225 cleared to fly in UK & Norway

[SASless]

The reality of what we are discussing here......we have had two occurrences like what the video of the shed rotor system slowly spinning on its way to earth and rising column of black smoke from the wreckage in the Bergen video shows.

Yet, today, there is no definitive explanation of exactly what caused the failures....we know pretty well HOW they happened...but not WHY they happened.

It might be that I missed the explanation in all of the posts in this thread that did clearly lay out the case of what actually caused the failures.

Lots of jabber about how to better monitor the MGB's for potential failures of the same sort, modifications of inspection schedules and component replacement, lots of testing of modified or different parts and components.....but no definitive statement of the exact root cause of the two failures.

Did I miss or over look it?

Am I the only one?

If that exact cause is not genuinely identified....should passengers and crew be flying in the aircraft?

[Fareastdriver]

If that exact cause is not genuinely identified....

The cause is that the orbital gear cannot contain a disassembling planetary gear. Containment is not new; I've flown helicopters with a containment ring around the turbines to stop them destroying the other engine and hydraulics.

The problem with the existing gearbox is that there is only one failure path; that goes and you are dead. It is IMHO impossible to guarantee that any gear assembly will last it's planned overhaul life however long you test it.

The 330 and 332 got away with it because the design of the gearbox had sufficient reserves. You pile in more torque and strain on affectively the same bottle then something has to go. I cannot see any other way than a complete redesign with multiple failure paths within the gearbox.

History may prove that this way will be the standard in the future.

[Geoffersincornwall]

My engineering colleagues have explained that this type of gear wheel is made of a steel that has rather a severe crack propagation characteristic and as such the failure mode is severe and is exacerbated by the lack of debris created during the failure process. This lack of debris leaves the airworthiness of the component design open to question as there is no effective way of preventing ultimate failure once cracks have appeared.

You may well ask how we progress from here. It rather looks as if the edge of the carpet has been raised and the offending question swept under it. A large notice stating DO NOT GO HERE has been stuck in the ground by the EASA lawyers who appear to have wagged at large and menacing finger at the brave chaps and chapesses in the UK and Norwegian CAA.

G

[SASless]

As I read it....Geoffers and Fareastdriver have arrived at two different causes already.

Both very experienced Pilots and professionals and each has carefully considered a lot of information about this.

We have had a lot of good discussion by some very sharp folks....but yet there is ambiguity when it comes to the actual mechanism of these two failures.....which I am not sure even AH really knows what is causing the failures.

Which is it.....wrong material that does not afford reasonable detection of a pending failure as Geoffer's Engineer buddies are saying or is it the Orbital Gear cannot contain a failing Planetary Gear as suggested by Fareastdriver?.

Or....is it something altogether different?

Is this apparent contradiction indicative of the problem confronting the Helicopter Industry, Operators, and Customers?

AH and the Authorities in conjunction with the Accident Investigators seem not to be able to explain this situation to the degree there is conformity of thought as to the actual root cause?

Until there is a definitive....proven....without any doubt....explanation....should the aircraft be certified by any authority as being safe for flight in general air transport operations carrying passengers?

[Twist & Shout]

Until there is a definitive....proven....without any doubt....explanation....should the aircraft be certified by any authority as being safe for flight in general air transport operations carrying passengers?

No.

It should be in the experimental category. With associated warnings and restrictions.

[Concentric]

Originally Posted by SASless

As I read it....Geoffers and Fareastdriver have arrived at two different causes already.

Their individual explanations are not mutually exclusive but in fact describe different phases in the failure sequence. That sequence is something of a chain reaction that ought to have been prevented at any of a number of places. It wasn’t and that is a failure of the designers to consider the unthinkable.

[SASless]

Concentric,

Are you agreeing with my statement that the definitive cause has not been fully identified and all EVERYONE (including AH Engineers) is discussing are symptoms, responses, and assumptions resulting from that lack of certainty about what kicks off that sequence of events?

[Pittsextra]

We are running the risk of accepting the nonsense of ambiguous communication. Ever thought that it exists so as not to generate an instant set of questions from the final report?

[riff_raff]

Originally Posted by Geoffersincornwall

My engineering colleagues have explained that this type of gear wheel is made of a steel that has rather a severe crack propagation characteristic and as such the failure mode is severe...

Geoffers - The planet gears in question were made from 16NCD13 double vacuum melt steel (AMS 6263) which is very high quality. You can find some data on the alloy here. It has excellent fracture toughness and was a good material choice for this application.

Having said that, there is an interesting situation described in the AIBN report issued on April 28, 2017 regarding the fracture analysis work used to certify the gearbox design. If you read the report beginning with section 1.17.6.3, there is some discussion of revisions to EASA CS 29.571 Fatigue tolerance evaluation of metallic structures, and how this relates to the EC225 MRGB.

If you look at the CT scan image of the failed planet gear outer bearing race section shown in fig. 38 of the report, you'll see that there is a sub-surface fracture (highlighted in red) passing thru several spall locations just ahead of the final rim fracture. This sub-surface fracture runs at a shallow depth (~.012") and lies entirely within the carburized case thickness (~.050").

Then consider this part of the report:
"1.17.6.4 Assessment of the MGB against CS 29.571 - The AAIB investigation into the G-REDL accident found that the phenomenon of crack formation within the carburized layer of the outer planet gear race had not been considered during the design and certification of the AS 332 L2 and EC 225 LP epicyclic reduction gearbox module or the development of the approved maintenance program of the MGB.

The AAIB stated in the G-REDL report (on page 95) that “although the design satisfied the certification requirement in place at the time of certification”, and further “it would appear that if the current requirements [CS 29.571 issued in 2003] were applicable they may not have been met."

The report is saying that the original fracture analysis performed to certify the gearbox design did not take into account the particular type of failure that occurred in this case. This may not provide the complete explanation many are looking for as to why this unfortunate accident happened, but it helps clarify things quite a bit.

[Concentric]

Originally Posted by SASless

Concentric,

Are you agreeing with my statement that the definitive cause has not been fully identified and all EVERYONE (including AH Engineers) is discussing are symptoms, responses, and assumptions resulting from that lack of certainty about what kicks off that sequence of events?

SASless,

Generally speaking, I am in agreement that the definitive cause (or ‘root cause’ as I would prefer to call it) has not yet been satisfactorily explained or reported. The regulators lifting the restrictions before the AIBN has reported seems to me like ‘putting the cart before the horse’. Hopefully since the ‘cart’ has already appeared, the ‘horse’ will not be too far behind. Mee3 claims inside knowledge that the report is coming soon. I can’t tell if that comes from the horse’s mouth though, given the apparent direction of travel.

What the other 2 posters described were both consequences and each was generally correct as such but neither is the absolute root cause. They were not quite adjacent ‘dominos’ in the failure sequence either but the speed with which the whole design came apart illustrates the vulnerability of the 1960’s era architecture of this MGB.

Those consequences illustrate the criticality of ensuring all possible failure modes of the planet gear/bearing are considered in its design and in particular fatigue tolerance, which leads into riff_raff’s excellent post above. One thing I would add to that is that the mechanical properties of the stock material can be considerably different to those of the finished heat treated component.

[Hompy]

Eurocopter Tiger

No sharing of gearbox components with the Tiger is there?

": Absturz Kampfhubschrauber Tiger
Am 26. Juli 2017 gegen 14.20 Uhr Mitteleuropäischer Sommerzeit ist ein Kampfhubschrauber Tiger aus bislang ungeklärter Ursache circa 70 km nordöstlich von Gao in Mali abgestürzt. An Bord des Hubschraubers befanden sich zwei Besatzungsangehörige, die bei dem Absturz ums Leben gekommen sind. "

My two pence: I doubt I will be asked to but I will not be flying the 225 until I am clear the problem that caused the failure has been clearly identified and a redesign has been implemented rendering further failure of the same type impossible. More frequent inspection does not cut it for me.

[Geoffersincornwall]

riff raff - would it be true to say that the fracture toughness you refer to means that the fracture dynamics are somewhat rapid as a result?
G.

[rjsquirrel]

sasless said, "As I read it....Geoffers and Fareastdriver have arrived at two different causes already. Both very experienced Pilots and professionals and each has carefully considered a lot of information about this."

The amazing thing is that the Airbus VP of Sales told a group of Industry experts 8 weeks ago (HeliOffshore Conference) that the cause was shipping damage to the gearbox. He was so certain that he advocated a sensor in shipping containers that could tell of shipping abuse.
So, perhaps, Sasless, we have THREE different causes, and we put them in a hat and select the agreed upon one.

I choose what's behind door #4, unknown. http://www.pprune.org/forums/images/.../eusa_wall.gif

[SASless]

I would suggest Door Four contains something lots of folks do not care to consider....that being the "root cause" is the basic design of the 225 MGB.

It is not capable of containing what should not be a catastrophic failure and fails in that regard due to its inability to transfer the load path to other parts of the gearbox following a failure within an individual module.

We know the existing detection and monitoring systems fail to give enough warning...so perhaps there must be a simpler explanation than what the Boffins are looking for.

The American MGB designs use "thru shafting" as a way of holding all the big pieces together unlike the AH 225 gearbox.

Or am I misunderstanding the various drawings I have been looking at?

Any of you design engineers able to help me with this?

[riff_raff]

Originally Posted by Geoffersincornwall

riff raff - would it be true to say that the fracture toughness you refer to means that the fracture dynamics are somewhat rapid as a result?

Predicting propagation of a fracture within a metal structure is a fairly complicated task, mostly due to the wide range of variables involved. The fracture propagation analysis is intended to demonstrate that a worst case size/shape flaw in the part structure will not produce propagation of a fracture originating from this flaw that results in a catastrophic failure event.

The rate at which a fracture will propagate is greatly influenced by the stress levels at the fracture tip. The fracture will only propagate if the stress level at the crack tip exceed the local tensile strength of the material. The design of planets gears like the example discussed, having a very thin section rim and internal radial support provided by just a few rollers over a limited angular sector of the bearing, is very difficult to get right. If the planet's spherical roller bearing has excessive internal radial clearance, it can produce higher bending stresses in the gear rim than it was designed for.

[Concentric]

Originally Posted by SASless

I would suggest Door Four contains something lots of folks do not care to consider....that being the "root cause" is the basic design of the 225 MGB.

It is not capable of containing what should not be a catastrophic failure and fails in that regard due to its inability to transfer the load path to other parts of the gearbox following a failure within an individual module.

We know the existing detection and monitoring systems fail to give enough warning...so perhaps there must be a simpler explanation than what the Boffins are looking for.

The American MGB designs use "thru shafting" as a way of holding all the big pieces together unlike the AH 225 gearbox.

Or am I misunderstanding the various drawings I have been looking at?

Any of you design engineers able to help me with this?

In Root Cause Analysis, the design features and detection or monitoring system failings you describe above would be regarded as causal factors but not the root cause. Those causal factors certainly affect the final outcome and ought to be identified at the design stage, at least in any new design and in Failure Mode Analysis.

The architectural differences compared to US-made MGBs you describe could make a significant difference in mast retention; however the containment requirements are different to those on a turbine which can be shut down. As you know, it is not enough to just retain the mast you also need rotor rpm and pitch control, probably with some hydraulics. Broken gears and other shrapnel rattling around in a gearbox with no quick escape path could jam or seize the drive or damage blades or dampers. Freewheels on the input side are designed to allow for an MGB to rotate faster than an engine, not slower, and there is no output freewheel allowing the rotor to maintain rpm in an auto with a seized gearbox. The ‘Lack of Lubrication’ thread gives a graphic account of a Huey with a seized MGB.

Having said that, I do think the Blackhawk and possibly the similar S-92A MGB architecture would be less likely to seize if a planet gear fractured, which I am not sure has ever happened.

The root cause is what caused the bearing to initiate a crack in the first instance but then propagate it in a manner that the designers had not expected, i.e. different to spalling, leading to catastrophic fracture.


There is a curious statement on Airbus' accident investigation status page that: "We understand the sequence of events that led to the fatigue fracture, but the exact cause is still being investigated." That might infer they think they know 'how' but do not yet know 'why'.

[albatross]

Originally Posted by rjsquirrel

sasless said, "As I read it....Geoffers and Fareastdriver have arrived at two different causes already. Both very experienced Pilots and professionals and each has carefully considered a lot of information about this."

The amazing thing is that the Airbus VP of Sales told a group of Industry experts 8 weeks ago (HeliOffshore Conference) that the cause was shipping damage to the gearbox. He was so certain that he advocated a sensor in shipping containers that could tell of shipping abuse.
So, perhaps, Sasless, we have THREE different causes, and we put them in a hat and select the agreed upon one.

I choose what's behind door #4, unknown. http://www.pprune.org/forums/images/.../eusa_wall.gif

Something like these? Impact Products | SpotSee

Was not the gearbox returned to Airbus for overhaul after the transport accident in Australia?

Well at least he did not mention their..bad maintainance..re installation "missing pins" accusation.

[albatross]

German helicopter lost rotors before Mali crash - report | World | Reuters

So is there any commonality between the Tiger and EC 225 rotor system/ gearbox? Question was asked in a previous post here but not answered.

[henra]

Originally Posted by albatross

German helicopter lost rotors before Mali crash - report | World | Reuters

So is there any commonality between the Tiger and EC 225 rotor system/ gearbox? Question was asked in a previous post here but not answered.

No.
And the word is the Tiger lost the Rotor blades not the entire head.
Looks more like too high rpm or (negative?) G- Overload.

[albatross]

Thanks for the reply.
I googled but was unable to find an answer.