Investigators probing the 29 April fatal crash of an Airbus Helicopters H225 on Norway’s west coast have narrowed their focus to three possible causes for the main rotor separation that doomed the rotorcraft.

In a 27 May update to its preliminary report, Norway's SHT accident investigation branch says that as a result of discussions with the manufacturer and “fault tree analysis”, the scenarios under consideration include failure of the epicyclic module, the “lift strut” – or suspension bar attachment – and the main gearbox (MGB) conical housing.

Airbus Helicopters, which appears to still be focusing on the failure of the suspension bar, says it is not ruling out “any of the scenarios described by [SHT]”. However, it adds: “Although significant progress has been made in analysing information regarding the suspension bar attachments, more work needs to be done on scenarios involving the epicyclic module and the MGB conical housing.”

In addition, the SHT says its inquiry is being hindered by the failure to recover “some key components”, namely the epicyclic planet gear and parts of the forward gearbox suspension bar. It says the search for wreckage will resume in June, but it will first “fine tune” the area to be covered, “based on a refined flight path and mapping of where different components have been found.”

Although investigators had previously successfully downloaded information from the combined voice and flight-data recorder, an additional 13s of data stored in the H225’s health and usage monitoring unit has also been recovered and analysed. “It provides valuable information about the sequence of events and will be analysed further,” says the SHT.

Maintenance records for the helicopter have been secured and “are currently the subject of a thorough analysis”, it adds.

Thirteen passengers and crew died when the CHC Helikopter-operated H225 (LN-OJF) came down near Turøy on its way from a North Sea oil platform to Bergen.