A preliminary report into the fatal crash of Coast Guard Rescue Helicopter 116 has found that the aircraft's on board warning system did not include data relating to Blackrock Island.
The helicopter crashed into the island off the coast of Co Mayo on March 14th, with the loss of all four crew members on board.
The bodies of Captain Dara Fitzpatrick and Captain Mark Duffy have been recovered but two others, winch operator Paul Ormsby and winch man Ciarán Duffy, remain lost at sea.
In the report the Air Accident Investigation Unit (AAIU) says that its initial inquiries have found that an Enhanced Ground Positioning Warning System (EGPWS) did not have the "lighthouse obstacle" included in an obstacle database and that "the terrain of the island" was not listed on a terrain database.
The AAIU says its inquiry is ongoing and a final report into the accident will be published in due course.
In the final seconds, the helicopter pitched up rapidly, impacted with terrain at the western end of the island and "departed from controlled flight", the report states.
Audio from the black box revealed a crew member identified an island in front of the helicopter, and recommended a right turn:
"K…looking at an island just in, directly ahead of us now guys, you want to come right", he said.
The commander, Dara Fitzpatrick, responded: "OK, come right just confirm?" The crew member then said to turn "about 20 degrees right". The commander then asked the co-pilot to 'select heading', which the co-pilot confirmed by replying 'Roger'.
Just two second later the rear crew member was recorded as saying: "come right now, come right, COME RIGHT." The last recorded audio from the flight deck was the co-pilot saying "we're gone".
As well as a carrying out a detailed analysis of the flight recorders, the wreckage of R116 will be inspected at the Unit's facility in Gormanston, Co Meath.
Mechanical anomalies on the helicopter have already been ruled out contributory factors and the AAIU focus is now understood to be on operational aspects of the flight.
These take in several headings but will include human behaviour, navigational controls and procedural issues.