Twelve hours. Four hundred flights. Zero communication.

Lady Luck, rather than an on-ground tech tool, helped avert a mid-air collision over Calcutta as key communication gadgets at the airport went on the blink from Wednesday night to Thursday morning.

Vital installations that enable the communication tower to guide an aircraft were out of order, in phases, between Wednesday 8pm and Thursday 8am.

The air traffic control (ATC) hit the panic button, and with good reason. “With around 400 aircraft operating from or flying over Calcutta during this period and the airport tower unable to guide them properly, there could have been a disaster,” said a senior airport official.

According to sources, for a significant period of those 12 harrowing hours, “there was no link between the ATC and aircraft” with all key communication gadgets and their back-ups suffering technical glitches.

Trouble started around 8pm on Wednesday when the Automatic Dependence Surveillance and Controller Pilot Data Link Communication malfunctioned.

“This enables the controller to know the position of an aircraft and communicate with the pilot, acting as a cushion for the existing surveillance and communication systems to ensure aircraft safety,” said an official.

The equipment kicked into life around 10pm, but spluttered again after midnight.

For about an hour, the system was non-functional.

If that was not bad enough, from 11pm to 11.30pm and then 12.15am to 1am, the Flight Data Processing System at the ATC — tracking all vital information about a flight, from type of craft to altitude — developed a snag. “If this does not work, controllers cannot provide the required separation between two aircraft needed for their safety,” said an official.

Things took an even more risky turn when the Remote Control Air Ground System tripped from 11.30pm to 8am. This hit direct communication between the ATC and a pilot, as this plays the most essential part of guiding an aircraft.

“The situation was very grim. Some aircraft did come close to each other on a few occasions,” said an official, but not enough to set off the Traffic Collision Avoidance System (TCAS) beep when two aircraft come within 40 nautical miles of each other.

A detailed report has been submitted to senior airport officials and an internal probe will be conducted, sources said.