A serious incident involved an AirAsia flight (flight number IAD811) and IndiGo flight (number IGO747) on January 29, 2021. The aircraft came in very close proximity of each other risking collision, as revealed by the report of the Aircraft Accident Investigation Bureau (AAIB) of India.

The AirAsia flight was operating from Sardar Vallabhbhai Patel International Airport (AMD) to Chennai International Airport (MAA), while the Indigo flight was on a route to Vadodara Airport (BDQ) from Kempegowda International Airport Bengaluru (BLR) when the incident occurred over the Mumbai airspace.

The Indigo flight maintained an altitude of 38,000 feet after receiving clearance from the Air Traffic Controller (ATC) in Mumbai when it was nearing its destination. The AirAsia aircraft, operating on a non-standard route, requested to climb from 32,000 feet to 39,000 feet during the same period. The flight paths of both aircraft were reciprocal.

Due to increasing traffic and a lack of situational awareness from the ATC, the narrow altitude difference between the aircraft was not acknowledged.

The conflicting paths generated a Predicted Conflict Warning (PCW) from the Air Traffic Controlling System (ATCS) . The ATC officer ignored this warning due to a preconceived notion regarding the Air Asia flight following a standard route.

"Controller had preconceived mind of regular routing of IAD811 but on the day of incident aircraft was following different route," stated the report. "Thus, controller did not realize the current scenario i.e. failed to evaluate the present situation under his preconceived mind."

The controller eventually realized the situation and tried to handle it accordingly. Still, the AirAsia flight Traffic Collision Avoidance System (TCAS) had already issued a warning when the aircraft climbed 38,008 feet and the IndiGo aircraft 38,000 feet.

The report concluded that the flights had a lateral separation of eight-kilometer at 300 feet vertical proximity and a six-kilometer lateral separation at a 500 feet vertical difference.

It was indicated that the serious incident occurred due to 'Loss of situational awareness by Controller' and 'Evaluation of the present situation under the influence of preconceived mind by the concerned controller' according to the report. Additional training for the controller was recommended.