The fatal accident inquiry into the North Sea Super Puma disaster has been told a blunder by the operator’s engineers was a key factor in the tragedy.
Their failure correctly to identify a tiny piece of metal found in the aircraft’s gearbox had “catastrophic” consequences.
A week after the discovery, Bond Flight 85N plunged into the sea like a “torpedo” – killing all 16 men on board – while returning from a BP platform in the Miller field.
An Air Accidents Investigation Branch report blamed a catastrophic gearbox failure for the crash.
The inquiry into the accident on April 1, 2009, which is being held at the Town House in Aberdeen, entered its third week yesterday.
Yannick Boyer, 37, a dynamic component specialist for Eurocopter – the French firm which built the doomed Super Puma AS332 L2 – told the inquiry Bond’s engineers got it wrong after a metal fragment was found in the gearbox on March 25.
After mistakenly identifying the sliver as silver or cadmium, they followed a Eurocopter “troubleshooter” – as advised by the manufacturer – to work out what to do next.
If the fragment had been bigger or there were others, or if the type had been identified correctly, they would have been referred to the aircraft’s maintenance manual and an instruction to open up part of the gearbox known as the epicyclic module and check internal magnets for more metal.
Instead, the helicopter went back into service under “close monitoring”, or extra gearbox inspections, at every engine shutdown during the next 25 flying hours.
Mr Boyer said the fragment – later identified as carbon steel – could not have been silver or cadmium as these metals had no magnetic properties.
His evidence also highlighted confusion between Bond and Eurocopter, which has since changed its troubleshooting advice, over where in the gearbox the metal particle was located and whether there was one or more.
Mr Boyer said fellow Eurocopter technical expert Brice Fernando called him that afternoon about the discovery of metal fragments and e-mailed him screen grabs of a vibration alert on the helicopter’s monitoring software.
He added: “I said to Brice that we should know what kind of particle was collected, which is why we referred the customer to the MTC (maintenance task card), which has descriptions.
“It is the customer’s responsibility to apply the work card correctly. It is up to them to apply the maintenance.”
Mr Boyer said he was later reassured that fragments found in the main gearbox – not the epicyclic module – were “nothing significant”.
He also said the helicopter’s maintenance manual, which Bond’s engineers did not refer to at the time, was “sufficient to check the airworthiness of the aircraft”.
In addition, he said Bond must have known about the potential for particles attaching themselves to magnets in the epicyclic module as one of its North Sea helicopter fleet had its epicylic unit replaced for the same reason in 2008.
The internal magnets – meant to prevent “contamination” of the main gearbox module – were removed from Super Pumas following the 2009 tragedy, he said.
Sheriff Derek Pyle said Eurocopter’s revised guidance for identifying metal fragments, including an option to have material analysed by experts, did not go far enough.
He added: “To my mind the instruction should be that if you find any kind of particle you must have it tested by someone with technical knowledge in order to identify what the material is.
“Identification of the material was a crucial matter in deciding whether or not to leave the gearbox on the helicopter or to change it.
“If they had changed it, the accident would never have happened.”
The crash showed that failing to correctly identify such a particle “can be catastrophic”, he added.
Sheriff Pyle said it was impossible to guarantee safety in all situations, but risk could be controlled.
He added: “The only way to guarantee the particular circumstance at the heart of this case could not happen again in relation to the misidentification of the particle would be to have a metallurgist see it.”
The inquiry continues.