A report into the Rescue 116 crash has found navigational issues were a major factor in the tragedy.
Dara Fitzpatrick, Mark Duffy, Paul Ormsby and Ciaran Smith all lost their lives in the crash off the Mayo coast in March 2017.
A report by the Air Accident Investigation Unit has found the crew were unaware a lighthouse and terrain were on the flightpath as neither databases nor imagery detected it.
The report says the helicopter operators CHC Ireland didn't have a formalised, standardised, controlled or periodic system in place to test flight routes.
Security auditor Retired Lt Col Kevin Byrne says there's also a question mark over whether the mission was necessary at all:
"There was definitely confusion in a number of elements of this. The medical doctor never actually asked, to my knowledge, for a rescue operation to take this person from the ship at sea to a hospital. He wasn't in immediate danger of loss of life. He may have lost a thumb but that was not a fatal matter."
