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Old 11th Apr 2009, 22:07   #1 (permalink)
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Main Gear Boxes and The Grand Lottery

We have had two Main Gear Box failures in the past few weeks that destroyed two aircraft and killed thirty three people causing much sorrow and loss to the families and friends.

Knowing Gann, I have to accept things like this will befall us as we pursue our career in aviation but yet we can question the reasons as to why they happen.

Is it "Fate"....a simple roll of the dice or are we assisted by the system letting us down?

Some questions that came to mind while thinking about this situation....

What does "Extremely remote" mean, and how was it missed by EC? By the DGAC? By Sikorsky? By the FAA?

That gear inside the 332's transmission is a "Primary Structural Element" and as such was certified in the 332L2 and the EC 225 as having a failure probability that was "extremely remote".

That means the probability of failure is less than one in a billion - a fleet of a thousand aircraft flying 1000 hours per year flying for 1000 years should not experience such a failure.

Yet, in spite of Eurocopter's promise and the aircraft certification, the gear broke, suddenly, and with little if any warning.

Clearly they missed it - both Eurocopter and the JAA/Easa, precisely as the oil leakage path on the S92 was missed by Sikorsky and the FAA.

How does one miss such things? Is there a blame? or is this something we have to live with?

Last edited by SASless; 11th Apr 2009 at 22:35. Reason: For clarity
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Old 11th Apr 2009, 22:30   #2 (permalink)
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If youre looking for 100% mechanical reliablity then youre in the wrong job.
It will just be another AD like the other mechanical issues causing fatalities.
Remember when the blades were flying off the A109s and the mast were coming off Bell mediums, when Arriel 5 modules were blowing up on Astars, and Robbie blades had all the inertia of a balsa model. Tail rotor tip caps came off 500s and all pretty much of the LTS101s early history?
Its that sort of thing.
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Old 11th Apr 2009, 22:42   #3 (permalink)
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Jim Beam

Jim, maybe the questions SAS is posing is valid for all those failures too. If we just sit back and accept it as part of the wallpaper of life then where, when and how do we progress.

There are stories about things going wrong at the factory that only come out years later. The saga of how the S76 tail rotor problems were suddenly solved overnight with a piece of kit that must have taken months to develop, the spindle saga, Bells combining gearboxes. Are they being straight with us or what?

When the Bell 412 gearbox (epicyclics) seized a couple of years back they managed an autorotation - why not the S92 and the 332L2?

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Old 11th Apr 2009, 22:43   #4 (permalink)
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Mmm ...

To an extent JB is correct ... we are at the mercy of the mechanical engineering .... BUT is that not the reason to examine the result of incidents and accident in our industry and then IMPROVE the design and reliability of of our machinery .... to allow the improvements of computer design tools to make our machinery better ....

SASless as far as transmissions are concerned ... was not the BA Chinook disaster also related to a transmission failure??.

Perhaps ... now the certifying authorities will revisit the criteria for Public Transport Helicopter transmission design and certification in view of recent events!

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Old 11th Apr 2009, 22:49   #5 (permalink)
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We do not know the precise failure of the 332L2 epicyclic gearing but I have concerns about Sikorsky using titanium studs on a vital part of the MGB oil system, why change from the usual steel studs used for years? Titanium is not very tolerant, the MRH on the Lynx has to be treated with extreme caution, any slip of a spanner causing a ding on the material is a big problem. I suspect that titanium might be similar to stainless steel (SS) in day to day use, we had to be very careful when using SS bolts not to strip the threads and sometimes it couldn't be helped.
With a filter cover that had to opened several times during the tbo of a S92 mgb perhaps the studs were inadvertantly overstressed causing an ultimate failure. We really need to know why the studs failed on the NL S92.
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Old 11th Apr 2009, 23:17   #6 (permalink)
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i think to some extent its a "learn from your mistakes" sort of industry.
A bit like, "wow Kapton wiring" and "..all aircraft will be made of composites in the future.."
You dont know about alot of this stuff until its been in service for a while. But then the 332L has been around for a long time, maybe my argument is only valid for the S92 accident...
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Old 11th Apr 2009, 23:41   #7 (permalink)
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Quote:- "That means the probability of failure is less than one in a billion - a fleet of a thousand aircraft flying 1000 hours per year flying for 1000 years should not experience such a failure."

It does state where in that one billion an incident will occur: it could be during the first 100 hours or equally it could be in the last 100 hours. Who knows? Don't get me wrong; this event must be thoroughly investigated and steps taken to try to make the odds better than one in a billion. As a current EC225 driver, I have a vested interest!

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Old 11th Apr 2009, 23:53   #8 (permalink)
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The BAH Chinook crash was a transmission failure.


You failed to catch the point I was making.

The FAA has a definition of "Extremely Remote" as does the DGAC and CAA and JAA and EASA....just to mention a few of the certifying organizations.

My question is how they apply that definition to critical parts and how they determine when to apply the definition to those parts as compared to requiring documented testing of the items.

Look at the Sikorsky situation with the 92 gearbox.....it runs for hours with almost any amount of oil left in the sump....but only minutes with no fluid. It might have run longer if the aircraft had been flown at a reduced power setting than was used. No one anticipated the oil filter failing and causing such a failure as has occurred twice now.

The EC gearboxes have shown themselves to be subject to an unexpected cracking of gears based upon one known event and probably this latest tragedy. Despite HUMS and top notch maintenance engineering we see this failure occur and we now have to question the risk assessment that was applied to the potential failure of the gear box and its critical components.

I am not suggesting we can achieve perfection although that should be the goal otherwise we should not be involved in aviation to begin with.

What we need to do is assess the thinking behind some of the decisons and promises made by the builder and certifying agencies.

In light of this MGB failure at Bond.....the "Extremely Remote" concept just flew out the window!


You posted as I was editing my post.

I feel you prove my point about the definition....if it can happen in the first hour or the last hour....then what does that say about statistical probability.

If we had two failures on the same type aircraft, on the same operation, on the same day.....would that definition still hold? All we would have to do is to have the fleet fly the rest of its service life without a another failure and everything would be okay I guess.
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Old 12th Apr 2009, 00:13   #9 (permalink)
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The "Problem" with aviation is that it is constantly changing.

Equipment, Materials, Personnel, Processes, Repairs Schemes, Regulations, Research, Development, Standards, Companies, Manufacturers. All changing, all the time.

And nearly all the time it is to save money.

I cant remember which astronaut it was (John Bell?) who I paraphrase as saying:

"All the time I was up there, I couldn't help thinking all the capsule's parts were made by the cheapest bidders!"

The same goes for all parts made for any aircraft. If you think a broken Main Rotor is a critical item - try flying a plane with a broken Spar!

Yes, I am being simplistic! But I am trying to say that it can be a matter of Luck. As it can be lucky just to get to work every day, and then get home safely too.

Every care is taken with getting these critical products right - Manufacturers do worry about getting it wrong!

Perhaps it's time to change the "business equations" of safety levels - but I think that would be sooo costly as to prevent future development of any aircraft.

The main point is that we all want to make money as easily as possible, with the most risk permitted. We all accept inflated amounts of money to take part in those risk mitigations too.

We don't accept a high level of risk in anything we do - because we partake in daily risk assessment and mitigation.

Sometimes, someone gets something wrong - but not often. Often, it has been found, the person who 'does it wrong' is not aware of what has happened or why it happened until years after it happened.

Aviation, like medicine, is not a science. It is a "best guess" with some maths thrown in as a check.

We must learn from it - and then move on. As we always do.

We are all as good as the last accident/incident proved.

My respects to those that died, and to their families.

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Old 12th Apr 2009, 00:32   #10 (permalink)
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I take your point SASless.
All these "one in a billion" statements are dervived mathmatically though.
Two things spring to mind, bigger and heavier.
Aviation parts, and particularly helicopter components, are built as light and as weak as possible.
Thats probably the reason Sikorsky used titanium studs, lighter than steel.
Build in more overload, more safety features, larger saftey margin.

I re read your first post about the death toll, 33 in two weeks, id imagine the oil companies (who pretty much drove up the aviation standards single handedly) will have quite a bit to say on this matter and things may change from their side rather than the regulators.
It will be interesting to see how this might affect the EC175s production testing.
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Old 12th Apr 2009, 00:53   #11 (permalink)
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Step One:

Put chip burning chip detectors to adequately monitor the gearbox and link it to pilot caution warning system. You do not need a $500K HUMS to do this. Let the crew either command a burn (Bell 222 technology from the 70's) or automate it with a CAS message alerting the crew. Multiple successful burns should be indication of bad things and prompt immediate crew actions. Unsuccessful burns should have the same result.

Step Two:

Implement step one as part of the regs (i.e. do step 1)

From the L2 design chips may have been accumulating on a HUMS read mag plug (not annunciated) which if annunciated could have given this crew actionable information.

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Old 12th Apr 2009, 01:34   #12 (permalink)
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Your post assumes the defective item is making metal.. It is possible, and reasonable that a crack in a gear will make an alteration to vibration signals, before it makes significant amounts of metal.. Thus $ 1/2 million HUMS systems.

Your dismissal of the system is in itself symptomatic of the kind of attitudes that prevent system improvement. If half a million bucks worth of HUMS isn't quite good enough, then why not spend another 1/4 of a mil making it better?? At least then when we start making the oily bits out of non-magnetic materials, we will have system maturity on the fault detection.

I'm not Rubbishing your statement, just highlighting limitations. I guess though, nothing that involves a human interpretation or judgement will ever be fool proof.

PS to whoever mentioned "spacecraft made of parts supplied by the lowest bidder"

One rather well known American manufacturer openly tells its supplier base that it will be insisting on a 2% year on year price reduction for the next 5 years.. How do you think that is going to be achieved?? A bit of lean manufacturing? Six sigma? Improved investment in new technologies? That must be it.. For sure.. Phew! No need to worry!
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Old 12th Apr 2009, 02:39   #13 (permalink)
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I am making no assumptions. I am very knowlegdable in HUMS and I am not dismissing it as not beneficial, as history has shown it is one piece of the safety puzzle. It is an extremely useful tool, and if it is the primary tool for chip detection it is critical.

The issue is that those who claim vibration monitoring is superior to, or can eliminate, debris monitoring border on delusional. It is only a great addition to the tool kit. For vibration to be generated you have to have an actionable level of damage. Problem with cracks on high stress gears are that they may propogate to a critical level before any loss of stiffness is noticed in a daily check of HUMS. It is basically good, good and bang. Relative to root cause of many failures debris generates before enough surface damage occurs to provide an actionable vibration signature. If the chips are magnetic and the chip detectors are well placed, chip lights will be the first indication. Obviously, if the result of the Bond accident is causing a more frequent check of the mag plugs, the root cause is made from magnetic material.

As stated above all detection methods have a place. The industry and regs need to pick the low hanging fruit which has been missed.

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Old 12th Apr 2009, 06:45   #14 (permalink)
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Low Hanging Fruit

One of the major oil companies has gone on record asking a major US manufacturer (and the message was, I believe, passed to their French, German and Italian brethren) ......"Here is a flight bag with 10 kgs of metal - please put that back in your helicopter where it will do the most good, we will gladly accept 10kgs less payload as a price worth paying".

That after having to live with fragile tail-rotor systems and components that are scrapped if they are even scratched.

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Old 12th Apr 2009, 08:34   #15 (permalink)
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Main Gear Box failure from the Past

A sad accident from the past, on Google type " Puma Helicopter accident 16th December 1980 near Kuala Belait Brunei ".

About 30 years ago.

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Old 12th Apr 2009, 09:57   #16 (permalink)
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Am I right in thinking some aircraft including some Eurocopter models have a weak link in the Main Rotor Shaft, that will break and allow autorotation in the event of a sudden gearbox failure ??

If this is the case, why is it not a feature of all helicopters. As a pilot, I've never missed it before, but as of February, I am beginning to consider it a necessity (if viable and available)
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Old 12th Apr 2009, 10:41   #17 (permalink)
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Ta for clarifying, slightly misunderstood your first post.

Concur with your thoughts on the 2nd one.

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Old 12th Apr 2009, 11:04   #18 (permalink)
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EC gearboxes

I think we are missing the point here. The original cause of these failures is often an overtorque that has caused damage to a load bearing surface. This eventually results in stress fractures or similair failures. Correct me if I am wrong but I seem to remember that after an event the IHUMS is redatumed and virtually continues from this new baseline.
Regarding the 330J Puma failure I was there and involved both in the recovery and investigation. There is more to the story than revealed.
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Old 12th Apr 2009, 11:40   #19 (permalink)
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I know this thread has concentrated on the two most recent accidents but can any 76 drivers please comment on what the 76 RFM requires us to do ie we can conceivably have all three MGB caution lights illuminated + gauge presssure zero + gauge temperature high and it is a LAND AS SOON AS POSSIBLE ! When we get Grinding , Abnormal Vibration or Torque increase then we go to a Land Immediately or Ditch. I've always thought that by then , even if we are at zot feet and reduced speed , we are stretching our luck.....comments ?

Answer to a previous query. The only machine I am familiar with that had , inside the gearbox) , a waisted section on the main mast was the AS350. Supposedly the gear box could fail/seize and the mast would shear below the main upper gear box bearing thus allowing an autorotation. I wonder how they tested that ???

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Old 12th Apr 2009, 11:49   #20 (permalink)
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Gearboxes again

Ref wasted portion in gearbox, the Alouette series had that feature in 1963 when I did the course so they have had plenty of time to test but have never heard of it been used in earnest.
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