Originally Posted by
Nadar
I think it's pretty clear. You must see this sentence in context of the previous:
They are saying that there were no warning signs before the accident neither from the chip detectors or HUMS for LN-OJF. They see this as a "certification flaw" as the certification was based on the premise that a fatigue crack would produce a warning from at least one of these systems.
What they are saying is that there were no warnings from the chip detectors or HUMS. That implies that they have checked the previous HUMS readouts. If I recall correcly, the HUMS data for the accident flight determined to be lost since these data aren't stored (but kept in RAM only which loose all information when it looses power).
The reason the problems the previous days aren't mentioned must be that it's seen as irrelevant to this accident. This is a preliminary report with just the current status of the major investigation areas.
When it comes to the chip detectors, they say this:
That seems pretty clear to me.
This is not what I read in the report. Quite the opposite, they say that the design philosophy assume that a spalling will not develop into a fracture because of the material properties, but that this accident indicates that this assumption is wrong:
This is clearly stated in the report. It was repaired/modified/inspected and approved for use by AH after being sent there as a result of the road accident in Australia were the gearbox was damaged.
Not as I read it. I think it's perfectly clear in the areas being addressed. That all remote, unlikely connections aren't included is to be expected as it is preliminary and the investigation is still ongoing.
Thanks for your detailed answer and citations!
If there where no HUMS readouts, how can they conclude it is not a sufficient way of monitoring?
In G-REDL there was indeed seen several HUMS readouts,
some of them failed to cause the attention of the engineer because
the datacard was incorrectly loaded.
So no reading, or not an abnormal reading, is not definitely answered yet?
When it comes to no chip detection, if that is the case it is indeed a strange condition.
For the 300 flight hours done on LN-OJF the gearbox had, it canīt be argued against.
But what about the 1080 hours before?
Do we know already that the gearbox had 1300 hours without any flaws at all? No.
We also know that the individual gears are not due to visual inspection before 2000 hours of operation.
Also, if they claim that it have proven against the theory that spalling does not induce any chips, it could be because the gear had already induced spalling and left the surface in such a shape that a crack would be induced.
It was then shipped out to LN-OJF
We can clearly see that the gears on the pictures have spalling.
especially the bottom left.
from the gear starts to get light pitting, which may cause hairthin
cracks in the surface, it will also soon start to spall.
Canīt say how long, but with normal lubrication and temperature it should have at least been ok for 300 hours.
So again, I donīt feel that the report brings a clear view to several
relevant aspects that should have been cleared out until now.
For sure there maybe reasons to hold off the information, until
further investigations are done, but still if the conclusion is that the gear is without warning cracked causing the gearbox total seizure. I don`t believe it.
And person with experience in risk assessement and engineering should not believe it either.
Concentric: I agree with your theory, it could be the underlying reason it happened.
And that could also be the reason they changed the material on the bevel gear from 16NCD13 to 32CDV13 to allow nitrating.
Especially in relation to the crack-prone area near the welding.
However, that should have been a failure mode, that they are perfectly aware of. Seeing the process a gear goes through during manufacturing,
it is not very likely to be a problem with only 1 gear.
And even if it was, the way we can see the surface pitting on the other gears, and also cracks due to deformation, if not occured in aspects of the gear development and certification, that process is for sure useless