© Anthony Pecchi The European Aviation Safety Agency (EASA) regulatory authority on July 10 approved the prevention and detection mea...
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© Anthony Pecchi
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EASA's validation was followed by the U.K. Civil Aviation Authority's lifting of operational restrictions on the same day, with the Civil Aviation Authority of Norway taking the step on July 19. This allows the full return to service of EC225s helicopters worldwide. The suspended flights will restart this month.
EASA also approved Eurocopter's modifications to the EC225s main gear box emergency lubrication system that ensure its full performance throughout the flight envelope.
Eurocopter’s opinion is that the crack initiation is the result of a combination of root causes leading to the degradation of the bevel gear shaft’s fatigue resistance.
The Eurocopter solution comprises a set of modifications and inspections which aim at monitoring and detecting vertical shaft crack conditions and reducing the likelihood of any shaft crack initiation. When complied with, this solution ensures safe and airworthy operations of the EC225 type.
Eurocopter’s opinion is that the crack initiation in the welded area resulted from the combination of 3 factors : all three factors must be present to initiate a crack
- Reduced fatigue strength due to residual stress introduced during the shaft welding process
- Reduced fatigue strength due to stress “hot spots” associated with the shaft geometry, in particular the surface roughness and the shaft shape
- Very exceptional active corrosion in localised areas of the shaft.
Root cause explanation
Factor 1 & 2: Residual stress & “hot spots”
Many parameters influence a part resistance and its safety margins: the geometry, the material, heat treatments, production process, flight loads and any alteration that could occur all along the life of the part.
In particular, residual stresses resulting from the welding process contribute to reduce the fatigue strength in this welding area of the Main Gear Box bevel shaft. On top of this, highly stressed areas, so-called “hot spots” exist on the welding area of the current EC225 shaft, which result from the shaft geometry.
These “hot spots” equally contribute to reduce the fatigue strength of the shaft.
Factor 3: “Active Corrosion”
Detailed examination of the crack showed the presence of corrosion pits at the origin of the cracks initiation.
Corrosion is an unusual phenomenon in a Main Gear Box as all the parts are highly lubricated. The occurrence of corrosion has a negative impact on resistance of the parts but it is not enough to explain such a reduction of bevel wheel safety margins. So many tests have been conducted to identify the additional parameters that cancelled the safety margins.
The main factor has been identified and explained: the presence of moisture, trapped in confinement of the welded area, in direct contact with the bevel wheel during in flight solicitation.
In the area where moisture is trapped, an exceptional phenomenon, called “Active corrosion” develops and grows over time, leading a weakened material after several hours of flight.
The Residual stress and “hot spots” factors even in presence of classical corrosion are not sufficient to initiate a crack.To initiate a crack, all three factors, including the active corrosion phenomenon must be present.
Once a crack has initiated, it gradually grows, or “propagates” around the circumference of the shaft.
How was moisture trapped?
In the first ditching : moisture got trapped behind the welding hole plug, due to a change in the welding area production process.
In the second ditching : Wearing of the splines generate fine metallic powder over time. This powder, mixed together with lubrication oil already presents in the Main Gear Box, forms a paste.
When the MGB wheel rotates, this paste forms along the internal shaft wall, and traps moisture in the welded area of the shaft.
In the second case, this phenomenon accumulated over several hours and after examination showed an extremely high amount of paste on the weld bead area of the shaft.
These helicopters have logged more than 20,000 flight hours since last October without incident.
Eurocopter is now supporting helicopter operators as they modify the aircraft in accordance with EASAs airworthiness directive, enabling the full EC225 return to flight.
The EC225 is an 11-ton-class rotorcraft in Eurocopters Super Puma family. With more than 300,000 flight hours accumulated in service worldwide, it is deployed in civil, military and para public operations that range from offshore transportation and cargo airlift to Search and Rescue (SAR) duties.
Source: www.eurocopter.com
