Originally Posted by
pax2908
Sorry, just an engineer, not even aero.
So there are components which "must not fail" - over their entire service life ... which is determined ... how?
One would hope that, for these parts, they would be periodically replaced so that the used ones can be analysed (metallography) while looking for any sign of accelerated degradation?
At time of certification an evaluation is done to determine which parts are critical. Obviously main hub and blades main shaft. Less obvious are things that will not immediately be catastrophic due to redundancy or even partial detectable fail modes. Planet would normally be in this category as there are several and initially the chips would come before total box failure.
Next is to determine fatigue life by test usually until failure or acceptable margin. It is an expertise all on its own.
The parts replaced should not actually show fatigue damage or else not enough margin was used. The damage is in the molecular crystalline structure long before crack formation.
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