From the point of view of a pilot and not an engineer:
I was heartened by the quality of the AAIB's forensic investigation that permitted establishment the point of failure to (part of) a specific second stage planetary gear in the epicyclic module. Further that the chip, collected on the 25th March, was from a 'second stage planet gear' - concluded, not just from its metallurgical composition but from the marks made during 'honing' process - was astonishing.
I was also astounded by the further analysis of that chip which appeared to indicate that the source of the stress leading to the fatigue cracks was some foreign agent whose indent could still be seen. Although not making any conclusive statements, there was a hint that the introduction of this foreign object could have resulted from the previous conical housing/rotor head replacement on the 1st March.
I agree with HC that the assignment of blame is futile but a system where such an important precursor is permitted to be discarded because the conclusion was reached that "the particle was a piece of 'scale'" - i.e. that it was believed to be "...silver or cadmium plating, which, in accordance with MTC 20.08.01.601, was '...unimportant...'" should be a wake-up call for management everywhere. Perhaps such areas of critical judgement should have far tighter control mechanisms if so many actions and consequences are likely to stem from them.
We have previously discussed the placement of the magnets on the epicyclic module separator plates (since rectified) - which might have captured debris from the stressed gear and thus prevented them from reaching the chip detector. Surely another failure of FMECA in the design process.
The reference to FARs (as opposed to JARs or EASA CS) is because the 332 was certificated to FARs (JARs were not in yet in place) and, specifically to amendment 24 of FARs. As you should be aware, the certification basis for an aircraft is frozen at a particular amendment version - which is why the FAA regulatory library has to provide all previous version on their regulatory web site.
Like others, I was disappointed with the failure of HUMS to provide adequate warning of failure (which confirmed experience from previous planetary gear failures in the S61); further, that it might never be an effective detector of bearing wear/failure in the epicyclic module (even with AAD). Clearly, we have to move back to debris monitoring using old and proven methods such as SOAP and other oil analysis techniques as is recommended in the report.
All-in-all a fascinating report and one which, if it is extensively studied, will add to the knowledge of helicopter professionals everywhere.
Jim
Last edited by JimL; 25th Nov 2011 at 14:26.