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The Accident Investigation Board of Norway (AIBN) has released an update to its preliminary report into the fatal crash of the CHC Helicopter H225 near Turøy, Norway, on April 29, and says the scenarios under consideration for the cause of the crash include failure of epicyclic module, suspension bar (lift strut) attachment, and main gearbox conical housing.
However, the AIBN admitted that several key components from the aircraft are still missing, despite “a comprehensive search” taking place in the first three weeks following the crash. The missing parts of particular interest to investigators are the epicyclical second stage planet gear carrier and parts of the forward suspension bar.
According to an Airbus Helicopters statement released following the publication of the update, the manufacturer’s analysis pointed to just seven potential initial events causing the accident aircraft’s main rotor to detach.
“Out of these seven scenarios, only one – the failure of the attachment of a suspension bar – can been assessed as probable by Airbus Helicopters, based on the information available to date,” the manufacturer stated.
The crash took place as the aircraft, which was carrying 11 passengers and two crew, was returning to Bergen Airport Flesland from the Gullfaks B platform in the North Sea. The detachment of the main rotor head and mast caused the fuselage to plummet into a small island below, killing all 13 on board. The main wreckage caught fire following the crash, and then fell into the sea, while the main rotor eventually came to a rest several hundred meters away from the crash site.
Investigators were able to quickly retrieve the aircraft’s cockpit voice and flight data recorder (CVFDR) and send it to the Air Accidents Investigation Branch in the U.K. for download. Within two days, they had a complete data set of both voice and flight data. According to the preliminary report, the data show that “everything appeared to be normal” until a catastrophic failure that developed in the space of one or two seconds — but the recordings end abruptly at that time.
The AIBN’s update was released following a three-day meeting of the official investigation team at Airbus Helicopters’ headquarters in Marignane, France, led by the AIBN, with Airbus Helicopters, BEA France, the Air Accidents Investigation Branch UK, the European Aviation Safety Agency (EASA), the Civil Aviation Agency UK, and CHC in attendance.
The AIBN said the meeting was held to agree on further investigation of the parts that the investigators had sealed and sent to Airbus Helicopters, and to discuss a list of possible scenarios that could explain the detachment of the main rotor.
The report also provided an update on the retrieval of data from the aircraft’s health and usage monitoring system (HUMS). It stated that due to HUMS data being saved on a memory card only after a helicopter has landed, data from the accident flight was not available.
However, the system also stores some flight data parameters that are used for flight data monitoring, and on May 12, French authorities were able to download FDM data that lasted approximately 13 seconds beyond the CVFDR data.
“A preliminary analysis was ready a week later,” the report states. “It provides valuable information about the sequence of events and will be analyzed further.”
The AIBN said detailed examination work continues to focus on the MRH suspension bar assembly, the main gearbox and the main rotor head. Other wreckage parts and components are also being examined in parallel.
The AIBN said it will also be resuming its search for the missing parts once it has studied all the information from searches so far, and refined the aircraft’s flight path and mapping of the locations of various components.