Naval Service divers will try to locate the wreckage of the downed Irish Coast Guard helicopter off the Mayo coast to allow air accident investigators piece together what caused the tragedy.
Divers will use special side-scan sonar in a bid to identify the resting place of the wreckage. The area in focus is between 8km and 10km off the Mayo coast.
However, weather conditions will dictate how long it will take to find and then bring the helicopter wreckage ashore.
Divers will then help salvage experts lift the helicopter body as well as its engine and gearbox assembly.
Air Accident Investigation Unit (AAIU) officials consider an examination of the helicopter's engine, gearbox and rotor assemblies as a critical part of their probe.
One of the theories being examined is that the S-92 crew encountered some kind of catastrophic incident as they prepared to land to refuel while providing 'top cover' for another Irish Coast Guard helicopter.
The Dublin-based helicopter vanished off radar at 1am yesterday - without a mayday being triggered or any indication of difficulty as it prepared to land for refuelling at the Black Sod base. Gearbox assemblies are a critical element of such helicopter operations.
One Irish marine industry source said helicopters have what is nicknamed "a Jesus bolt unit". The sudden failure of this unit can be catastrophic to flight operations.
However, the Sikorsky S-92 at the centre of the Mayo tragedy boasts one of the best safety records of its type.
The US-built helicopter, which forms the backbone of the Irish Coast Guard five-strong fleet, is the workhorse of the long-haul helicopter aviation market worldwide.
Around 280 S-92s are in service around the world - the bulk in operation with emergency services or oil industry use.
The search continued until darkness fell last night and will resume again this morning.
The Irish Air Corp Casa aeroplane, LÉ Róisín naval ship, fishermen from all over the area, and Garda divers have all been on the scene.