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Old 13th May 2017, 00:53   #1801 (permalink)
 
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That statement about finding such a relatively large piece of gear steel debris inside the heat exchanger core seemed odd to me at first, since it would have had to pass thru the oil pump inlet screen to end up there. However, after looking at this image of the oil pump inlet screens it does seem possible that such a relatively large piece of debris could pass thru due to the coarse mesh size.

One thing that AH might consider is using a finer mesh size for the pump inlet screens. To me, the existing inlet screen mesh seems too coarse. Obviously using a finer mesh size would require increasing the screen area to maintain acceptable flow characteristics. Logically, the size of the screen mesh should be capable of stopping any debris particles that could not pass through the pump without causing unacceptable damage to the steel pump elements. I don't believe it could be demonstrated by analysis that this would be the case with the relatively large piece of gear steel debris found in the heat exchanger.

Would be interesting to see the condition of the oil pump elements, assuming they were recovered.
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Old 13th May 2017, 23:29   #1802 (permalink)
 
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I think you are missing the point completely Riffraff. There shouldn't be debris of this magnitude anyway to warrant any change of filter design. The root cause needs addressing, not the consequences
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Old 15th May 2017, 12:51   #1803 (permalink)
 
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Comment in this week's Flight Global ...

"But it seems clear that opportunities were missed. The frankly startling revelation that parts sourced from one of two suppliers have a failure rate three times as high as the alternative is troubling; that no one thought to analyse this data before last year’s Norway crash is almost beyond belief".

Yes, the H225 complies with all the certification requirements, but that begs the difficult question as to whether those standards are still fit for purpose.
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Old 15th May 2017, 15:43   #1804 (permalink)
 
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Yes, the H225 complies with all the certification requirements, but that begs the difficult question as to whether those standards are still fit for purpose
If you look back at comments to prior incidents/accidents with the type you'll find a number who would seek to put aviation into some special category where normal logic doesn't seem to count.
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Old 15th May 2017, 16:14   #1805 (permalink)
 
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@Pitts: if all you are interested in is providing a wind-up it might be best to avoid hitting the "post" button. (I have more frequently of late overcome the urge to post, which I find is a healthy thing).


It is troubling to me, the idea that of two suppliers for a precision component in a drive train/transmission, one had 3x failure rate (or significantly less reliability) and yet was still retained as a supplier/vendor by the manufacturer. While there may be more to that story, on the face of it someone didn't take reliability as seriously as they ought to have done, or, a batch/lot of material of considerable size was produced and distributed and it took a long time for people to discover and try to remedy that.
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Old 15th May 2017, 17:33   #1806 (permalink)
 
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Not a wind up. The issue here is fuelled by people, some of whom would be a useful voice within the industry who in the past have shown what one might call ambivalence. As your post goes on to say it is troubling and yet that process can not be a sudden revelation to those embedded in the various organisations. Your attention should be less upon my voice and more upon those whose concern in the recent past extended to selecting a large size of under crackers.
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Old 16th May 2017, 06:17   #1807 (permalink)
 
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Quote:
Originally Posted by Lonewolf_50 View Post
It is troubling to me, the idea that of two suppliers for a precision component in a drive train/transmission, one had 3x failure rate (or significantly less reliability) and yet was still retained as a supplier/vendor by the manufacturer. While there may be more to that story, on the face of it someone didn't take reliability as seriously as they ought to have done, or, a batch/lot of material of considerable size was produced and distributed and it took a long time for people to discover and try to remedy that.
I think we need to be careful not to conflate spalling with ‘failure’. I am not even sure the term “less reliable” is appropriate either, as Time Since New is not listed against each unit withdrawn, only collective fleet hours fitted with respective bearing types (apart from the Angolan one).

The two catastrophic failures were by fatigue crack propagation beyond the hardened depth. Not all spalling results in deep fatigue cracking, as evidenced by the 20 FAG bearings that have spalled ‘harmlessly’. Remember that 7 SNR bearings have spalled ‘harmlessly’ too. Given the total production numbers of bearings, most of both types would actually appear to have either been still in satisfactory service or have reached design SLL when these statistics were gathered. In that sense the behaviour of all except the REDL and LN-OJF bearings has been ‘reliable’ in that they followed the expected end of life degradation process.

You can look at those same statistics presented in Table 6 a very different way. Look at the total number of units of each type and how many of them DID NOT produce spalling. That would give “reliability” scores of 99.35% and 99.76% for FAG and SNR respectively. Not so clear cut now is it? Remember also the definition of L10 life for bearings (see 1.6.8.2).

You cannot equate that minor difference in reliability simplistically with Hertzian contact stresses of 1800MPa and 1550MPa (86% of 1800MPa) respectively. There is far more to it than that. A lower Hertzian contact pressure (Edit: by widening the path) in a spherical raceway can involve more slippage and, depending on lubrication, generate more surface friction that is more likely to turn a crack inwards.

Further investigation should not only focus on metallurgical differences but the whole tribology of these bearings and their operational conditions.

Last edited by Concentric; 16th May 2017 at 11:13.
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Old 16th May 2017, 17:39   #1808 (permalink)
 
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Possibly of greater interest than comparing FAG bearing spalling with SNR bearing spalling statistics over the same period, is to compare the information in 1.6.11.4 with that in 1.6.11.5. Excluding the crash items, between 2001-2016, 27 bearings have been removed in service due to spalling or micro-pitting but (according to Airbus) only 2 of those were between 2009 and 2016.

FAG stated (in 1.6.8.3) that there have been no differences to the design or manufacture during the production life of their bearings, yet for the first 8 years 25 bearings (incl. both types) were removed (average 3.125p.a.) and for the next 7 years only 2 were removed (0.286 p.a.), despite any heightened concerns or vigilance there may have been following the REDL crash in 2009. I can’t quite figure that reduction of 91% out.

What, if anything, has changed materially in that time and in particular perhaps in, or shortly before, 2009? Was there any change to lubricants, additives or to internal spray nozzle arrangements?
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Old 19th May 2017, 06:04   #1809 (permalink)
 
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One photo in the report shows an obvious difference in roller face width between the FAG and SNR bearings. But the difference in face width alone may not be responsible for the different hertzian contact stress levels noted. The roller/race osculation ratios can also have a significant effect.

The M50 material used for aircraft bearing rollers/races typically allows a higher material reliability adjustment factor to be used in a conventional rolling element bearing fatigue life analysis than a race using carburized VIM-VAR 9310 gear steel would.
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Old 19th May 2017, 07:49   #1810 (permalink)
 
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I agree, the difference in face width is not the only factor. But even the difference in Hertzian contact stress I believe may not be the only driving force behind this failure mechanism. Some bearings in wind turbine applications run up to 3 GPa. Normally fatigue damage is actually stress range raised to the power of 3, or higher depending on the S-N curve assumed for the damage calculation, so such a difference in Hertzian stress would produce even more contrasting results than we are seeing here. That approach would seem to apply more to dry rolling contact.

As I understand it the prevention or delay of damage in rolling/sliding contact is highly dependent on the ability to maintain a thin film of lubricant between the contacting surfaces, most significantly for ball or barrel roller bearings with their element of sliding within the Hertzian contact area. Without changing the bearing design or roller/race osculation ratios, the best way for a manufacturer to improve this would be a change of lubricant or introducing certain additives (some of which can be aggressive). That could significantly prolong surface contact fatigue life and delay the onset of spalling; producing an improvement for both FAG and SNR bearings similar to what we see in the reported statistics post 2009.

I am no expert but the profile of the crack running deeper into the material ahead of the travel of the rollers looks uncannily similar to that caused by lubricating fluid seepage into a crack with pressurization and possible entrapment effects. There are several recent research papers available here and here.

To put it in layman’s terms, this mechanism is similar to hydraulic fracturing (fracking) of highly compressed rock formations in shale gas recovery. Could it also overcome the residual compressive stress in the carburised raceway, even the higher compression on the FAG bearing?

So my big question is – did AH achieve that dramatic improvement in spalling statistics post-2009 by changing the lubricant to one better able to maintain that thin film but potentially also better able to seep into micro-defects and be pressurized behind the crack tip?

By reducing the degradation process of spalling which, at the time of certification was considered to be benign, could AH have removed a crucial indicator and (unwittingly) simultaneously introduced a lethal failure mechanism that leaves no trace?


I really hope I am wrong.

Last edited by Concentric; 19th May 2017 at 08:52.
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Old 19th May 2017, 10:05   #1811 (permalink)
 
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Concentric

This does appear to be a plausible mechanism to explain the crack propagation in the gears. It cannot be the sole reason because a crack has to be initiated by some other event or process first, but once that opening or pit is created it would seem that eventual failure is inevitable. Every compression cycle would be the equivalent of driving an incompressible hydraulic wedge deeper into the material. As the process continues the hardness of the surface layer would assist in driving the crack propagation deeper into the softer underlying material.
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Old 19th May 2017, 10:37   #1812 (permalink)
 
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Gouli,

Indeed, the crack has to be initiated somehow and it appears from the report that it was from one of a series of micro-pits 15mm - 16mm from the top face of the bearing, so all of them slightly below the line of maximum Hertzian pressure at 14mm but where sliding contact might begin. That it was not one isolated micro-pit (a rare occurrence in itself) but one of several in a row is, I think, a clue as to how it came to exist. Those other micro-pits should give the metallurgists something to analyse.
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