Human error led to the sinking of the INS Sindhurakshak submarine, Indian Navy Chief Admiral R.K. Dhowan said Wednesday, indicating that the crew's failure to follow standard operating procedures could have caused the accident."In case of a submarine, aspects related to procedures need to be followed very strictly. There could have been a mistake while handling of equipment or weapons," Dhowan said."When procedures are not followed, there is room for error," he said at a press conference on the eve of the 43rd Navy Day.Eighteen navy personnel - three officers and 15 sailors - were killed when blasts ripped through the torpedo compartment of the INS Sindhurakshak while the submarine was berthed in Mumbai harbour."It is a very, very serious accident and has been taken by the navy very seriously. It is not normal for a submarine to explode like that. It is not because of one cause, but a series of issues," Dhowan said."All aspects need to be looked into. The board of inquiry is looking into it, but we have a fairly good idea about what happened. We are not waiting for an outcome and direction has been sent to all submarines to be extra careful in following standard operating procedures," he said.