A report into the Rescue 116 tragedy has found databases didn’t detect the island which the crew crashed into.
Four members of the Irish Coast Guard lost their lives in the tragedy off the Mayo coast in March 2017.
The Air Accident Investigation Unit (AAIU) final report was published today, following a lengthy investigation.
It contains harrowing details of the final moments of Rescue 116, where the winchman was heard announcing 'come right now, come right, COME RIGHT'.
Captain Dara Fitzpatrick, Captain Mark Duffy, winch operator Paul Ormsby and winchman Ciaran Smith all lost their lives in the crash.
They had been tasked to help airlift a casualty from a fishing vessel and were going to refuel at Blacksod before proceeding.
The investigation has found the probable cause of the crash was that the helicopter crew members were flying at 200 feet in poor weather just before 1am and were unaware that a 282-foot obstacle - namely Blackrock island – was on the flight path.
A total of 12 contributory causes were also identified.
Among them was that neither their databases nor imagery indicated the presence of Blackrock.
It also found that helicopter operators CHC Ireland didn’t have a formalised, standardised, controlled or periodic system in place to test flight routes.
The report notes neither flight crew member had operated recently in Blacksod, while the commander had been awake for 18 hours and the co-pilot for 17.
It notes: "Databases did not indicate the presence of Blackrock, and neither did some Toughbook and Euronav imagery.
"It was not possible for the Flight Crew to accurately assess horizontal visibility at night, under cloud, at 200 ft, 9 NM from shore, over the Atlantic Ocean.
"The Flight Crew members’ likely hours of wakefulness at the time of the accident were correlated with increased error rates and judgment lapses."
The report contains a total of 42 safety recommendations for the various bodies involved in search & rescue missions and helicopter flights, including several recommendations for the Department of Transport to implement.
The AAIU stresses its reports should not be used to "assign fault or blame".
The bodies of Dara Fitzpatrick and Mark Duffy were recovered, but the bodies of Paul Ormsby and Ciaran Smith remain lost at sea.