It has emerged that a pilot had flagged the absence of Black Rock from the on board warning system of Rescue 116 four years ago.
According to an RTÉ Prime Time report, the error wasn't corrected despite it's potential lethal danger.
Captains Mark Duffy and Dara Fitzpatrick; winch operator Ciarán Smith and winchman Paul Ormsby died when their Coast Guard helicopter crashed off the Mayo coast in March.
The report also revealed that nine days after the crash, it was discovered that the aeonautical data relating to Skellig Michael was also inaccurarte.
A revised map of the island was issued last month which increased the height of it from 174 feet to its true height of 712 feet.