CHC Helikopter Service Flight 241

Investigation

The Norwegian Accident Investigation Board (AIBN) is responsible for investigating aviation accidents in Norway. The British Air Accidents Investigation Branch (AAIB) and French Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile (BEA) each dispatched a team of investigators to Norway to assist the AIBN in its investigation. Representatives from Airbus Helicopters and engine manufacturer Turbomeca were part of the BEA team. The European Aviation Safety Agency (EASA) also participated in the investigation.

The aircraft's combined cockpit voice recorder (CVR) and flight data recorder (FDR) was sent to the AAIB in the United Kingdom for data retrieval. Data were successfully downloaded on 1 May and sent back to the AIBN which confirmed that the received data were of good quality and useful for further investigation.

In May the BBC reported an earlier issue when the helicopter had returned to base in late April after the illumination of a cockpit warning light.

In a press conference on 3 May, the AIBN stated that the initial investigation and analysis of data from the combined FDR and CVR convinced them that pilot error could be ruled out and that there were no indications of any malfunction until one second before the end of the recording, which they assumed was the moment when the rotor detached. An AIBN spokesman said the accident was down to a technical fault and "not an accident caused by human error". Later that day Airbus Helicopters updated their press release bulletin with a request for verification of the correct installation of all main gearbox suspension bar attachments for the EC225LP. They also stated that "similar measures will be published shortly for the EC725AP in a specific ASB" (Alert Service Bulletin).

Within a week of the accident, an underwater sled with magnets had been developed and deployed to find small critical metal fragments from gearbox and bearings to support the investigations.

On 27 May the AIBN confirmed that scenarios under consideration included failure of epicyclic module, suspension bar (lift strut) attachment and MGB (gearbox) conical housing. On the same day Airbus Helicopters updated their press release bulletin and stated that in their opinion only failure of the attachment of a suspension bar could be assessed as probable based on the information available by that date, an opinion also stated by Airbus during a safety meeting on 20 May.

On 1 June the AIBN released an update to the preliminary investigation report including an urgent safety recommendation to the European Aviation Safety Agency. The recommendation was based on metallurgical examinations where signs of fatigue in parts of the second stage planet gear were found.

The gearbox had suffered "unkind treatment" (road accident) during transport in Australia, and was repaired before being mounted in LN-OJF. On 15 June, Airbus requested operators to check for metal residue in oil and to report unusual gearbox events.

On 28 June the AIBN released a new preliminary report where they stated that the most likely cause for the accident was a fatigue fracture in one of the second stage planet gears. They had not yet determined what initiated the fracture. AIBN draws similarity to the 2009 Bond Helicopters Eurocopter AS332 crash, also caused by a gearbox fracture. Whereas particles had been detected in the Scottish gearbox's oil prior to the crash, no such indication was present for the Norwegian gearbox., AIBN continued investigations with no indication of when a conclusion could be made. Also in February 2017, EASA issued a notice for operators to investigate the oil cooler for 16NCD13 alloy from the gearbox.

On 28 April 2017 the AIBN released a new preliminary report with an update of the investigation progress one year after the accident. In this report they stated that the accident was a result of a fatigue fracture in one of the eight second stage planet gears in the epicyclic module of the main rotor gearbox and that the crack initiation appeared to be a surface micro-pit. The origin of the micro-pit was considered unknown at the time when the report was published. Also unknown is whether the fracture occurred momentarily or over several flight hours, and whether fracture fragments were spalled for detection by maintenance systems as happened in G-REDL. The issue is related to the airworthiness certificate of the aircraft.

On 5 July 2018 the AIBN released the final report, they determined the cause as the following:

The accident was a result of a fatigue fracture in a second stage planet gear in the epicyclic module of the main rotor gearbox. Cracks initiated from a micro-pit at the surface and developed subsurface to a catastrophic failure without being detected.

12 recommendations were made, one of the recommendations stated that Airbus should take another look at the design of the main gearbox of the Super Puma. By September 2019, Airbus had replicated the root cause in testing.