An offshore helicopter crash in which four people died was caused by pilot error, an inquiry has ruled.
Sheriff Principal Derek Pyle said the Super Puma had not maintained the correct speed as it approached its landing in Shetland in 2013.
He said the reason for the error remained unknown – but that there had been “no wilful neglect” by the pilot.
A total of 18 people were on board when the helicopter hit the sea on its approach to Sumburgh.
The Super Puma overturned and filled with water, but it did not sink due to its flotation devices.
Sarah Darnley, 45, from Elgin; Duncan Munro, 46, from Bishop Auckland; and George Allison, 57, of Winchester, drowned in the accident.
Gary McCrossan, 59, from Inverness, who had cardiac disease, died from heart failure following the crash.
The inquiry also heard that one survivor, Sam Bull, took his own life four years later aged 28 after suffering post-traumatic stress disorder (PTSD).
Pilot Martin Miglans and co-pilot Alan Bell were among those who escaped.
Mr Bell was praised during the inquiry for being able to arm the flotation devices before impact with little warning.
In his findings, Sheriff Principal Pyle said the cause of the accident had been pilot error, but the reasons for this remained unknown.
“At the end of the day we know that for whatever reason or reasons the commander failed to maintain the target approach speed,” he said.
He said one possible reason was in the developing knowledge of the inability of the human brain continuously to monitor flight instruments.
“There was plainly no wilful neglect,” he added.
“Rather, there was, as one witness described it, a perfect storm of circumstances which resulted in all the safety barriers in place not preventing – or remedying – his one failure, to maintain the correct speed.”
During the inquiry Martin Richardson, for the Crown, argued that the accident had been caused by a failure to maintain the helicopter’s target approach speed.
He said that failure arose because the flight instruments were not effectively monitored by the flight crew, and that the Crown believed the “principal cause” was pilot error.
The inquiry heard differing opinions about the effectiveness of the submerged escape training which was undergone by the crew and passengers on North Sea flights.
Sheriff Principal Pyle said a balance has to be struck between realism and the safety of those being trained.
“In truth it would be impractical, perhaps impossible, to provide realistic training which would ensure that passengers had sufficient experience to make a significant difference,” he said.
He added: “This was a dreadful accident with long-term repercussions for the survivors and the families of the deceased which no determination by this court can properly describe.
“My condolences go, in particular, to the families of the deceased, including – lest it be forgotten – the family of Mr Bull.”
The fatal accident inquiry, which had been delayed earlier this year by coronavirus, was described as the largest virtual fact-finding case of its kind.
The seven-year wait for the inquiry to take place had previously been described as “deplorable” by Sheriff Principal Pyle.
He said it was a “great credit” to all the legal teams involved that the hearing itself took less than the four weeks it had been expected to last.
Jonathan Nicholson, assistant director at the UK Civil Aviation Authority (CAA), said a major safety review had been launched in 2014.
He said breathing equipment was improved for passengers in emergencies, along with escape procedures. There were also restrictions on some of the extreme weather in which helicopters could operate in, and efforts to prevent accidents taking place.
“Offshore helicopters work in an extreme environment”, Mr Nicholson said.
“But we want to make sure, working with everybody involved, that they have the safest possible measures in place for all those that use them.”