EC225 crash near Bergen, Norway April 2016
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From what I read there was a fracture thru the rim of an output stage planet gear that originated at a spall on the spherical roller bearing outer race surface. Normally, bearing race surface spalls are fairly benign events that propagate slowly, and do not result in a rapid catastrophic failure. They also produce ferrous debris which is quickly and easily picked up by a lube oil magnetic chip detector. On the other hand, fractures thru a gear rim (as opposed to thru a gear tooth) are almost always catastrophic and tend to propagate fairly rapidly. This is one reason rotorcraft drivetrain gears use an L2 reliability rate for tooth bending fatigue, while rolling element bearings can safely use an L10 reliability rate for contact fatigue.
One thing I noticed looking at this cross section of the gearbox is how thin the rim section of the output stage planet gear is. This type of planet gear and bearing configuration having a thin rim without adequate radial stiffness can present a fatigue issue. The planet gear is subject to fairly large opposing inward radial forces twice per rev, from the mesh contacts at the ring gear and sun gear. Basically this means the planet gear rim gets squashed into a slightly oval shape twice per revolution. In my opinion, based simply on what I see in the drawing, it seems like the planet gear rim could have been made a bit thicker. Since this gear is a fracture critical component, I'd think this particular condition of crack propagation from a race surface spall would be a case considered in the fracture analysis.
There is one other thing that seems a bit strange to me based on what I read. If the planet gear rim fracture initiated at a spall in the race surface, there should have been enough ferrous debris generated by the early spall formation to be picked up by a chip detector long before it became a problem. Once again, looking at the cross section drawing of the gearbox linked above, there is an oil tray below the planetary stages. I wonder if the shape of this tray, combined with swirling motion imparted to the return oil flow by windage from the close proximity rotating components, effectively trapped the ferrous debris above the tray and prevented it from reaching the chip detector in the sump.
I'd just like to state the comments above are all just speculation on my part, and I admit to having no detailed knowledge of the design of the gearbox in question.
One thing I noticed looking at this cross section of the gearbox is how thin the rim section of the output stage planet gear is. This type of planet gear and bearing configuration having a thin rim without adequate radial stiffness can present a fatigue issue. The planet gear is subject to fairly large opposing inward radial forces twice per rev, from the mesh contacts at the ring gear and sun gear. Basically this means the planet gear rim gets squashed into a slightly oval shape twice per revolution. In my opinion, based simply on what I see in the drawing, it seems like the planet gear rim could have been made a bit thicker. Since this gear is a fracture critical component, I'd think this particular condition of crack propagation from a race surface spall would be a case considered in the fracture analysis.
There is one other thing that seems a bit strange to me based on what I read. If the planet gear rim fracture initiated at a spall in the race surface, there should have been enough ferrous debris generated by the early spall formation to be picked up by a chip detector long before it became a problem. Once again, looking at the cross section drawing of the gearbox linked above, there is an oil tray below the planetary stages. I wonder if the shape of this tray, combined with swirling motion imparted to the return oil flow by windage from the close proximity rotating components, effectively trapped the ferrous debris above the tray and prevented it from reaching the chip detector in the sump.
I'd just like to state the comments above are all just speculation on my part, and I admit to having no detailed knowledge of the design of the gearbox in question.
riff raff and buzz66, you are both spot on.
Failure to detect sufficiently is the one bold claim made in the preliminary report.
The gear design does not account for a rim fracture as a design point, relies on this mode to simply never show up. An overtorque test is the standard used, but that would not consider degradation of raceway over time, just tooth bending primarily. Other factors could still include mis-assembly leading to uneven load share (although at least two planets have the cracks. Worse, there may not be enough control of planet load share. We really speculate without knowing details, but fatigue can still happen in a matter of hours if loads are unusually high. In that case - would we not expect severe loads to be picked up by the HUMS? I said early on that I suspect that we have multiple failed oppurtunities that would have prevented this. We have an idea on two, details will follow, but it will take a couple months anyway. Urgency to report initially is about maintaining fleet safety. Only urgency now is money, and the investigators are not losing any, so they are not the ones in a hurry. We won't be seeing more tidbits until someone inside the investigation leaks it. Until then, we rumor anyway.
Failure to detect sufficiently is the one bold claim made in the preliminary report.
The gear design does not account for a rim fracture as a design point, relies on this mode to simply never show up. An overtorque test is the standard used, but that would not consider degradation of raceway over time, just tooth bending primarily. Other factors could still include mis-assembly leading to uneven load share (although at least two planets have the cracks. Worse, there may not be enough control of planet load share. We really speculate without knowing details, but fatigue can still happen in a matter of hours if loads are unusually high. In that case - would we not expect severe loads to be picked up by the HUMS? I said early on that I suspect that we have multiple failed oppurtunities that would have prevented this. We have an idea on two, details will follow, but it will take a couple months anyway. Urgency to report initially is about maintaining fleet safety. Only urgency now is money, and the investigators are not losing any, so they are not the ones in a hurry. We won't be seeing more tidbits until someone inside the investigation leaks it. Until then, we rumor anyway.
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To summarize: It seems useless, hypothetical and philosophical to discuss about chicken and egg, if the chicken turns out to be a cuckoo! The rest is ’PPRu’ and entirely worth reading, because the crash of LN-OJF for sure will become a crucial experience for future helicopter technics and design.
Can anyone explain to me why the EASA and FAA directives cover the 225LP and the 332L2 but not the 332L1 and none of the short Puma variants, the civil version being now called the H215C1e? I know the MRH is different on the 332L1 but is the main transmission the same? Between the many variants and the Airbus model name changes it is difficult to know what some of the differences are. I am an engineer/mechanic and may become involved with the short Pumas. Thanks.
roscoe1
I believe the MRGB changed significantly after the L1.
Further, I understand the L2 and EC225 MRGBs are substantially the same design as each other. Hence in this, and the previous grounding, the L2 and 225s were grounded, but the earlier variants continue/continued to fly.
I believe the MRGB changed significantly after the L1.
Further, I understand the L2 and EC225 MRGBs are substantially the same design as each other. Hence in this, and the previous grounding, the L2 and 225s were grounded, but the earlier variants continue/continued to fly.
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Shaft Problems & Co
Because I'm not particularly familiar with the development of the SUPER PUMA I browsed a bit in the Web and found the following interesting documents
In this case all helicopters being equipped with ’Shaft 3’ must be grounded and those might be besides the already grounded H225 and AS 332L2, due to overhauls of MRGB, a majority of AS332L1 and AS 332L worldwide!
With regret and the wishing of a competent and happy knack in tackling the various problems of the PUMA / PUMA SUPER series it is no longer done.
Too much has accumulated over the decades at the "Design Approval Holder" AIRBUS Helicopters France and it seems to me more likely to go to "To be, or not to be, that is the question”.
The "shaft problem" and/or the total MRGB problem might result in the desaster that the majority of the approx. 800 PUMA / SUPER PUMA operated worldwide is multiple affected.
- Probe into EC225 North Sea Ditching Centers on Gearbox | Business Aviation News: Aviation International News
- Probe into EC225 North Sea Ditching Centers on Gearbox | Business Aviation News: Aviation International News
- In the L, L1 and L2 models is (was) "Shaft 1" of original design installed.
- For the H225 model was "Shaft 2" developed and installed
- In the course of product care, maintenance and overhaul "Shaft 2" as retrofit could also be installed in the L, L1 and L2 models.
- However, "Shaft 1" can not be installed in the H225 model.
- As one of the results of the Aircraft Accident Investigations of Ditching of G-REDW and of G-CHCN a change in the manufacturing process of specific batches of "Shaft 2" was thought to be the cause.
- Since 2012 and the groundings, L, L1 and L2 aircraft are re-fitted with ‘Shaft 1”, the original design of shaft which had accumulated 4.5 million flight hours without obviousely corresponding incidents or accidents
- New evidence is coming to light which suggests that the scopes of the problems associated with ’Shaft 2’ are wider. Use of aircraft which feature this shaft is not supported by the aircraft operators and is also not permitted by the CAA of UK since the incidents and accidents of 2012.
- H225 remained grounded till development, evaluation, production and availability of ’Shaft 3’ was successful in 2013 and all H225 were equipped / retrofitted with ’Shaft 3’.
In this case all helicopters being equipped with ’Shaft 3’ must be grounded and those might be besides the already grounded H225 and AS 332L2, due to overhauls of MRGB, a majority of AS332L1 and AS 332L worldwide!
With regret and the wishing of a competent and happy knack in tackling the various problems of the PUMA / PUMA SUPER series it is no longer done.
Too much has accumulated over the decades at the "Design Approval Holder" AIRBUS Helicopters France and it seems to me more likely to go to "To be, or not to be, that is the question”.
The "shaft problem" and/or the total MRGB problem might result in the desaster that the majority of the approx. 800 PUMA / SUPER PUMA operated worldwide is multiple affected.
Last edited by AW009; 4th Jun 2016 at 15:02. Reason: formatting
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So the MRGB was changed from the L1 version, is this now the same gearbox that is in the 175???
Are there new evidences given, that ’Shaft 3’ might be one of the multiple causes of latest accident in Norway?
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What can't be ignored regardless of what the cause, is the fact it was never detected by HUMS, CHIPS, FDR, CVR, SOAP, ENGINEERS, PILOTS
Whatever failed first it gave no warning at all!
I am not an expert on the Super Puma but have friends flying them. I agree with Buzz66 on the fact that there was no warning thus no pilots reaction time to this accident. This is a major set back in confidence on the platform for both pilots and passengers whatever the outcome of the investigation.
Whatever failed first it gave no warning at all!
I am not an expert on the Super Puma but have friends flying them. I agree with Buzz66 on the fact that there was no warning thus no pilots reaction time to this accident. This is a major set back in confidence on the platform for both pilots and passengers whatever the outcome of the investigation.
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@Never Fretter, @Pablo332: Do You both definitely know, that the bevel gear and its shaft was 100% o.k.?
Then you might know more as AIBN!
Or are you AIBN?!
Then you might know more as AIBN!
Or are you AIBN?!
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@Never Fretter, @Pablo332: Do You both definitely know, that the bevel gear and its shaft was 100% o.k.?
Then you might know more as AIBN!
Or are you AIBN?!
Then you might know more as AIBN!
Or are you AIBN?!
Would not surprise me. It would be interesting to know the material used. I expect that will feature in AIBN reports in due course. (I am a former automotive gear design guy.)
Trolling?
@Never Fretter, @Pablo332: Do You both definitely know, that the bevel gear and its shaft was 100% o.k.?
Then you might know more as AIBN!
Or are you AIBN?!
Then you might know more as AIBN!
Or are you AIBN?!
we need, as a whole industry, a serious investigation, not self appointed experts.
Besides, the issue of the bevel gear has been discussed in other long threads, and your "x files" are not, in fact, the end of that story. You should do your homework better, and read AAIB accidents reports, AH SINs, ADs, introduced interim modifications and redesigned gear description, on this topic.
Another thing: do you really need to use "AW009" nick? You know, for me, working in AW, now Leonardo, it is a bit annoying that a glorious name is associated to such posts...
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AW009,
Why do you not get it??? It is the only thing that Airbus has 100% verified. After 48 hours, they stated that this was NOT a repeat of the issue that caused the 2012 ditchings. In which, they were talking about the bevel gear and its shaft. That is about the only thing that has been confirmed at this point.
@Never Fretter, @Pablo332: Do You both definitely know, that the bevel gear and its shaft was 100% o.k.?
seems some of you need to see this
https://www.youtube.com/watch?v=iJKZPbf_kPI
and
https://www.youtube.com/watch?v=oD4jKBOIBwc
its about the new vertical shaft
https://www.youtube.com/watch?v=iJKZPbf_kPI
and
https://www.youtube.com/watch?v=oD4jKBOIBwc
its about the new vertical shaft
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Bad Parts again?
The refurbishing of the grarbox, and probably the epicyclic gears too, makes me think of old stories about bad spare parts being resold as new. The husband of by secretiry died as pilot in an accident over the North Sea when a bolt broke making the plane lose its vertical stabilser.
Is there a system with numbering of each part in a gearbox which would certify that old parts are not saved and reused several times instead of new parts.
Is there a system with numbering of each part in a gearbox which would certify that old parts are not saved and reused several times instead of new parts.