Fourteen oil workers and two crew died when the Bond Super Puma plunged into the water off the Aberdeenshire coast on April 1, 2009.
Sheriff Principal Derek Pyle in a fatal accident inquiry report yesterday said precautions which might have avoided the deaths included following agreed procedures and communications regarding possible issues with the gearbox a week before the accident. He concluded that on the balance of probabilities spalling - the fracturing of metal - in the gearbox was the probable cause of the accident.
Solicitors acting for the families believe the determination shows there is a "case to answer" with regard to a criminal prosecution under health and safety legislation. But the Crown Office says evidence put to the FAI had not changed its view that there was insufficient evidence to justify criminal charges saying Mr Pyle makes clear that a "reasonable doubt" remained over the technical cause of the crash.
Caroline Doyle, 35, whose father Raymond, 57, of Cumbernauld, North Lanarkshire, died, said: "How this is not deemed a breach of health and safety is beyond comprehension."
Audrey Wood, whose son Stuart was one of those killed, said: "How they arrived at that decision will haunt us.
"The decision was also made without all the evidence being present, as vital witness statements had not been given."