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EC225 crash near Bergen, Norway April 2016

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EC225 crash near Bergen, Norway April 2016

Old 29th Jun 2016, 13:11
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Hydrogen embrittlement is certainly a very important concern in all metallic rotorcraft components. Halogenated cutting fluids can cause this issue while machining metallic parts and there is nearly no way to test for this effect in a fatigue lab setting.

Some OEMs learned this the hard way

All that said, the fracture pattern from hydrogen embrittlement is fairly telltale, and it does not appear to be the case on these gears.
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Old 29th Jun 2016, 14:32
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@Lonewolf_50:

My question was purely hypothetical as you know, and proposed on the observation that hundreds of these planet gears have been manufactured and not failed catastrophically in the same application. But this one did fail, so why? Transit damage might be one answer and is being further investigated but other answers may also exist. Fatigue obviously has propagated a crack but it would need a source, which could be quite small, probably outside the residual compressive stress zone from the carburising, and if so might not cause sufficient spalling at an early enough stage to be detected. That seems to be what AIBN are reporting.

I would not rush to the conclusion that there exists a ‘bad batch’ of components and with the affected type(s) currently grounded any worrying would be commercial and not safety based. The failed bevel gear shaft of G-REDW was initially put down to a bad batch with roughly drilled holes and we know the rest of that story.

The manufacture of these safety critical gears would be subject to many controls and inspections. Baking would most likely be among them and that process would be tightly controlled (possibly empirically), but I don’t know how you could actually determine the removal of hydrogen had been homogeneous and irreversible. Is baking done only after carburising or also after etching for final polishing?

There are so many factors that need to be achieved in a complex manufacturing system to make this gearbox safe. The holes in the Swiss cheese do not necessarily need to be concentric, just the slightest overlap may do it, like a build up of tolerances. That the AS332 up to L1 variant has never suffered this fate once in 4m hours says a lot for the system, but does the L2/EC225 just occasionally nudge one cheese a little too far?
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Old 29th Jun 2016, 14:59
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Originally Posted by SansAnhedral
Hydrogen embrittlement is certainly a very important concern in all metallic rotorcraft components. Halogenated cutting fluids can cause this issue while machining metallic parts and there is nearly no way to test for this effect in a fatigue lab setting.

Some OEMs learned this the hard way

All that said, the fracture pattern from hydrogen embrittlement is fairly telltale, and it does not appear to be the case on these gears.
Thanks for your feedback. My thoughts (in my hypothetical scenario) were that the hydrogen embrittlement would not be across the bulk of the fracture surface but just at the source. Would the interstitial carbon restrict diffusion of hydrogen out through the hardened raceway?
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Old 29th Jun 2016, 16:09
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@Miles Gustav,

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.

...

Firstly, there are no professionals out there who look to put people in danger!
Well, if a constructive comment helps, the regulators should always ask themselves whether the conditions they impose are sensible. Mandating a deep technical inspection of a critical component after every flight is at tacit admission of a lack of faith in that component, an acknowledgment that within the timescale of a flight it might fail. Regulators should in future apply poppycock filters to their advice.

As for professionals deliberately putting people in danger, we'll of course they don't do that. But circumstances (commercial pressures, can do attitudes, etc) have a nasty way of resulting in a conspiracy of optimism, and it's the regulator's job to spot those and put a stop to them. I think what's happened here is that the regulator has not acknowledged that they have the power to prevent aviation as well as the duty to enable it.
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Old 29th Jun 2016, 17:17
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"the AS332 up to L1 variant has never suffered this fate once in 4m hours"

doesn't go far in establishing 1x10^-9 scale safety
#provesnothing #

"the regulators should always ask themselves whether the conditions they impose are sensible."
#haha
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Old 29th Jun 2016, 17:57
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Originally Posted by Nadar
I think it's pretty clear. You must see this sentence in context of the previous:
They are saying that there were no warning signs before the accident neither from the chip detectors or HUMS for LN-OJF. They see this as a "certification flaw" as the certification was based on the premise that a fatigue crack would produce a warning from at least one of these systems.


What they are saying is that there were no warnings from the chip detectors or HUMS. That implies that they have checked the previous HUMS readouts. If I recall correcly, the HUMS data for the accident flight determined to be lost since these data aren't stored (but kept in RAM only which loose all information when it looses power).

The reason the problems the previous days aren't mentioned must be that it's seen as irrelevant to this accident. This is a preliminary report with just the current status of the major investigation areas.

When it comes to the chip detectors, they say this:
That seems pretty clear to me.


This is not what I read in the report. Quite the opposite, they say that the design philosophy assume that a spalling will not develop into a fracture because of the material properties, but that this accident indicates that this assumption is wrong:

This is clearly stated in the report. It was repaired/modified/inspected and approved for use by AH after being sent there as a result of the road accident in Australia were the gearbox was damaged.


Not as I read it. I think it's perfectly clear in the areas being addressed. That all remote, unlikely connections aren't included is to be expected as it is preliminary and the investigation is still ongoing.

Thanks for your detailed answer and citations!

If there where no HUMS readouts, how can they conclude it is not a sufficient way of monitoring?

In G-REDL there was indeed seen several HUMS readouts,
some of them failed to cause the attention of the engineer because
the datacard was incorrectly loaded.

So no reading, or not an abnormal reading, is not definitely answered yet?

When it comes to no chip detection, if that is the case it is indeed a strange condition.
For the 300 flight hours done on LN-OJF the gearbox had, it canīt be argued against.
But what about the 1080 hours before?
Do we know already that the gearbox had 1300 hours without any flaws at all? No.

We also know that the individual gears are not due to visual inspection before 2000 hours of operation.

Also, if they claim that it have proven against the theory that spalling does not induce any chips, it could be because the gear had already induced spalling and left the surface in such a shape that a crack would be induced.
It was then shipped out to LN-OJF

We can clearly see that the gears on the pictures have spalling.
especially the bottom left.
from the gear starts to get light pitting, which may cause hairthin
cracks in the surface, it will also soon start to spall.
Canīt say how long, but with normal lubrication and temperature it should have at least been ok for 300 hours.

So again, I donīt feel that the report brings a clear view to several
relevant aspects that should have been cleared out until now.

For sure there maybe reasons to hold off the information, until
further investigations are done, but still if the conclusion is that the gear is without warning cracked causing the gearbox total seizure. I don`t believe it.
And person with experience in risk assessement and engineering should not believe it either.

Concentric: I agree with your theory, it could be the underlying reason it happened.
And that could also be the reason they changed the material on the bevel gear from 16NCD13 to 32CDV13 to allow nitrating.
Especially in relation to the crack-prone area near the welding.
However, that should have been a failure mode, that they are perfectly aware of. Seeing the process a gear goes through during manufacturing,
it is not very likely to be a problem with only 1 gear.
And even if it was, the way we can see the surface pitting on the other gears, and also cracks due to deformation, if not occured in aspects of the gear development and certification, that process is for sure useless
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Old 29th Jun 2016, 21:32
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Originally Posted by turboshafts
Thanks for your detailed answer and citations!

If there where no HUMS readouts, how can they conclude it is not a sufficient way of monitoring?

In G-REDL there was indeed seen several HUMS readouts,
some of them failed to cause the attention of the engineer because
the datacard was incorrectly loaded.

So no reading, or not an abnormal reading, is not definitely answered yet?

When it comes to no chip detection, if that is the case it is indeed a strange condition.
For the 300 flight hours done on LN-OJF the gearbox had, it canīt be argued against.
But what about the 1080 hours before?
Do we know already that the gearbox had 1300 hours without any flaws at all? No.

We also know that the individual gears are not due to visual inspection before 2000 hours of operation.

Also, if they claim that it have proven against the theory that spalling does not induce any chips, it could be because the gear had already induced spalling and left the surface in such a shape that a crack would be induced.
It was then shipped out to LN-OJF

We can clearly see that the gears on the pictures have spalling.
especially the bottom left.
from the gear starts to get light pitting, which may cause hairthin
cracks in the surface, it will also soon start to spall.
Canīt say how long, but with normal lubrication and temperature it should have at least been ok for 300 hours.

So again, I donīt feel that the report brings a clear view to several
relevant aspects that should have been cleared out until now.

For sure there maybe reasons to hold off the information, until
further investigations are done, but still if the conclusion is that the gear is without warning cracked causing the gearbox total seizure. I don`t believe it.
And person with experience in risk assessement and engineering should not believe it either.

Concentric: I agree with your theory, it could be the underlying reason it happened.
And that could also be the reason they changed the material on the bevel gear from 16NCD13 to 32CDV13 to allow nitrating.
Especially in relation to the crack-prone area near the welding.
However, that should have been a failure mode, that they are perfectly aware of. Seeing the process a gear goes through during manufacturing,
it is not very likely to be a problem with only 1 gear.
And even if it was, the way we can see the surface pitting on the other gears, and also cracks due to deformation, if not occured in aspects of the gear development and certification, that process is for sure useless
Turbo shafts

Just a couple of points

REDL s HUMS was analysed by both the AAIB, EC and the system manufacturer, all of who concluded that the CIs observed gave no prediction of catastrophic failure of the 2nd stage planet gear. There were also issues with the epicyclic chip detection system which called its correct function into question

We may not know the history of the MGBs 1300 hours since overhaul, but you can be absolutely sure that the AIBN and AH do, it probably hasn't been reported because there is nothing of significance.

You can't predict how long the gear should run as the complete gear is not available to fully quantify the damage to the raceway as (I assume) you do not know enough about the gears operational load cycle, effect esn of lubrication, rate of damage progression etc.

SPalling Is the progressive failure of a bearing surface due to rolling contact fatigue, surface damage continually increases, as does particle production. Carburisation of the surface introduces comressive stresses in the bearing surface, to a depth of about 2mm. Spalling in this area will result in particle release from the raceway but not crack progression into the body of the gear/outer race. THence the use of MCDs as the primary method of detecting epicyclic deterioration (HUMS doesn't work well due to the constant movement of the planets relative to the accelerometers).

The 225 and 332 have plenty of history of detecting spalling of planet gears without catastrophic failure, therefore it is possible that this failure is not due to normal spalling. If a crack forms at the interface of the carburised layer, or in the body of the gear it will only be detectable when it breaches a surface and results in particles being released. This was what the AAIB theorised with REDL and may be a factor in the accident.

Re the jamming of the gearbox, the epicyclic ring gear damage suggests that something was entrained between a planet gear and the ring gear which burst the case, If the epicyclic had jammed the ring gear and planets would have had significant stripping of gear teeth. There was no suggestion that the REDL MGB jammed
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Old 30th Jun 2016, 03:57
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turboshafts:

If there where no HUMS readouts, how can they conclude it is not a sufficient way of monitoring?
Prior to the accident flight the Airbus HUMS system was serviceable and being analysed IAW company and customer procedures, AH guidance and regulation.

AIBN are indicating that HUMS did not detect any anomalies, which is the reason for their statement "HUMS appears unable to identify symptoms of such degradation in the epicyclic module."

Where did you get the idea from that there were no HUMS readouts?
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Old 30th Jun 2016, 04:39
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Certainly going to be interesting to see the root origin of the failure.

Reminded of the fan disc failure in a DC-10. Was found the disc had been machined from a piece that came from the near end of a billet, which was found to contain inclusions. Fix, discard the ends of the billet.

In the early 60's worked in a shop manufacturing steam turbines for ships. For the gear cutting shop battery back up was supplied to the cutters/grinders. Once a cut/grind on a tooth was begun it was not permitted to be interrupted, for it introduced a point of future failure.
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Old 30th Jun 2016, 10:45
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AH EASB and SIN issued yesterday

For those who have not seen it:

EASB - MAIN ROTOR DRIVE - Epicyclic module Replacement of the epicyclic module second stage planet gears:

Summary:
For helicopters which are not subject to EASA Airworthiness Directive No. 2016-0104-E, this ALERT SERVICE BULLETIN requests that you identify the P/Nos. of the epicyclic module second stage planet gears and replace the module if its planet gears have the P/Nos. concerned.

Compliance:
Airbus Helicopters renders compliance with this ALERT SERVICE BULLETIN mandatory.
...

SIN No. 3053-S-00- Update on EC225LP accident in Norway dated April 29th:

-Deleted-

Last edited by GenuineHoverBug; 1st Jul 2016 at 09:40. Reason: SIN deleted by AH
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Old 30th Jun 2016, 15:23
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So the Rev B planet gear bearing is expected not to show the mystery non-spalling fatigue crack growth that should have never occurred in the original version.

Metallurgically, I suspect there is little to no difference between them. If it was a material-driven failure, the lower loads observed may serve only to delay the onset of this undetectable failure.

I do very much wonder exactly what "In service experience shows enhanced reliability" means
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Old 30th Jun 2016, 23:17
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Originally Posted by Apate
turboshafts:
Where did you get the idea from that there were no HUMS readouts?
Originally Posted by nadar
If I recall correcly, the HUMS data for the accident flight determined to be lost since these data aren't stored (but kept in RAM only which loose all information when it looses power)
This is what Nadar wrote. I asked a question, wether it is clearly known if there are no HUMS readouts, or no abnormal readouts.

Originally Posted by n305fa
You can't predict how long the gear should run as the complete gear is not available to fully quantify the damage to the raceway as (I assume) you do not know enough about the gears operational load cycle, effect esn of lubrication, rate of damage progression etc.
Thatīs is why there is an immense amount of durability testing of components.
I would tend to agree with you if it was a completely new design, but this design has operated for years with 1000's flying hours. If you look at the failure modes and
wear prior to failure modes, you are able to accurately predict the life of components, adding a safety factor to it.

Originally Posted by n305fa
THence the use of MCDs as the primary method of detecting epicyclic deterioration (HUMS doesn't work well due to the constant movement of the planets relative to the accelerometers).
The accelerometers are able to measure vibration frequency. As I understood
monitoring a given treshold for the frequency should tell you if it is abnormal operation. It might be the case, exceeding my knowledge, if the G-REDL
and LN-OJF has different positioning of the accelerometers?

Originally Posted by n305fa
The 225 and 332 have plenty of history of detecting spalling of planet gears without catastrophic failure, therefore it is possible that this failure is not due to normal spalling. If a crack forms at the interface of the carburised layer, or in the body of the gear it will only be detectable when it breaches a surface and results in particles being released. This was what the AAIB theorised with REDL and may be a factor in the accident.
Read the REDL report closely. The length of the spalling close to where the crack started is very similar on the two gears

Originally Posted by n305fa
Re the jamming of the gearbox, the epicyclic ring gear damage suggests that something was entrained between a planet gear and the ring gear which burst the case, If the epicyclic had jammed the ring gear and planets would have had significant stripping of gear teeth. There was no suggestion that the REDL MGB jammed
If you look at the outer ring gear of the secondary stage planet,
you will clearly see that on G-REDL it is broken on the same place as on LN-OJF.
What is different as I see it is the fair amount of teeth crushing on the sun gear of the LN-OJF.

In both cases the outer ring is cracked open.
In both cases 1 gear is cracked into several pieces.

Such a failure is fairly stochastic. If there was a piece of the failed gear that
broke and jammed between itself and the sun gear on LN-OJF which cracked
open the case and on G-REDL it jammed between itself and the outer ring gear.
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Old 1st Jul 2016, 00:27
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Originally Posted by Concentric
I am just going to throw this speculative idea up in the air for consideration. Is it possible there may have been hydrogen embrittlement of the gear material? I noted from AAIB report 2-2011 that the carburised gear raceway is chemically etched before final polishing. I am not a materials specialist but I wonder if a combination of materials susceptibility, environment and stress have combined with the presence of diffused hydrogen trapped underneath the carburised layer to initiate a defect and cyclic stress has done the rest?
Carburized surfaces of the planet gear that get finish ground after heat treat are given a surface temper etch inspection procedure (AMS 2649 or similar) to check for manufacturing damage such as overheating (re-hardening or over-tempering) caused by abusive grinding, or to detect areas of local discontinuous carburization. The surface temper etch inspection is performed after finish grinding but prior to any honing/lapping/polishing operations on the surface. The etchant used is typically a 3-5% nitric acid solution with an anti-smut additive containing hydrochloric acid. Immediately after etching the parts are cleaned with an alkaline solution followed by hot water rinse. After inspection the parts are given hydrogen embrittlement relief baking (AMS 2759/9 or similar). The time between etching and baking should be less than 24 hours.

So the short answer to your question is that it is possible there may have been some hydrogen embrittlement issues from the temper etch inspection, but only if the procedures were not performed correctly.
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Old 1st Jul 2016, 07:19
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Originally Posted by turboshafts
This is what Nadar wrote. I asked a question, wether it is clearly known if there are no HUMS readouts, or no abnormal readouts.



Thatīs is why there is an immense amount of durability testing of components.
I would tend to agree with you if it was a completely new design, but this design has operated for years with 1000's flying hours. If you look at the failure modes and
wear prior to failure modes, you are able to accurately predict the life of components, adding a safety factor to it.



The accelerometers are able to measure vibration frequency. As I understood
monitoring a given treshold for the frequency should tell you if it is abnormal operation. It might be the case, exceeding my knowledge, if the G-REDL
and LN-OJF has different positioning of the accelerometers?



Read the REDL report closely. The length of the spalling close to where the crack started is very similar on the two gears


If you look at the outer ring gear of the secondary stage planet,
you will clearly see that on G-REDL it is broken on the same place as on LN-OJF.
What is different as I see it is the fair amount of teeth crushing on the sun gear of the LN-OJF.

In both cases the outer ring is cracked open.
In both cases 1 gear is cracked into several pieces.

Such a failure is fairly stochastic. If there was a piece of the failed gear that
broke and jammed between itself and the sun gear on LN-OJF which cracked
open the case and on G-REDL it jammed between itself and the outer ring gear.
Turbo shafts

Re the prediction of remaining gear life with spalling present, I was referring to your assertion that the gear should have lasted 300 hours, not the ability of the design team to predict residual life based on certification testing.

The use of case mounted accelerometers to measure deterioration in epicyclic planet gears is close to impossible, the planet system is inherently noisy, any defect in a planet race and hence the signature it produces is constantly changing position in relation to a fixed accelerometer, the planet rotates around the ring gear and it is also rotating about its mounting point. HEnce current research programs to try to develop a monitoring system that can defect epicyclic deterioration.

The REDL report clearly states that no spalling was present on the bits of the failed gear recovered, if there was spalling it would have been on the 25% of the gear not recovered. The only evidence of spalling was the particle recovered on 25 March and the evidence it gave.

Jamming means that a section of the gearbox monentarily stopped rotating, If you read the REDL report there is no suggestion of jamming and the latest AH report into OJF says that there is no evidence of jamming. The REDL report talks about debris being "entrained" between the remaining planets and ring gear.
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Old 1st Jul 2016, 09:13
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Off Topic – The fascinating thing in the ‘Pink Panther’ series of films is that the culprit is invariably caught, in the end.

On Topic – So now there is consensus that (probably) suspension bars were pinned. Connecting bolts were (probably) torqued. Now AH EASB mandates removal of all 2nd stage planetary gears of one design (there were 2 – who knew?).

The EASB does not stipulate what is to be done with the removed planetary gears, does it? Send them back to AH? Or forward to AIBN for scanning and testing to see if they yield any ‘gems’ or hold any clues so sought after?
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Old 1st Jul 2016, 11:01
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@riff_raff:

Thank you for that enlightening explanation of the carburizing and etching processes. It illustrates well the point I mentioned about complex processes required to achieve a safe finished product, and that is just one such process.

Who would imagine that to control dimensions of the bearing raceway and hardened layer that so many chemical and thermal processes would also be required, some of which to counteract potentially harmful effects of a preceding process?
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Old 1st Jul 2016, 11:24
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Concentric
You are spot on. I was chilled to read the number of critical processes that are required. So many opportunities for error.

That process has to happen how many times (eight of those gears) and as you say thats before you mention all the bearings, raceways and other processes.

I dare say if you add up all the processes, from ensuring that no one has a biro in their pocket at the foundary (non melting ball can cause component failure) to design, to regulatory errors, to assembly for all components in series.

Far from saying that these things are safe it is an absolute miracle that they are as safe as they are, showing what an amazingly reliable job all the characters involed are performing despite the whole exercise being so inherently dangerous.

2 crew performing the softest of flying tasks goes some way to

Serial risk of 1000 processes, maybe 10000? Still hoping to acheive 1x10^-9? daft unrealistic nonsense
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Old 1st Jul 2016, 11:55
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Originally Posted by AnFI
Concentric
You are spot on. I was chilled to read the number of critical processes that are required. So many opportunities for error.

...

Serial risk of 1000 processes, maybe 10000? Still hoping to acheive 1x10^-9? daft unrealistic nonsense
Concentric/AnFi

Given the complexity of the manufacturing process and the many steps necessary to ensure that a pristine product performs safely, it struck me as odd that a unit that had been damaged in a road accident was repaired and returned to service.

Now I know we have no idea whether it was a contributing factor and clearly we can't just throw away expensive kit at the drop of a hat, but safety critical components with very limited opportunities for redundancy such as an MGB? If such a thing receives a shock loading outside of its normal operational environment which causes damage which needs to be repaired, surely it would be prudent to write it off as an insurance loss. I am not familiar with the overhaul that was carried out but it would be close to impossible to ensure the safety standards inherent in the manufacturing process or a routine overhaul from normal wear and tear.
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Old 1st Jul 2016, 12:13
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This is my first post - thanks to everyone over the 73 pages for an informative read.

I'm interested in the latest revelation about the two types of second stage gears - it raises these questions:

1. Did the two types exist prior to the REDL accident and if so why was analysis of the differences not a feature of the AAIB investigation? Which type was REDL using?

2. If the second type post-dated the REDL accident, why was it introduced, and if it was a quiet response to internal AH findings with respect to REDL, why was the immediate retrofit of the second type not mandated?
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Old 1st Jul 2016, 14:07
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Originally Posted by Brucci2000
This is my first post - thanks to everyone over the 73 pages for an informative read.

I'm interested in the latest revelation about the two types of second stage gears - it raises these questions:

1. Did the two types exist prior to the REDL accident and if so why was analysis of the differences not a feature of the AAIB investigation? Which type was REDL using?

2. If the second type post-dated the REDL accident, why was it introduced, and if it was a quiet response to internal AH findings with respect to REDL, why was the immediate retrofit of the second type not mandated?
You ask a couple of very good questions. I think it raises further questions too.

I do not mean this as criticism of the AAIB but if you look at their report on G-REDL, section 1.18.2 it states that:

“Data provided by the helicopter manufacturer indicated that between 2001 and 2009 there were nine recorded cases of planet gear spalling on the AS332 L2 (see Table 2)”

and then goes on to add that:

“The information provided by the manufacturer regarding the number of planet gear rejections due to spalling was incomplete. During the investigation anecdotal evidence was provided that indicated that overhaul facilities disposed of rejected gears without routing them for investigation”.

As the EC225 and AS332L2 are widely reported to share the same epicyclic gearbox module, isn’t it surprising that only data on L2 gears with spalling was considered relevant? Was the Type 2 gear introduced for the EC225 to use exclusively, therefore considered to not be relevant to the 2009 investigation? Something doesn’t add up here.
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