Four months after Rescue 116 crashed into Blackrock Island off the coast of Co Mayo, questions have been raised over when it was known that the island was not included on the onboard warning system.
RTÉ's Prime Time has reported that a pilot had flagged the absence of Blackrock Island from the system four years ago - but the error was not corrected.
It also reported that nine days after Rescue 116 crashed on 14 March, it was discovered that the aeronautical data relating to one of the country's most famous islands, Sceilg Mhichíl, was also dangerously inaccurate.
All four crew members of R116 - Capt Dara Fitzpatrick, Capt Mark Duffy, winch operator Paul Ormsby and winchman Ciarán Smith - died in the crash.
The bodies of Cpt Fitzpatrick and Cpt Mark Duffy have been recovered but the two other crew members remain lost at sea.
The Coast Guard's Search and Rescue service is run by a private operator, CHC Ireland, which is a subsidiary of a global helicopter services provider based in Canada.
It won a ten-year €500 million contract to provide the service in 2012.
All of the Coast Guard's Search and Rescue helicopters have been equipped with an Enhanced Ground Proximity Warning System since 2013.
The EGPWS is designed to warn pilots to take corrective action to avoid a collision if they get too close to terrain or a known obstacle. But the system can only work if its database has a complete picture of the terrain and obstacles on a flight path.
Blackrock Island was not in the system. The pilots flying Rescue 116 did not receive a warning and did not see the island until it was too late.
Prime Time revealed there was a chain of emails between Sligo base Coast Guard pilots and a senior CHC manager referring to Blackrock Island and/or other omissions in the EGPWS in 2013.
A source recalled that the emails were sent after a pilot spotted the omission of Blackrock Island from the EGPWS while carrying out test flights to ensure that the new aircraft's safety systems were all operating as they should.
This was a particular safety concern as the 85m-long island was on the route approved by the company for approaching the refuelling base at Blacksod Bay from the south.
According to a source quoted in the Prime Time programme, Coast Guard personnel were told at a meeting last April, six weeks after the Rescue 116 crash, that management was trying to establish if this information had been passed on to the company that supplied the database for the system.
That company, Honeywell, told Prime Time it could not comment on an ongoing investigation.
It said: "Until that investigation is complete, any inferences or conclusions drawn at this time would simply be speculation."
It was also revealed that search and rescue pilots based in Dublin had been complaining that the EGPWS was alerting them to an obstacle that did not exist.
A CHC spokesperson told Prime Time that the company is precluded from responding to its questions under international protocols governing air accident investigations.
The spokesperson said the company "has multiple systems and programmes that encourage employees to report any safety concerns ... Our focus remains on supporting the families and friends of all of those affected by this accident".
The Irish Aviation Authority's State Safety Plan says that the authority provides terrain and obstacle data for use by database suppliers.
It told Prime Time that "Blackrock Island was not shown as it does not constitute an obstacle under ICAO [International Civil Aviation Organisation] Standards".
The lighthouse was shown but there was no indication that it was located on an island with a highest point of 85m.
The authority said it has no record that it was advised that Blackrock Island was not in the Coast Guard's EGPWS system.
The authority confirmed to Prime Time that it only learned that Sceilg Mhichíl was inaccurately depicted on its official maps nine days after the Rescue 116 crash.
Up until 24 March the official Aviation Authority aeronautical map indicated that Sceilg Mhichíl was just 56m high.
This was actually the height of the lighthouse on the island and not the island's highest point. The Authority issued an immediate alert to pilots notifying them of the error.
The authority issued a revised map last month which increased Sceilg Mhichíl to its true height of 217m.
Sceilg Mhichíl, off the coast of Kerry, is a UNESCO world heritage site and has recently been used as a location in the latest installments of the Star Wars franchise.
The Irish Air Line Pilots' Association said it would be inappropriate to comment on the possible contributing factors giving rise to the crash while the accident investigation is still under way.
However it added: "Separately from the Rescue 116 accident and its ongoing Annex 13 investigation, the Irish Air Line Pilots' Association has raised concerns with successive Ministers for Transport on the effectiveness of the Irish Aviation Authority in fulfilling its Safety Regulatory and Oversight obligations."
It said the concerns raised by IALPA with successive transport ministers are germane to the questions and issues raised in the Prime Time report.
The Aviation Authority rejected this.
It said: "We note IALPA's view. However, this is not shared by anybody in the industry, specifically the international organisations that continuously audit Ireland's safety oversight regime. The IAA is one of Europe's most respected aviation bodies and is clearly recognised as a leader in its field."