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One year after the fatal crash of an Airbus Helicopters H225 (EC225 LP Super Puma) operated by CHC near Turoy, Norway, investigators from the Accident Investigation Board of Norway (AIBN) are still trying to fathom why a crack initiated in the main rotor gearbox (MGB) and how it propagated.
In a preliminary report on the crash released today, investigators also raised questions about the certification process, as the failure mode “seems to differ from what was expected.”
The AIBN also confirmed “many similarities” with the MGB failure that led to the fatal crash of an Airbus Helicopters AS332 L2 (G-REDL) off the coast of Peterhead, Scotland, in 2009.
“The AIBN will continue the investigation into how and why two similar catastrophic accidents could happen to near identical helicopters only seven years apart,” the AIBN report states. “Further assessment of the followup on the G-REDL safety recommendations and the continuing airworthiness of the gearbox after 2009 is a relevant issue.”
The investigation has shown that the crash on April 29, 2016, which resulted in the deaths of all 13 people on board, was the result of a fatigue fracture in one of the eight second-stage planet gears in the epicyclic module of the MGB. The fatigue had its origin in the upper outer race of the bearing (inside of the gear), propagating towards the gear teeth. The crack initiation appears to be a surface micro-pit. However, the AIBN does not yet understand why the micro-pit formed and how and why the crack continued to grow sub-surface, thus preventing detection. No material conformity or manufacturing issues have been revealed during the investigation.
As previously revealed, the MGB had been involved in a road accident during transport in 2015, but the AIBN said it found no connection with the crack.
As to the similarity with the 2009 accident, the AIBN notes there was one warning of possible gear fracture in that case. Unfortunately, the actions taken did not recognize the degradation of the second-stage planet gear, which subsequently failed. It said there was no advance warning before last year’s crash.
The AIBN said it will continue metallurgical examinations and seek to understand the underlying driving mechanisms of the fatigue fracture. This includes studying the recently salvaged second-stage planet carrier with the inner race from the fractured planet gear. The investigators, however, say they cannot estimate a completion date for the final report.
In a statement reacting to the publication of the AIBN’s preliminary report, the European Aviation Safety Agency (EASA) said it continues to provide its full support to the investigation. “EASA continues to implement robust and proven certification processes taking into account all available information,” the regulator added. “EASA proactively takes all necessary actions to mitigate identified possible contributory factors, in order to guarantee the safety of flights.”
Meanwhile, in a statement issued following the release of the report, Airbus Helicopters CEO Guillaume Faury said the company was not aware of any issue related to the 2009 crash at the time of the Turoy crash.
“The information available to us from the 2016 accident has allowed us to take protective measures that we could unfortunately not have put in place in 2009 based on the knowledge and evidence available at the time, and also because significant parts from the 2009 accident [aircraft] were never recovered,” he said.
“In the course of the investigation into the 2016 accident, we have implemented a set of protective measures which have been requested and validated by EASA. Nothing in this preliminary report alters this.”
Faury added that Airbus was “totally committed to transparency” in all matters regarding aviation safety and international helicopter regulations.