FL290. Because both aircraft were directed to the same altitude while on opposing headings, a Short Term Conflict Alert (STCA) was triggered when they were approximately 17.4 NM apart.
Despite the controller issuing immediate heading changes to divert the aircraft, the separation between the two jets was breached. Both flight crews reported receiving Traffic Alert and Collision Avoidance System (TCAS) Resolution Advisories (RA). The closest point of approach saw the aircraft separated by only 200 feet vertically and 5.8 NM horizontally. The conflict ended once the aircraft followed the TCAS instructions, with one climbing and the other descending.
The investigation
An investigation by the AAIB examined the actions of the Radar Surveillance (RSR) North controllers. The inquiry established that the Executive Controller assigned the identical flight level of FL290 to both aircraft. Furthermore, the Planning Controller failed to identify this duplication in the data block.
The investigation also noted that after the initial STCA alert was generated, the controller did not take immediate corrective action because they were focused on analyzing the cause of the alert. By the time the error was recognized and heading changes were issued, the vertical and lateral separation minima had already been violated.
Findings
- The primary cause of the separation breach was the assignment of the same flight level to two aircraft on reciprocal tracks.
- The controller failed to maintain standard separation minima and adequate surveillance.
- The Planning Controller did not detect the altitude conflict in the data block.
- The controller's delay in responding to the STCA alert contributed to the breach.
- There was no damage to either aircraft and no injuries to the crew.
Safety action
- The Area Executive Controller is to receive corrective training focusing on surveillance, separation techniques, and situational awareness.
- The Planning Controller is to undergo corrective training regarding their specific duties and responsibilities.