Here's your 10-point cheatsheet to this big story:
The "breach of separation" was not reported by the Airports Authority of India, or AAI, sources said, quoting a preliminary report by the Directorate General of Civil Aviation, or DGCA. "Breach of separation" happens when two aircraft cross the minimum mandatory vertical or horizontal distance in an airspace.
Two IndiGo flights - 6E 455 going from Bengaluru to Kolkata, and 6E 246 going from Bengaluru to Bhubaneswar - were involved in the scare, sources said. Both were variants of the Airbus A320.
Bengaluru airport operates two runways - north and south. On January 7 morning, flights were taking off from the north runway and landing on the south runway, the DGCA's preliminary report said.
A shift in-charge of runway operations decided to use a single runway, the north one, for both landings and take-offs, the report said. The south runway was then closed, but it was not told to the south tower controller.
The south tower controller allowed the flight going to Kolkata to take off. At the same time, the north tower controller also gave permission to the flight going to Bhubaneshwar to depart.
The clearances by the south and north tower controllers were given without coordination, the DGCA's preliminary report said. The report indicated a communication gap between air traffic controllers after one of the runways was closed.
Both jets should not have been allowed to take off simultaneously in the same direction, DGCA sources have said.
"As both aircraft after departure were on converging heading i.e. moving towards each other, an approach radar controller gave diverging heading and avoided a mid-air collision," the report said.
The matter was not recorded in any logbook and not reported by the AAI. "We are investigating and will take strict action against those responsible," a DGCA official said. The AAI runs air traffic control.
The Bengaluru-Kolkata flight carried 176 passengers and six crew, while the Bengaluru-Bhubaneswar flight carried 238 passengers and six crew - a total of 426 passengers.