Originally Posted by
turboshafts
I donīt understand this statement:
"Also, the HUMS appears unable to identify symptoms of such degradation in the epicyclic module"
From the G-REDL report, it is quite clear that there was abnormal HUMS readouts in the days before the accident.ī
But it was not known how to interpret the data,
that showed increased vibrations. According to Eurocopter the debris should be determining the Health of the gearbox and not the HUMS readouts.
I think it's pretty clear. You must see this sentence in context of the previous:
Originally Posted by AIBN
AIBN believes that a sub-surface crack has propagated without creating a significant amount of magnetic debris from spalling. Also, the HUMS appears unable to identify symptoms of such degradation in the epicyclic module.
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.
Originally Posted by
turboshafts
So it is strange to me no HUMS readouts on LN-OJF. Still no mention about the problems with the helicopter in the days before the accident.
Would be interesting to understand why AIBN concludes on the HUMS?
What about the chip detectors?
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:
Originally Posted by AIBN
No findings indicate any malfunctions to the magnetic debris detection system on LN-OJF, or fail to follow procedures for visual inspection and checks before flight. Neither are there any records of magnetic debris findings from inspections made since the gearbox was installed on LN-OJF in January 2016.
That seems pretty clear to me.
Originally Posted by
turboshafts
I already read that Visual inspection of gears are done at 2000 hours. up to that spalling can occur, that is not dangerous to the gearbox operation.
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:
Originally Posted by AIBN
An essential design philosophy regarding a possible failure inside the epicyclic module has been that propagation of a crack would be suppressed by the compressive surface stress. Thus a crack in the surface area should grow outboard and create spalling that would produce magnetic debris, which will be detected on the magnetic plugs (chip detectors). The optional HUMS1 is an additional means for detecting developing degradation.
Originally Posted by
turboshafts
"The main gearbox (MGB) was received from Airbus Helicopters after modification, inspection and repair before it was installed in LN-OJF 15 January 2016. At the time of installation, the MGB had accumulated 1 080 hrs since new"
Why is the gearbox modified, inspected and repaired by Airbus at 1080 hours?
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.
Originally Posted by
turboshafts
I am disappointed in AIBN, a very weak preliminary report
with vague statements.
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.