The company that runs the search and rescue helicopter service, CHC Ireland, received repeated warnings over five years that the life jackets they supplied to their pilots were not fit for purpose.
A number of pilots had warned management that the personal locator beacons on the life jackets would not work as they were not installed according to the beacon manufacturer’s guidelines, according to RTÉ's Prime Time.
The manufacturer advised that the beacon should be located at least 30cm apart from the GPS antenna. However, in the pilots' life jackets both elements were in the same pouch.
One pilot said that unless the life jackets were modified to address this fault "pilots are wholly exposed in the event of a ditching".
A safety report filed by a crew member in 2014 stated: "Effectively this means that the beacon could produce absolutely zero receivable transmissions."
The Rescue 116 helicopter crashed off the Co Mayo coast in March, claiming the lives of all four people on board.
The bodies of Captain Dara Fitzpatrick and Captain Mark Duffy have been recovered but winch operator Paul Ormsby and winch man Ciarán Smith remain lost at sea.
The Air Accident Investigation Unit issued a preliminary report four weeks after the crash, which identified this crucial issue and issued a safety recommendation to the life jacket manufacturer to review the installation of this equipment.
A report, which will be broadcast on RTÉ’s Prime Time tonight, reveals internal documents that show that CHC Ireland was aware of this fault for at least five years before the accident.
When Rescue 116 crashed, none of the personal locator beacons worn by the four crew members activated.
Prime Time also has documents that show there had previously been ongoing issues with the beacons on the life jackets supplied to winch crew.
The life jackets were tested eight weeks after the crash. The type of life jacket worn by the winch crews passed the test.
However, the life jacket that pilots had been wearing failed to activate. CHC Ireland immediately withdrew those life jackets from service.
"Given that the Rescue 116 accident continues to be the subject of a formal investigation ... it would be inappropriate for us to respond to specific questions of a technical nature.
"CHC is subject to oversight and regular independent audit by the Irish Aviation Authority, as well as audits undertaken on behalf of the customer - the Irish Coastguard," said CHC Ireland in a statement.
Life jacket manufacturer Beaufort Ltd was "aware of the findings of the AAIU report, which are at this stage preliminary".
"Investigations are continuing and we are liaising closely both with the AAIU and the Irish Coast Guard. We cannot make any further comment until the final outcome of the investigation."
Coast Guard helicopter crews repeatedly raised concerns about the quality and accuracy of the maps on board their helicopters, the Prime Time report will also reveal.