Originally Posted by
Geoffersincornwall
My engineering colleagues have explained that this type of gear wheel is made of a steel that has rather a severe crack propagation characteristic and as such the failure mode is severe...
Geoffers - The planet gears in question were made from 16NCD13 double vacuum melt steel (AMS 6263) which is very high quality. You can find some data on the alloy
here. It has excellent fracture toughness and was a good material choice for this application.
Having said that, there is an interesting situation described in the AIBN report issued on April 28, 2017 regarding the fracture analysis work used to certify the gearbox design. If you read the report beginning with section 1.17.6.3, there is some discussion of revisions to
EASA CS 29.571 Fatigue tolerance evaluation of metallic structures, and how this relates to the EC225 MRGB.
If you look at the CT scan image of the failed planet gear outer bearing race section shown in fig. 38 of the report, you'll see that there is a sub-surface fracture (highlighted in red) passing thru several spall locations just ahead of the final rim fracture. This sub-surface fracture runs at a shallow depth (~.012") and lies entirely within the carburized case thickness (~.050").
Then consider this part of the report:
"1.17.6.4 Assessment of the MGB against CS 29.571 - The AAIB investigation into the G-REDL accident found that the phenomenon of crack formation within the carburized layer of the outer planet gear race had not been considered during the design and certification of the AS 332 L2 and EC 225 LP epicyclic reduction gearbox module or the development of the approved maintenance program of the MGB.
The AAIB stated in the G-REDL report (on page 95) that “although the design satisfied the certification requirement in place at the time of certification”, and further “it would appear that if the current requirements [CS 29.571 issued in 2003] were applicable they may not have been met."
The report is saying that the original fracture analysis performed to certify the gearbox design did not take into account the particular type of failure that occurred in this case. This may not provide the complete explanation many are looking for as to why this unfortunate accident happened, but it helps clarify things quite a bit.