Navy and Coast Guard, which have launched a search and rescue operation to locate the missing pilots and wreckage, have sighted a door of the aircraft.
Two Immediate Support Vessels of the Navy have reached the area, about 82 nautical miles off south Mumbai coast, sources said. ONGC vessels have also been pressed into service.
The 14-seater VTPWF Dauphin aircraft of 2001-make took off from the ONGC installation at 7:12 PM and contact with it was lost eight minutes later, a top official of the state-run Pawan Hans said.
Since the chopper was not on crew duty, there were no passengers onboard, ONGC Director (offshore) T K Sengupta told PTI. ONGC hires Pawan Hans helicopters to ferry its officials from Mumbai to its offshore oilfield.
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"Destroyer 'INS Mumbai' with integral helicopter will reach the area after midnight to join the search and rescue efforts," it said.
In order to augment the search, a night capable helicopter Seaking C is being launched, while a second helicopter will be placed on standby, a defence spokesman said.