A preliminary report into the Rescue 116 crash has recommended that the onboard technology used by helicopters needs to be reviewed and upgraded.

The warning system on Coast Guard helicopter Rescue 116 did not have data related to Blackrock Island, off the Mayo coast, where it hit rocks.

The helicopter crashed on March 14th, with four crew members on board.

The bodies of Captain Dara Fitzpatrick and Captain Mark Duffy have been recovered.

Two other crew members - winch operator Paul Ormsby and winchman Ciarán Duffy - remain lost at sea.

The Air Accident Investigation Unit (AAIU) has been examining data from the black box flight recorder since it was found among the wreckage.

There was no mechanical failure in the aircraft before the crash, the report indicates.

The report also states that the helicopter pitched up rapidly in its final moments, impacted with terrain at the western end of the island and "departed from controlled flight".

The AAIU's former head of engineering Graham Leddy says crucial information was not relayed to the pilot.

Speaking to Newstalk Breakfast, Graham explained: "They have this new system - Enhanced Ground Proximity Warning System (EGPWS)... All obstacles - heights of pylons, rocks and mountains - are in this database.

"It looks ahead electronically and says 'in front of you there is these obstacles. At the moment it may be below you, but you need to fly a little bit higher to increase your safety altitude'.

"Now, it would transpire from this preliminary report that Blackrock - which is a very substantial piece of terrain - is not actually in this database. That is extremely concerning."

The report suggests CHC Ireland - which is contracted by the Coast Guard to conduct operations - should "review/re-evaluate all route guides in use by its [search and rescue] helicopters in Ireland, with a view to enhancing the information provided on obstacle heights and positions, terrain clearance, vertical profile, the positions of waypoints in relation to obstacles and EGPWS database terrain and obstacle limitations."

AAIU has stressed that this is a preliminary report, and does not contain any conclusions or analysis.

The investigation is ongoing, and a final report will be published when that is concluded.