Synopsis
On 15 October 2018, a serious incident (Airprox) occurred between two Airbus A320 aircraft, VT-DEL (Air Asia flight IAD583) and VT-IHJ (Indigo flight IGO797), over Kolkata. The incident took place at approximately 0549 UTC. No injuries to persons or damage to either aircraft were reported.
History of the Flight
VT-DEL was operating scheduled flight IAD583 from Bagdogra to Kolkata, while VT-IHJ was operating IGO797 from Kolkata to Bagdogra. Both aircraft were under the control of the RSR North radar controller on frequency 133.75 MHz.
At 054426 UTC, IGO797 reported climbing through FL258 en route to a requested FL320. The controller instructed IGO797 to stand by due to reciprocal traffic. At 054520 UTC, IAD583 requested descent. At 054653 UTC, the controller instructed IGO797 to stop climb at FL290 and simultaneously cleared IAD583 to descend to FL290, entering FL290 in the data block for IAD583. The planning controller did not notice the same level assignment.
At 054736 UTC, a Short Term Conflict Alert (STCA) was generated when the aircraft were 17.4 NM apart with 57 seconds to crossing. IAD583 was descending through FL309 and IGO797 had just reached FL290.
At 054743 UTC, the controller directed IAD583 to turn left heading 090 and IGO797 to turn left heading 270. Both aircraft read back the instructions. However, IAD583 was observed to have changed heading to 120, and IGO797 continued its previous heading.
At 054825 UTC, both aircraft reported receiving Traffic Alert and Collision Avoidance System Resolution Advisories (TCAS RA) and followed RA procedures. IAD583 climbed to FL294 while IGO797 descended to FL285.
The minimum separation recorded was: at 054822 UTC, 5.8 NM horizontal and 200 feet vertical; at 054827 UTC, 4.9 NM and 500 feet; at 054840 UTC, 2.8 NM and 800 feet vertical.
By 054945 UTC, both aircraft were clear of conflict. IAD583 subsequently entered a conflict with another aircraft (BTN715), which was resolved by instructing BTN715 to stop climb at FL270. Both flights then continued normal navigation.
Findings
The investigation found that the required standard vertical and lateral radar separation was breached, reducing to 800 feet and 2.8 NM at 054840 UTC. The STCA alert was generated, and TCAS RAs were issued. No other contributing factors were identified.
Probable Cause
The probable cause of the serious incident was the issuance of the same level (FL290) to both aircraft by the executive controller and inadequate supervision by the planning controller.