European regulators have mandated a number of “precautionary” checks covering the main gearbox of the Airbus Helicopters H225 rotorcraft in the wake of the 29 April fatal crash in Norway.

The European Aviation Safety Agency says that although the investigation into the root cause of the accident – in which 13 passengers and crew lost their lives remains ongoing, “the partial information available so far indicate[s] in-flight separation of the main rotor hub from the main gearbox”.

Footage of the incident clearly shows the main rotor assembly spinning to earth having detached from the rest of helicopter.

In its airworthiness directive, EASA says that prior to the next flight operators must check the correct installation of the main gearbox suspension bars; examine the chip detectors and oil filters for the presence of metallic particles; and download data from the vibration health monitoring system for threshold exceedances.

The agency says the measures are “an interim action” and further mandatory requirements may follow.

The breadth of the checks appears to indicate that no single root cause has yet been identified. However, the absence of any reference to the bevel gear vertical shaft – the component behind two ditchings, and a subsequent grounding in 2012 – seems to rule this out.

Although the dynamic components of the H225 – previously the EC225 – share similarities with other variants EASA’s directive covers only the later model.

Earlier on 3 May, Norwegian investigators from the SHT investigation board said they had eliminated an error on the part of the flightcrew and instead were focussing on a “technical issue” as the root cause.

However, they warn that the investigation was likely to be complex and lengthy. The effort to locate all of the wreckage also remains ongoing.

The combined flight-data and cockpit-voice recorder has been recovered, however, and stored information downloaded, says the SHT.

The CHC Helikopter Service-operated rotorcraft (LN-OJF) came down west of Bergen following a flight from an oil platform.

Although a restriction on commercial flights using the H225 remains in place in Norway and the UK – accounting for the vast majority of the type operated in Europe – Airbus Helicopters on 1 May said that for the moment its decision “is not to suspend flights of any nature for the EC225”.

But it “continues to stand by the decision taken by the Norwegian and UK authorities” to restrict commercial flights, it says, “out of respect for all those affected by the accident”.

Bristow Group, another major operator of the H225, says that it has withdrawn its fleet from service except for those providing search and rescue cover. Although the majority of these aircraft are based in Norway and UK, the restriction also covers helicopters located in Australia.