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Old 15th May 2017, 12:51   #1801 (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   #1802 (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   #1803 (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   #1804 (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   #1805 (permalink)
 
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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   #1806 (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   #1807 (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   #1808 (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   #1809 (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   #1810 (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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Old 13th Jun 2017, 20:07   #1811 (permalink)
 
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Plan B

Dear All
As we are waiting for the final report , and modification / inspection-cycle or change of manufacturer of certain parts: How many ac are effected ( Norway and UK)
And what have the operators done.
I have followed this thread with great interest , but have little grasp of the local , regional and worldwide consequences for the SuperPuma.

I do compare this with the rudderhardover that B737 did suffer long time ago.
It is do or die, but they found the likely cause and did a good fix!

Anyway, I understand the Norwegian NTSB are mostly respected for not bowing to undue external pressure.

At the end of the day: we can never have main rotors depart without warning like this again. Your part of aviation is inherently more challenging than airliner ops is , never mind the wings going AWOL.

Regards
Cpt B
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Old 1st Jul 2017, 08:34   #1812 (permalink)
 
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There was a couple of low key press releases by Airbus claiming the 225 will be returning to service soon.
https://www.flightglobal.com/news/articles/h225-helicopter-to-return-to-full-flight-status-soo-438783/

https://www.flightglobal.com/news/articles/uk-caa-sees-no-prospect-of-quick-return-for-grounded-438857/

This survey has appeared on one of the offshore sites.
http://www.airbushelicopterssurvey.com/
I dont think they realise how things have moved on
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Old 1st Jul 2017, 08:38   #1813 (permalink)
 
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Originally Posted by S92PAX View Post
I dont think they realise how things have moved on
Not exactly sure to understand what you mean here?
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Old 1st Jul 2017, 08:46   #1814 (permalink)
 
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I meant the 225 has been replaced and even if was introduced again the strength of feeling against flying in it is too great to overcome.
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Old 1st Jul 2017, 09:07   #1815 (permalink)
 
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Interesting articles. Regardless of one's individual opinion, the market doesn't really need the H225 at the moment. S-92s took up most of the slack and the Super Mediums are taking slots where they fit which will free up more S-92s to use where a heavy type is definitely required.
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Old 1st Jul 2017, 17:35   #1816 (permalink)
 
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TM is probably right about the current market needs.

Super Puma on the front page of the P&J again today. Plenty of opinions inside from people who know less than nothing about helicopters. Plenty of mistakes as well. (H255?!)
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Old 2nd Jul 2017, 22:09   #1817 (permalink)
 
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https://www.energyvoice.com/oilandga...rth-sea-skies/

Quote:
The spokesman said: “Airbus Helicopters has launched an online helicopter users’ survey.

“We are keen to hear directly from these users, mainly pilots and passengers, regarding their thoughts on helicopter features in general and the H225 in particular.

“This broad and valuable feedback is key to helping us address the concerns, priorities and requirements of those who use our helicopters on a daily basis.”
I wonder if they will share the results?
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Old 3rd Jul 2017, 10:23   #1818 (permalink)
 
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Originally Posted by rotor-rooter View Post
https://www.energyvoice.com/oilandga...rth-sea-skies/



I wonder if they will share the results?
I am sure that if a lot of E&P workers were to reply positively then they would.

More relevant is that the industry in the North Sea changed. Operators/customers increased the use of medium types and many more of these have been introduced. I can see the use of the S92s being reduced and the mediums taking up a bigger share of the market.

What the Bergen crash seems to have been is a catalyst for this change.

AH have a decision to take on their new large helicopter, the mock-up shown in Paris asking as many questions as it answered. This survey is likely to have been issued partly to tell them how closely it can resemble the 225 and still sell, as evolution rather than revolution is always the preferred design approach.

Returning the 225 to the North Sea in any significant quantity has always seemed unrealistic in my opinion - but when you have a lot of history and personal in investment in a product it is very hard to let go.

Last edited by birmingham; 3rd Jul 2017 at 10:56.
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Old 4th Jul 2017, 10:32   #1819 (permalink)
 
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I think a number of the operators are able to claim compensation from Airbus during the time the 225 is grounded - much of this income is then ploughed into medium types, with the remainder kept for other operational costs or, in the rare case given the poor state of the industry, profit. When they are no longer able to make a claim (if/when the 225 is cleared to fly again), they may have to revert back to using it purely for financial reasons. I think there is still a market for them there - the S92 is not without its own problems.
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Old 4th Jul 2017, 11:45   #1820 (permalink)
 
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While the oil price is in this current regime, there will be increasing moves away from heavies as for most oil companies, the main driver in all decisions is price. The costs of heavies make them uneconomic in comparison. The H225 is dead in the water commercially unless Airbus can cut their hourly DOC/ PBH dramatically and the leasing companies accept the hit and drop the lease rates dramatically. Even then, it's reputation is still in the toilet.

It goes back to the Air Advisers and C&P people at the oil companies - who is going to sign up their workers to an H225 contract? Answer - no one for the foreseeable future.

IMHO AH would be better off focusing on selling what they have to militaries and spending their time and money creating something else.
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