All four radar at Calcutta airport and two satellite-based surveillance systems malfunctioned simultaneously on Thursday evening, forcing pilots to bank entirely on verbal instructions for landing and take-off.

Three flights bound for Calcutta had to be diverted and at least 10 aircraft hovered over the city for 30 to 40 minutes, officials said.

"A power cut believed to have been caused by a cable fault triggered the systems failure. Air traffic control had to depend on voice communication to operate flights," a senior airport official said.

The airport has power back-up for the radar but only for 30 minutes. On Thursday, the power failure occurred around 6.30pm and the radar went on the blink when supply could not be restored by 7pm, the official said.

The first of the four radar malfunctioned at 6.55pm. Power supply was fully restored at 8.45pm.

"One moment there were dots marking the flight positions in the air and the next moment there was nothing. The monitors went blank," an ATC official said.

"I was terrified for the entire duration landing was delayed," said a flier stepping out of the airport.

The airport has three surveillance radar for east, west and south and one for approach. There is also a surface-control radar to monitor the ground movement of aircraft.

The automatic dependent surveillance system and the controller-to-pilot data link system, both satellite-based, also malfunctioned. By 7pm, there was no visual surveillance of air traffic.

"We had to fall back entirely on voice communication, wherein the pilot of an aircraft gives his position and based on that, the course and speed of another aircraft is fixed," an official said.

The system is fraught with risks when traffic is high. "There is a chance of two aircraft being on a collision course," the official said.

In the manual system, the lateral separation between two aircraft is much more than that when ATC personnel can see flight positions on the radar.

Under normal circumstances, a five-nautical mile (9.26km) separation is used within a radius of 60 nautical miles (111km) of the airport. The separation is 10 nautical miles (18.52km) when two aircraft are beyond 60 nautical miles. The gap is extended to 80 nautical miles (148.16km) when surveillance is manual.

Jet Airways' flight from Mumbai was diverted to Dhaka and IndiGo's Patna flight to Bhubaneswar on Thursday evening. Air India's Bhubaneswar-Calcutta flight was sent back to base. Between 7 and 8pm, only three out of the 10 scheduled flights were allowed to take off.