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EC225 crash near Bergen, Norway April 2016

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EC225 crash near Bergen, Norway April 2016

Old 11th Oct 2016, 14:47
  #1581 (permalink)  
 
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Alas though there is politics for taking us into needless wars.

I think the place of safety though is too often clouded with the commercial and financial desires of those above. It does to me give the impression that this aircraft has one (another) last chance to not kill more people.
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Old 12th Oct 2016, 10:55
  #1582 (permalink)  
 
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I would be curious to know what AH understand by the term “reliability”.

"There are two configurations of planet gear within the current type design. In depth review of the design and service data showed that one configuration has higher operating stress levels that result in more frequent events of spalling, associated with rolling contact fatigue, while the other exhibits better reliability behaviour. By limiting the type design to the gear configuration with lower stress levels and better reliability and specifying a reduced life limit, combined with more effective oil debris monitoring procedures and other operational controls, an acceptable level of safety can be restored."

They state that one type has more frequent events of spalling but the certification of the H225 and Super Puma range MGB is reliant on magnetic chip detection. The problem identified by AIBN is that one 2nd stage planet gear fractured without prior warning by chip detection. Similarly the only warning on G-REDL was one single chip detected and referred to AH for advice. According to the AAIB Report 2/2011 section 1.18.3, the advice later given by AH when a chip of planet gear raceway was detected on another aircraft, G-REDN, in March 2011 (23 months post G-REDL and long after magnets were all removed from oil separator plates) was to continue to fly and monitor, which it did for another 87 FH until further chips were detected. It is fortuitous that G-REDN survived and it is therefore important to know what type, A or B, was its planet gear.

As far as published investigation reports go to date, no gear fragment recovered from either crash exhibited notable spalling. So, if operators replace these planet gears with ones that are even less likely to give prior warning by particles from raceway spalling, how exactly does that improve reliability? It seems there are 2 possible failure modes, one with spalling and one without.

What AH appear to be doing is attempting, desperately, to perpetuate the myth that the only cause of fatigue cracks can be spalling, and spalling extensive enough at that to be detected before a crack propagates to failure (which in the case of G-REDL the AAIB estimated the total time of crack propagation from initiation to failure as “possibly more than 100 flying hours”). The thing you have to ask yourself, punk…

Last edited by Concentric; 12th Oct 2016 at 11:54. Reason: Corrected quoted date.
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Old 12th Oct 2016, 13:29
  #1583 (permalink)  
 
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Well said Concentric!

As a nobody who used to overhaul a lot of AH epicyclics, I find it difficult to believe that AH seem to think the gear will spall before the inner race. The inner race has the load concentrated in one area, whereas the gear is rotating, spreading the load.

If the gear really does spall before the inner race and then this spalling has the possibility of inducing a fatigue failure, then there is definitely something wrong with their gear design/metallurgy.

The inner race should be spalling well before anything shows up on the gear, producing lots of nice flakes to set off the appropriate alarm, before anything catastrophic happens.

I'm not saying it doesn't happen, but I never saw an epicyclic gear with spalling. The inner race would lunch itself and contaminate the gear, leaving it with a pitted look, but not spalling on the gear itself.
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Old 12th Oct 2016, 23:22
  #1584 (permalink)  
 
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then we also come back to that of interpreting the facts...

I still don´t agree to the fact that LN-OJF had "no" spalling Before the breakdown.

the pics shown are as well spalling evident and exactly in the same shape
and position related to the depth of the nitrated Surface as with the G-REDL.

what can be said is that it is claimed that no significant spalling had occured
during the time the box was installed in LN-OJF

moreover, we don´t know if there was spalling at an earlier stage of overhaul, since the MGB was inside the TBO of checking and replacing the gears.

moreover, I Think we should say, we don't know... but AH knows

what we also don't know is the reason that the two types of gears are produced.. we also don't know how many TBO each gear set have had, in total with the H225.

lets's get the facts straight Before we go into that discussion.

it might be that many many of the 225's that are not operated by British or Norwegian would not in the next year(s) be in the flight time of MGB TBO.

I would assume that AH knows this, and that is the reason EASA is allowing it to fly.. But of course, they would never say that in the ongoing
investigations, or to any customers that have bought this chip maker of a french disaster.

I would not be suprised if the old gear-sets have passed many more flight hours and TBO's than the other one..


It is still nonetheless disrespectfull to the victims and their families to
still play with Dirty hands and not telling the truth
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Old 13th Oct 2016, 12:27
  #1585 (permalink)  
 
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Originally Posted by turboshafts View Post
I still don´t agree to the fact that LN-OJF had "no" spalling Before the breakdown.
I should say that I did not mean to imply in my post that there was ‘no’ spalling (i.e. zero) on the recovered fragments, but rather 'no notable spalling' and not enough to be detected. To my mind a notable or significant extent of spalling would be similar to that shown in Fig 33 in the AAIB report 2/2011 and that was associated with a crack that had not yet reached fracture. My personal experience is with much larger machinery bearings where spalling would often be measured in several square inches.
The extent of spalling in the CT scan Fig 6 in the AIBN preliminary report is minimal in comparison to Fig 33.
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Old 15th Oct 2016, 10:59
  #1586 (permalink)  
 
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Politics

Anybody else find it interesting that the French president was so annoyed that Poland cancelled an order for 50 225s that he cancelled a trip there? This within days of EASA lifting a flight ban on the type. Amazing coincidence......

France's Hollande scraps Poland trip over lost Airbus deal
http://www.bbc.co.uk/news/world-europe-37586911
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Old 15th Oct 2016, 13:17
  #1587 (permalink)  
 
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Originally Posted by Hompy View Post
Anybody else find it interesting that the French president was so annoyed that Poland cancelled an order for 50 225s that he cancelled a trip there? This within days of EASA lifting a flight ban on the type. Amazing coincidence......

France's Hollande scraps Poland trip over lost Airbus deal
France's Hollande scraps Poland trip over lost Airbus deal - BBC News
Perhaps he was expected to make the journey in an EASA225(‘B’) as a show of confidence in the type but he is a reader of PPRuNe? Not like a politician to look after their own skin…Comment allez-vous, Francois?
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Old 16th Oct 2016, 05:03
  #1588 (permalink)  
 
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Originally Posted by noooby View Post
Well said Concentric!

As a nobody who used to overhaul a lot of AH epicyclics, I find it difficult to believe that AH seem to think the gear will spall before the inner race. The inner race has the load concentrated in one area, whereas the gear is rotating, spreading the load.

If the gear really does spall before the inner race and then this spalling has the possibility of inducing a fatigue failure, then there is definitely something wrong with their gear design/metallurgy.

The inner race should be spalling well before anything shows up on the gear, producing lots of nice flakes to set off the appropriate alarm, before anything catastrophic happens.

I'm not saying it doesn't happen, but I never saw an epicyclic gear with spalling. The inner race would lunch itself and contaminate the gear, leaving it with a pitted look, but not spalling on the gear itself.
noooby,

Your point about a sector of the planet gear bearing inner race (which is fixed wrt radial loads) being subject to most fatigue cycles is correct. And normally one would expect this to be the most likely location for contact fatigue spalling to occur. But there can be other problems that would cause the planet gear to suffer the type of radial fracture propagating outward from a race surface spall like this example.

First, it is not possible to pre-load the type of spherical roller bearing used in this case. So there will normally be some amount of radial clearance present. If this radial clearance is excessive, the gear rim will be subject to greater reverse cyclic bending loads than it was designed for. Think of a rotating cylinder subject to opposing local inward radial loads as it gets squeezed between the ring and sun gear. The planet gear race surface will experience a tension/compression load cycle every 180 degrees of rotation, adding to the normal contact stresses. If the fatigue/fracture analysis of the planet gear did not consider a condition with excessive radial clearance in the bearing, this problem would not have been exposed. Normally, the analysis assumes the bearing is manufactured within certain tolerances.
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Old 16th Oct 2016, 06:36
  #1589 (permalink)  
 
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Originally Posted by riff_raff View Post
noooby,

Your point about a sector of the planet gear bearing inner race (which is fixed wrt radial loads) being subject to most fatigue cycles is correct. And normally one would expect this to be the most likely location for contact fatigue spalling to occur. But there can be other problems that would cause the planet gear to suffer the type of radial fracture propagating outward from a race surface spall like this example.

First, it is not possible to pre-load the type of spherical roller bearing used in this case. So there will normally be some amount of radial clearance present. If this radial clearance is excessive, the gear rim will be subject to greater reverse cyclic bending loads than it was designed for. Think of a rotating cylinder subject to opposing local inward radial loads as it gets squeezed between the ring and sun gear. The planet gear race surface will experience a tension/compression load cycle every 180 degrees of rotation, adding to the normal contact stresses. If the fatigue/fracture analysis of the planet gear did not consider a condition with excessive radial clearance in the bearing, this problem would not have been exposed. Normally, the analysis assumes the bearing is manufactured within certain tolerances.
A good clear description. If I may add a little:

There are further complexities that would make analysis of stresses in these gears a difficult 4D problem when considering fatigue. The squeezing described above is not equal but rather the 'cylinder' is wrapped around 3 quarters of the bearing. All the clearance bunches up behind the bearing (in advance of the bearing as it orbits the sun gear) which will happen once in 360 degrees. So you have effects once per rev and effects twice per rev of the planet.

The 2 races of rollers per gear are inclined and bear against the concave inner surface of the gear (the outer raceway of the bearing). As well as radial forces, these also impart axial tensile forces into the body of the gear around the loaded sector creating axial tensile stresses that vary with the thickness of the 'cylinder'. The opposite effect occurs on the inner race which is axially compressed.

As was revealed in the G-REDL accident report, the barrel shaped rollers only truly 'roll' at one position over their length, with some of the contact under loading conditions involving some sliding and frictional force components that had not originally been considered in the design and which can lead to a crack radiating outwards.

Whilst we can envisage these forces and stresses, actually calculating and combining them all accurately requires sophisticated software tools. Even then there have to be certain assumptions made regarding utilisation load spectra, load sharing, temperature effects etc. and the validity of assumed boundary conditions in any model. Then you have to consider material and manufacturing consistency that the whole model is based on. Factors that affect the various heat treatments have previously been discussed.

Last edited by Concentric; 16th Oct 2016 at 17:31.
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Old 16th Oct 2016, 21:23
  #1590 (permalink)  
 
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As a mere pilot these technical explanations appear to reveal a situation in which the manufacturers of these gearboxes know from the outset that they are full of imperfections due to the very concepts behind their design. They then rely on being able to detect the inevitable failure courtesy of the infamous chip detector system. When this detection mechanism is found to have holes in it surely that puts the whole design philosophy behind this type of gearbox into question. How can the certification authorities support the notion that all is well in the helicopter transmission business? Is this behind the UKCAA and NCAA approach? How come the EASA experts don't get it? What kind of world are we living in?

G.
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Old 17th Oct 2016, 20:38
  #1591 (permalink)  
 
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Tell you what, Geoffers, whilst I miss flying terribly, I am glad I am now retired!!

TC
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Old 18th Oct 2016, 05:09
  #1592 (permalink)  
 
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Originally Posted by Geoffersincornwall View Post
As a mere pilot these technical explanations appear to reveal a situation in which the manufacturers of these gearboxes know from the outset that they are full of imperfections due to the very concepts behind their design. They then rely on being able to detect the inevitable failure courtesy of the infamous chip detector system. When this detection mechanism is found to have holes in it surely that puts the whole design philosophy behind this type of gearbox into question. How can the certification authorities support the notion that all is well in the helicopter transmission business? Is this behind the UKCAA and NCAA approach? How come the EASA experts don't get it? What kind of world are we living in?
Geoffers-

As Concentric pointed out, the analysis of this particular bearing case is a very complex non-linear problem. Even with the best computational analysis tools currently available, there are lots of assumptions and simplifications that must be used to make the analysis practical. Normally the assumptions and factors-of-safety used tend to give a very conservative result. But sometimes a "perfect storm" of problems occurs that the analysts did not consider, and the result can be what we witnessed in this situation.

Don't blame the gearbox designers. They seemed to have done a professional job designing this gearbox given the engineering tools available at the time, and meeting existing certification requirements.
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Old 18th Oct 2016, 08:12
  #1593 (permalink)  
 
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riff
The manufacturers employ gearbox designers who presumably know about the shortcomings of making an epicyclic gearbox, which, if I have understood the situation, is one that harbours basic conceptual floors given the manufacturing tolerances currently achievable. Small imperfections are magnified by the very nature of the relationship between sun, planet and ring gears when under load. It seems that the design philosophy is that 'we know it's going to break but we will replace/overhaul it BEFORE it breaks. Premature failure will be detected by HUMS (or equivalent) and by monitoring for debris using a system that gathers debris on a sensor and indicates the presence of debris to the end user.

Seems to me (pilot remember) that a gearbox overhaul period is a major key element. It may also be that existing gear wheels are NDT'd and returned for another spell at the sharp end at overhaul and not swapped for a complete new set. The TBO, in my simple mind, should be a fraction of the period before a test article produces debris. Say... half that period. (what methodology to calculate TBO's is used currently???).

So that brings into question the certification criteria with regard to initial TBO and overhaul protocols. Presumably the authorities have their own experts in gearbox design so the manufacturers cannot 'snow' the pen-pushers with a scientific overload. Can we use the lessons of the 225 to change the certification criteria.

History can teach us some lessons. As a type enters maturity the TBO's are extended as much as possible so reducing the safety margins in pursuit of reducing the costs of ownership. At the same time new types put pressure on manufacturing facilities so older types may have their vitals farmed out to sub contractors. I think Sikorsky have had that as an issue over the years with the S61 and S76. Sub contractors can cut corners to make money, particularly when i comes to the detailed checking of components. Who's to know if you check one in five instead of every one. One in 100 instead of one in 10.

So history sets us up to fail unless the rules are changed or existing rules properly enforced.

Seems to me that we need to invest in a little more reality from every organisation involved.
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Old 18th Oct 2016, 11:08
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TC. You and me. both.
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Old 18th Oct 2016, 17:17
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Originally Posted by Geoffersincornwall View Post
riff
Seems to me (pilot remember) that a gearbox overhaul period is a major key element. It may also be that existing gear wheels are NDT'd and returned for another spell at the sharp end at overhaul and not swapped for a complete new set. The TBO, in my simple mind, should be a fraction of the period before a test article produces debris. Say... half that period. (what methodology to calculate TBO's is used currently???).

History can teach us some lessons. As a type enters maturity the TBO's are extended as much as possible so reducing the safety margins in pursuit of reducing the costs of ownership. At the same time new types put pressure on manufacturing facilities so older types may have their vitals farmed out to sub contractors. I think Sikorsky have had that as an issue over the years with the S61 and S76. Sub contractors can cut corners to make money, particularly when i comes to the detailed checking of components. Who's to know if you check one in five instead of every one. One in 100 instead of one in 10.
Geoffers
TBO is typically driven by the life limited component with the lowest fatigue life, be it a gear, a bearing, a shaft, what-have-you. The fatigue life is calculated based on the loads modeled and then the safety factors ...
A variety of other things arise which lead to "on condition" overhaul decisions, which means that something didn't reach the predicted MTBF/TBO. It is in the ability to detect "on condition" conditions -- wear, pitting, fretting, corrosion, ect. -- that an on condition decision is made. If conditions are hidden or masked, as appears to be the case with this crash, a surprise failure, rather than "graceful degradation" is what the aircrew are confronted with.

A TBO change as a design matures would, in a perfect world, be based on parts or components remaining serviceable beyond its initially predicted/calculated fatigue life.


I am not sure I understand your point on broad brush TBO changes. Over time I'd expect them TBO to go down, not up, as parts age, and wear, and as actual replacement intervals are discovered in service (as compared to design/calculated fatigue life).
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Old 18th Oct 2016, 18:17
  #1596 (permalink)  
 
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LoneW. - My only knowledge of TBO's is in association with a new product where conservative TBO's are set on day one and the manufacturer seeks to grow those as real world experience increases. I didn't realise that they could be reduced as a type aged.

I guess the gearbox designers are struggling with weight issues so how much metal goes into the vital components is clearly a critical part of the design. Well, here is one pilot that would beg the designers to put 15kg (my flight bag) of metal back into the gearboxes or any where else where margins have been pared to the bone.

G.
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Old 18th Oct 2016, 18:19
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"We don't know what we don't know yet."
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Old 18th Oct 2016, 23:07
  #1598 (permalink)  
 
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Originally Posted by Geoffersincornwall View Post
LoneW. - My only knowledge of TBO's is in association with a new product where conservative TBO's are set on day one and the manufacturer seeks to grow those as real world experience increases. I didn't realise that they could be reduced as a type aged.

I guess the gearbox designers are struggling with weight issues so how much metal goes into the vital components is clearly a critical part of the design. Well, here is one pilot that would beg the designers to put 15kg (my flight bag) of metal back into the gearboxes or any where else where margins have been pared to the bone.

G.
Geoffers: our experience seems to vary, so I suppose it may very much depend on the model, the safety factors used, and what they see in service. My comments are biased by my experience, so let's not pretend that my model is universal, and your experience has now taught me something.

Thank you.
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Old 19th Oct 2016, 06:27
  #1599 (permalink)  
 
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Goffers,

Both and/or any case of TBO/Finite life can change in the life cycle of most models. It more than likely starts out low as you say then increases with empirical evidence then inevitably it possibly goes down again as MTOW's increase during the life cycle of the type. ATA Chapter 4 and 5 are normally constantly changing.

Or not.

As usual being the customer we probably get about 1% of the real picture.

The "code" written in manufacturer service bulletins between the lines is "priceless" in some cases.
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Old 20th Oct 2016, 07:28
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Geoffers,

Just before my son started his course in mechanical engineering, I vividly remember his professor (the head of department at the University) quoting this:

"Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse, so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance."

Apparently this was taken from a lecture by a distinguished engineer in 1946. We have made lots of progress since then, but I doubt that the fundamental truth has changed.
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