Airprox between Indigo and AirAsia India A320s in Mumbai Airspace (VT-IAY)

Incident OverviewOn 21 March 2022, a serious airprox occurred in Mumbai airspace between an Indigo Airbus A320, registration VT-IAY, operating flight IGO6261 from Delhi to Mumbai, and an AirAsia India Airbus A320, registration VT-HYD, operating flight IAD773 from Delhi to Goa. The event was classified as a serious incident and investigated under Rule 11(1) of the Aircraft (Investigation of Accidents and Incidents) Rules, 2017, with the investigation ordered on 29 March 2022 under reference INV12011/3/2022-AAIB. No injuries or aircraft damage were reported for either flight.## Sequence of EventsFlight IAD773 was cruising at flight level 360 (FL360) and contacted Mumbai Radar at 154907 UTC after being handed over from Ahmedabad. It was instructed to proceed direct to waypoint MABTA while maintaining FL360. Flight IGO6261 was at FL380 and contacted Mumbai Radar at 155045 UTC. The pilot in command requested a descent, and at 155105 UTC the controller cleared the aircraft to descend to FL370. The crew read back the clearance incorrectly as “descent level 310 IFLY6261 confirm,” and the controller acknowledged with “IFLY6261 Affirm.” The pilot then set FL310 in the flight management system, and the aircraft began descending at a faster rate than intended.## Radar Conflict and Separation LossAt 155142 UTC, while issuing heading instructions to IGO6261, a Predicted Conflict Warning (PCW) was generated in the automation system at 155147 UTC. The controller checked the data block and observed that the pilot-selected altitude was FL310 rather than the cleared FL370. The controller transmitted a correction instructing IGO6261 to turn right, fly heading 217, maintain FL370 on reaching, and check the selected altitude, noting that FL370 was the cleared level. The flight crew did not respond to this transmission or a subsequent instruction to maintain FL370. When the crew eventually responded, they reported having already crossed FL370 and reached FL360, the same level as IAD773. A Current Conflict Warning (CCW) was then generated, and standard vertical and lateral radar separation reduced to zero feet and 3.8 nautical miles, respectively.