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EC225 crash near Bergen, Norway April 2016

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EC225 crash near Bergen, Norway April 2016

Old 17th Jun 2016, 23:21
  #1361 (permalink)  
 
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Radial roller bearings outer races are heated up and pressed oval in a die when mounting the balls.
They would not crack with a hammer hit. The standard bearing material is 100Cr6 and has a surface hardness of around 60 HRC.

here are two interesting reports that gives a bit more insight in the materials used

https://www.google.de/url?sa=t&rct=j...,d.ZGg&cad=rja

https://www.google.de/url?sa=t&rct=j...,d.ZGg&cad=rja

And here it refers the material of the 332L2 planet gear outer race material:
Fractured Gear | Flight Safety Foundation
Subsequent testing determined that the particle was not silver or cadmium but 16NCD13 steel, planet gear outer race/gear material.
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Old 18th Jun 2016, 05:59
  #1362 (permalink)  
 
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Originally Posted by voando View Post
Probably off-thread sorry but was fowl at the time: RTA lowloader carrying S76 (all less blades) colliding with swerving articulated trailer filled with live chickens; re-named KFC ........

Attachment 563

Attachment 564
Gota feel sorry for the Chickens and the Truck
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Old 18th Jun 2016, 07:06
  #1363 (permalink)  
 
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Not sure how relevant this is, but I recently had to remove the inner race from a BMW rear wheel drive axle that just wouldn't move using any bearing puller available to me. It didn't split easily, I had to cut (with a Dremel) almost through the race and then use a hammer and chisel in the groove. After quite a lot of (hard) hammering it finally cracked. I'd say that's a long way from cracking simply from a moderate blow. It all comes down to the alloy and hardening process used, and I doubt (or hope not) that they would make the epicycle bearings in a MGB to "a lower standard"/more brittle than the wheel bearings on a car.
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Old 18th Jun 2016, 10:21
  #1364 (permalink)  
 
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Voando: Note this event with the S76 and the chicken truck happened about 8 years ago.
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Old 19th Jun 2016, 04:32
  #1365 (permalink)  
 
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The planet gear teeth and bearing outer race surface are case hardened by carburizing. The spherical bearing rollers and inner race are through hardened bearing steel. The cage is quenched and tempered alloy steel with a coating like silver electroplating.

The article linked by turboshaft was very interesting. Learned quite a bit about actual maintenance procedures used by commercial operators.

The following comment caught my attention, "Although the particle was removed from the epicyclic chip detector, maintenance personnel did not remove the epicyclic module or recover any particles that might have accumulated on the magnets that were part of the gearbox separator plate." This is something I had asked about in previous posts. Whether the gearbox separator plate arrangement was trapping ferrous debris from the epicyclic stages contained in the return oil flow.
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Old 19th Jun 2016, 12:55
  #1366 (permalink)  
 
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@riff raff: it is useful to specify that the maintenance personnel you refer to are not related to the Bergen Crash, but the crash in 2009. What the maintenance details were, or weren't, in the case of this crash have yet to be reported as part of the current investigation. (I realize you understand this, but some of our readers may not).
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Old 20th Jun 2016, 03:34
  #1367 (permalink)  
 
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Originally Posted by turboshafts

Radial roller bearings outer races are heated up and pressed oval in a die when mounting the balls.
They would not crack with a hammer hit. The standard bearing material is 100Cr6 and has a surface hardness of around 60 HRC.

here are two interesting reports that gives a bit more insight in the materials used

https://www.google.de/url?sa=t&rct=j...,d.ZGg&cad=rja

https://www.google.de/url?sa=t&rct=j...,d.ZGg&cad=rja

And here it refers the material of the 332L2 planet gear outer race material:
Fractured Gear | Flight Safety Foundation
Subsequent testing determined that the particle was not silver or cadmium but 16NCD13 steel, planet gear outer race/gear material.
That is the type of info we need, thanks

I am a layman and speak as a layman but when a chip is produced in a MRGB it is not because a gear is too soft but because it is too hard (brittle)

Obviously soft steel gears would not be used because they would wear out quickly so they harden them to a point where they have a long life at the risk of being brittle and designers walk a fine line trying to find the best balance.

In an earlier post I gave some (poor) examples of witnessing bearing races shatter from impact and I did not make my point properly so I will try a different slant ...

If planetary gears were made of mild steel we could place them on an anvil and hammer them flat and they would never crack or break .... but if they were made of hardened steel they would never bend or distort , they would shatter into several pieces

The failed gears in these MRGB's are never deformed , they break into several pieces as the photos show.
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Old 20th Jun 2016, 13:35
  #1368 (permalink)  
 
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According to the AIBN/AH timetable they should now have some of the initial metallurgy and be trying to make sense of what it tells them. I would think it is going to be a difficult job to prove the chain of events - so many variables in the equation.

Last edited by birmingham; 20th Jun 2016 at 14:11.
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Old 20th Jun 2016, 22:40
  #1369 (permalink)  
 
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Originally Posted by Arnie Madsen View Post
That is the type of info we need, thanks

I am a layman and speak as a layman but when a chip is produced in a MRGB it is not because a gear is too soft but because it is too hard (brittle)

Obviously soft steel gears would not be used because they would wear out quickly so they harden them to a point where they have a long life at the risk of being brittle and designers walk a fine line trying to find the best balance.

In an earlier post I gave some (poor) examples of witnessing bearing races shatter from impact and I did not make my point properly so I will try a different slant ...

If planetary gears were made of mild steel we could place them on an anvil and hammer them flat and they would never crack or break .... but if they were made of hardened steel they would never bend or distort , they would shatter into several pieces

The failed gears in these MRGB's are never deformed , they break into several pieces as the photos show.
From the REDL report the epicyclic planets are made of the following steels: Gear/outer raceway 16NCD13, rollers M50, inner race M50.

The REDL gives the reasons for the different materials and details manufacture.

From experience a normal spalling failure would start on the inner race if all of the materials were the same due to the increased loading of the inner race. The gear/outer race on the 225 planet is a "softer" material due to the load cycle of the gear teeth, therefore there is a tendency of the 225/332 planets to start with outer race spalling
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Old 21st Jun 2016, 02:06
  #1370 (permalink)  
 
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Originally Posted by Arnie Madsen View Post
That is the type of info we need, thanks

I am a layman and speak as a layman but when a chip is produced in a MRGB it is not because a gear is too soft but because it is too hard (brittle)

Obviously soft steel gears would not be used because they would wear out quickly so they harden them to a point where they have a long life at the risk of being brittle and designers walk a fine line trying to find the best balance.

In an earlier post I gave some (poor) examples of witnessing bearing races shatter from impact and I did not make my point properly so I will try a different slant ...

If planetary gears were made of mild steel we could place them on an anvil and hammer them flat and they would never crack or break .... but if they were made of hardened steel they would never bend or distort , they would shatter into several pieces

The failed gears in these MRGB's are never deformed , they break into several pieces as the photos show.
Arnie you are right and I take your Point.

However, there are several types of heat treatment that can be done to
a steel.

When you say hardened and brittle. There are several types of hardening.
I assume you mean through-hardened.
If you take a normal Construction steel and harden it, that is about
what you would get a brittle steel.

but for gears there could be several heat-treatments depending on the type of application.

you have through hardening, case hardening, quenching, carburizing etc.

through hardening is normally not favourable on a gear,
allthough in low-cost Products in may also be used.

if you have a carburizing steel, you heat treat it to
make the core though and the outer Surface hard.
If you look at the Pictures of the broken gears from the preliminary report,
you can actually see that the gears are lightly oval from deformation Before they crack.

you can see from the reports I linked the different characteristics of the failure modes in a gear cog.

what is evident here is fatigue cracks.
from my understanding is not occuring due to overload of the gear
but after a number of repeated cycles Close to its yield strength
it will sooner or later start to spall or crack

anyway after looking again at the epi pics from AIBN,
with out any chip detection alarm. oil pressure?
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Old 21st Jun 2016, 02:26
  #1371 (permalink)  
 
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Originally Posted by n305fa View Post
From the REDL report the epicyclic planets are made of the following steels: Gear/outer raceway 16NCD13, rollers M50, inner race M50. The REDL gives the reasons for the different materials and details manufacture. From experience a normal spalling failure would start on the inner race if all of the materials were the same due to the increased loading of the inner race. The gear/outer race on the 225 planet is a "softer" material due to the load cycle of the gear teeth, therefore there is a tendency of the 225/332 planets to start with outer race spalling
Good post.

16NCD13 is equivalent to AMS 6263 (9315), and the material is likely supplied in the form of a roll forged ring. This material is carburized, quenched, and tempered at a temperature that gives optimum core strength for the intended operating conditions. There is large difference between the case and core hardness.

You are also correct about the loaded sector of the inner race surface being the most likely area to experience a spalling failure under normal conditions. The inner race is fixed with respect to the applied radial load, the roller/race profile contact is convex/convex, so there is one small area of the race surface in-line with the load vector that is subject to a large number of load cycles at high stress every time a roller passes by.

The bearing roller/race contact is hertzian, and as noted, the most common failure is a race surface spall initiated by a sub-surface shear fracture. One very important consideration with carburized rolling element bearing race surfaces is ensuring the case depth after finish grinding is adequate for the hertzian contact conditions. The depth of max shear stress from hertzian contact must lie well within the very high strength carburized case. If the depth of max shear stress lies at the case/core transition or even within the core, a sub-surface shear fracture initiated spall will occur fairly quickly.

There are also a couple issues that can produce a local area of reduced strength in the gear/race material. One is excessive heating of the material during grinding operations that causes local de-tempering. This is fairly easy to do if the grinding is not performed carefully, since the gear material (9315) is tempered at a fairly low temperature (typically <400degF). Of course, there are normally NDI procedures used after finish grinding to check for this problem. Another potential cause of de-temper is excessive local heating of the gear/race material during operation, due to lack of cooling oil flow, or abnormal function of the bearing such as skidding/sliding.

One reason the inner race and rollers of this bearing were made from M50 is that M50 can operate at much higher temperatures (>500degF) without loss of strength/hardness.. There are some new carburizing gear alloys that have recently become available, such as C64, that can operate at very high temperatures (~900degF) without significant loss of strength/hardness. So we'll likely see greater use of these alloys in new gearbox designs.
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Old 21st Jun 2016, 16:35
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The KFC S76 happened here in Brazil...
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Old 21st Jun 2016, 16:48
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Am I understanding right???? Are they considering the transportation as potential factor?
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Old 21st Jun 2016, 20:02
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Originally Posted by cteneto139 View Post
Am I understanding right???? Are they considering the transportation as potential factor?
Yes but only in so much as they have to understand the entire history of the MGB (among many other things). Transport damage is one of many factors they have to consider - standard procedure I would imagine
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Old 22nd Jun 2016, 06:28
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This info on steels, bearings, etc. is excellent.

But isn't it time for a reality check? It seems clear that the combined talent of the design authority, crash investigators, regulators and operator maintenance staff is unable to fully understand or control what is happening to these aircraft. Whatever it is that we know about materials, operating conditions, wear rates, etc, that knowledge is inadequate in this case.

I know that the aviation sector is generally run with great maturity and care by all those involved, and takes pride in knowing all aspects of aircraft behaviour and how to operate safely.

However for this type they imposed a regime of intensive examination after every flight, after a few other accidents occurred. To me that sounds like "we don't really, truly know what's happening, so we'll sample the gearbox condition at every opportunity and maybe find out". In retrospect that's probably going to look like a bad idea.

The most important part of the investigation will be its focus on how it came to this.
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Old 22nd Jun 2016, 10:38
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Msbbarratt: "We don't really, truly know what's happening, so we'll sample the gearbox condition at every opportunity and maybe find out"
I don't see this as a very constructive comment and feel that this is fundamentally a miss-understanding of aviation and expectations of performance.
While any fatality is a horrendous occurrence I feel that it is important to understand that there is a lot about aviation we, as a species, don't understand.

Firstly, there are no professionals out there who look to put people in danger!
As an industry we are dealing with a truly amazing level of variables and we, as an industry, don't know what we don't know!
More importantly there are whole rafts of everyday aviation based upon best practice, consistency of outcome and historical approaches, we do this because of the law of unintended consequence, specifically if we stray of the established line we will be less certain of the outcome. Unfortunately this means that as an industry we do tend to have to learn a lot of 'new' things as a result of accidents, ideally not fatal ones and as an industry we have what must be the ultimate in hazards that we manage on a daily basis, gravity and it is an unforgiving mistress. So when something new, novel or different occurs we are all very lucky when we learn it without loss of life.

As an industry we model, plan, design, re-design, review those plans and re-designs, train, retrain and retrain some more until we understand our machines as much as is possible. We build test models and prove that they work as best as we can, to the limit of technical knowledge and skill. We then manufacture them and expose them to thermal ranges from -50 degrees to +50 degrees, stress loads that would rip a house apart, moisture, people and a truly vast number of other variables. As a result we can't, hand on heart tell you what we will find in a few years’ time when we pull it apart for a big maintenance check, we can't tell you what will happen if several really obscure failure modes happen at the same time, we can’t guarantee a zero accident rate.

It is this fundamental inability to control the aviation operating environment in its totality that separates us from other disciplines. I have talked with mechanical engineers who are critical of the aviation industry and its perceived inability to produce gearboxes that never fail. Ask them to explain how they would manufacture a gearbox that is a primary part of a large, variable load path, is subjected to a horrible amount of vibration and doesn’t weigh as much as a truck. And if it goes wrong could kill a dozen plus people, I haven't heard a credible answer yet.
Saying aviation shouldn’t have accidents is not an ideology that anyone in aviation would argue with, it is however easier said than done.

On a positive note, we have embraced our deficiencies and compensate. Due to our high levels of regulation and consistency of approach, once a failure mode is established the whole industry has the ability to enact the corrective action to reduce the possibility of re-occurrence in a very short space of time. But we do this one failure mode discovery at a time.

But is the aviation industry every going to be able say 'X will never have an accident'. No, and it is this that needs to be understood. We can only reduce the probability of one happening.

Last edited by Miles Gustaph; 22nd Jun 2016 at 10:53.
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Old 22nd Jun 2016, 12:18
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Originally Posted by Miles Gustaph View Post
It is this fundamental inability to control the aviation operating environment in its totality that separates us from other disciplines. I have talked with mechanical engineers who are critical of the aviation industry and its perceived inability to produce gearboxes that never fail. Ask them to explain how they would manufacture a gearbox that is a primary part of a large, variable load path, is subjected to a horrible amount of vibration and doesn’t weigh as much as a truck. And if it goes wrong could kill a dozen plus people, I haven't heard a credible answer yet.
Miles, well said, but I'll add a different angle to this point. Even fixed wing design and production folks often don't "get" how helicopter design and production and operation is orders of magnitude more complex/difficult. (Per pound, it's about 10x as expensive to create a helicopter than a fixed wing aircraft). The continual interaction in all three axes of loads, on the flight controlling surfaces and on the airframe, as well as the vibration problem to sort out, complicates achieving MTBF goals.

Wear and fatigue are facts of life. Where the commercial market finds itself bumping into sharp edges is in seeking to increase MTBF for major dynamic components while also chasing that weight reduction problem. If you have to change the main transmission every 500 hours, as opposed to every 2500 hours, has a big impact on how well your transport service operates, or if it even survives. Designing and maintaining in good health the critical systems is a never ending effort.

The other matter, and IMO the critical matter in a mature aircraft, is how the system is set up to warn the operator of impending failure or in many ways present wear and tear as "graceful degradation." What makes this accident such a shock to the system is the few seconds of warning and then it all came apart on a fairly mature aircraft design.

Monitoring of dynamic component health and usable signs of things wearing out: that's were accident prevention may learn something valuable when this accident is fully understood.
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Old 22nd Jun 2016, 15:57
  #1378 (permalink)  
 
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Devil learning effects

Just have a look in Latest Engine Problem May Impact Airbus A400M Delivery Plans and in http://www.pprune.org/9416209-post174.html

Last edited by AW009; 23rd Jun 2016 at 00:44.
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Old 22nd Jun 2016, 16:17
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With warmest regards, neither of those is on topic for this thread, which addresses a fatal helicopter accident. The A400M's engine problems don't seem to be related, as the 225's engines do not appear to be a culprit in this accident. (As far as we know today) . Your other post has to do with automobile transmissions, in the "end of 225" thread.
I believe you have already gone on record in this thread with a dislike for AB and AB's helicopter division. We get it. ( I have no dog in that fight ...)
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Old 22nd Jun 2016, 16:46
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@all: Arn't we relatively shure that the upper epicyclic module of H225 / AS332L2 MRGB with only 8 planet wheels might be one of the culprit?

That's really not a question of dislike or like of ABH, that's a question of design, layout, choose of material, metallurgy, tempering, finishing and tribology in transmisson technology and in general, no matter where the gearbox is installed. Gearboxes always are characterized by 3-dimensional dynamic and static loads.

Sorry, I have flown in my life too many AB/EC/AS/SA & SE helicopter types and have a little bit to much grey cells instead of being put in the corner by a submitted general aversion to some very good HC and to be accosted by shallow arguments of a lad who obviously missed a good upbringing.

Argumentes like "my wife doesn't like the Mini, … Cars only function in two dimensions unless one is stunt driving for a movie. Their altitude design spec is 0 feet AGL, … The A400M's engine problems don't seem to be related, as the 225's engines do not appear to be a culprit in this accident, … your other post has to do with automobile transmissions“ and I don't "fight with dogs", are unimpeachable evidence for a smart-aleck phrase monger, who didn’t latch on to the point that MRGB of H225 and PGB of TP400-D6/A400M are of the very same manufacturer and therefore a close technical fit is given.

Blustering to be an engineer (in profile) provocates not only the question „which sort of engineer?“ (*),but also due to beforesaid highly intelligent statements the question „Is the vaguest notion of gear technology and of mechanical engineering given?
--------------------------------------------
(*) Many pilots are also „engineer“ and commonly with an university degree.



Last edited by AW009; 23rd Jun 2016 at 08:27. Reason: wording
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