EC225 crash near Bergen, Norway April 2016
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It is actually not unusual for salvaged aircraft components to be returned to service. If they pass inspection requirements, they can be sold as used replacement components. Quite often perfectly serviceable commercial aircraft are purchased and parted-out because they are worth more as spare parts. A used large commercial helo MRGB in serviceable condition is probably worth a couple hundred thousand dollars.
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Despite there being no certainty about the source of the failure trajectory in this case yet, the whole question of engineering and design intent is still worth reflecting on.
We can always look back and cite S76 lightning struck MRBs, OEM repaired pressure domes on B747s, Shuttle O rings and brand newish A380 wings (though there is a scurrilous story that Filton told Toulouse that oversize rivets would be an issue into machined rib feet before design release - but I digress). These things - whether an Apple iPad or a helicopter MGB have to make compromise decisions when the design phase is initiated. These considerations about compromise usually have some sort of cost associated with the discussion - it's unavoidable - even human lives have an applied value that is considerably lower than that chosen by the nearest and dearest. This could be termed 'greed' and perhaps rightly so. But as the examples indicated at the top show, there is some person who finally signs off on a decision on the behalf of some organisation. I am sure if any organisation had an inkling that what they sign off on would potentially ground a fleet, then there would be some sort of design review. Sadly none of us are granted this futuristic revelation.
In 99.9999 times out of a hundred there are no implications or comeback. We get to focus on the 'accident porn' and assess where things went wrong - afterwards.
Given the number of B212 skid sets I abused in my years, I suspect that if a cross bar had been clattered against the side of a ship or pushed out the back of an Antonov and it was later fastened to something I later flew, I am almost certain that there would be no implication. How many times could you radiographically slice and inspect a post 'sudden stop' transmission before everyone is satisfied that three years down the line, all will be well?
We are all in the risk business, its not pleasant being confronted with those moments where the risk assessment did not flag an event and you trip over the 1 in whatever accident probability. My drive to work has a hundred fold risk of an accident over my next flight to Kuwait or Houston (actually probably a bit more here), my personal focus remains obstinately on the flight and not the drive.
Take care out there
TOD
We can always look back and cite S76 lightning struck MRBs, OEM repaired pressure domes on B747s, Shuttle O rings and brand newish A380 wings (though there is a scurrilous story that Filton told Toulouse that oversize rivets would be an issue into machined rib feet before design release - but I digress). These things - whether an Apple iPad or a helicopter MGB have to make compromise decisions when the design phase is initiated. These considerations about compromise usually have some sort of cost associated with the discussion - it's unavoidable - even human lives have an applied value that is considerably lower than that chosen by the nearest and dearest. This could be termed 'greed' and perhaps rightly so. But as the examples indicated at the top show, there is some person who finally signs off on a decision on the behalf of some organisation. I am sure if any organisation had an inkling that what they sign off on would potentially ground a fleet, then there would be some sort of design review. Sadly none of us are granted this futuristic revelation.
In 99.9999 times out of a hundred there are no implications or comeback. We get to focus on the 'accident porn' and assess where things went wrong - afterwards.
Given the number of B212 skid sets I abused in my years, I suspect that if a cross bar had been clattered against the side of a ship or pushed out the back of an Antonov and it was later fastened to something I later flew, I am almost certain that there would be no implication. How many times could you radiographically slice and inspect a post 'sudden stop' transmission before everyone is satisfied that three years down the line, all will be well?
We are all in the risk business, its not pleasant being confronted with those moments where the risk assessment did not flag an event and you trip over the 1 in whatever accident probability. My drive to work has a hundred fold risk of an accident over my next flight to Kuwait or Houston (actually probably a bit more here), my personal focus remains obstinately on the flight and not the drive.
Take care out there
TOD
Last edited by Thridle Op Des; 9th Jul 2016 at 10:09. Reason: afterthought
Pablo - if memory serves, there was a pressure valve in the MRGB line that opened when the normal gearbox pressure dropped - this allowed the feed from the torquemeter pumps (Tq meters worked on oil pressure) to be diverted to both the high speed inputs and the top of the MRGB. No electrical pump required.
Sasless, the FAA accepted SAC's assertion that, since all the oil pathways were inside the MRGB on the S92, there couldn't be a catastrophic loss of oil - that got them a pass/bye/wild card so they didn't have to meet the 30 mins test. Surely someone with such responsibility as the FAA engineers must have asked the question about the filter housing since that was clearly the only external element and should have been subject to some failure mode analysis. To the cynical, it seems like a fudge by a US federal department to help a large US company get their product into the market.
Sasless, the FAA accepted SAC's assertion that, since all the oil pathways were inside the MRGB on the S92, there couldn't be a catastrophic loss of oil - that got them a pass/bye/wild card so they didn't have to meet the 30 mins test. Surely someone with such responsibility as the FAA engineers must have asked the question about the filter housing since that was clearly the only external element and should have been subject to some failure mode analysis. To the cynical, it seems like a fudge by a US federal department to help a large US company get their product into the market.
Looks and reality can be quite different.
What we know for sure is it was a flawed design that was approved by every Authority, all of whom will very quickly roll a shoulder and point to someone else.
This gets back to my assertion that the current system that allows things like this to happen is flawed and needs its own overhaul.
What we know for sure is it was a flawed design that was approved by every Authority, all of whom will very quickly roll a shoulder and point to someone else.
This gets back to my assertion that the current system that allows things like this to happen is flawed and needs its own overhaul.
@riff_raff What aircraft have C64 gears in them at present, that you are aware of?
That doesn't really give a fair or even interesting basis for risk comparison for many reasons. The typical air trip is much longer than the typical road trip. I'm pretty confident that the comparison looks very different if you for example compare the risk of death per trip.
There are also other factors of the equation. The survivability of an accident with a car is much higher than with an airplane. Most technical faults on a car isn't critical, you can usually just stop. On an aircraft that might not be true, you have to trust whoever designed and manufactured the aircraft much more than with a car.
I think the fact that you have much less influence on the outcome also makes us consider flying more risky, even though that isn't expressed through statistics. When you drive a car you can to a large extent decide the risk yourself. In an aircraft you're to a larger degree depending on procedures made by someone else that you just have to trust that is safe. To me at least, the degree of control I have if something bad happens is a vital part of my risk assessment.
I'd also say that the "quality" of the drivers is much higher on aircrafts than in a car. If you lowered the bar for flying an aircraft to the same level as for driving a car, we would have a LOT of accidents. That impacts the statistics, but doesn't really apply when you compare the risk when the same person drive a car or fly a plane.
I don't intend to take this off-topic by making this a discussion, my point is simply that I think our brains has some good reasons for judging flying more risky than driving.
Here is a couple of links for anyone interested in the subject:
Flying Or Driving: Which Is Safer?
Driving Versus Flying: The Debate Is Settled! | Observer
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Most technical faults on a car isn't critical, you can usually just stop.
I will admit the rotorhead detaching is hard to accept. However, in both, the 330 and 332 instances of the past, both aircraft showed signs and provided time to correct. This one seems different in that manner (though we may find out more in the final report).
When you drive a car you can to a large extent decide the risk yourself. In an aircraft you're to a larger degree depending on procedures made by someone else that you just have to trust that is safe. To me at least, the degree of control I have if something bad happens is a vital part of my risk assessment.
...
Here is a couple of links for anyone interested in the subject:
Flying Or Driving: Which Is Safer?
Driving Versus Flying: The Debate Is Settled! | Observer
...
Here is a couple of links for anyone interested in the subject:
Flying Or Driving: Which Is Safer?
Driving Versus Flying: The Debate Is Settled! | Observer
Illusionary Superiority
By the way, if anybody reads your first citation, please also read the comments of the author (scroll to the end way past the links with the curvaceous girls). There the author admits that he miscalculated when he stated in his initial paragraph that the 'usual' stats where off by a factor of 66!
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True enough. But it reminds me of the well known statistic that the large majority of motor vehicle drivers believe they are above average drivers ;-)
Illusionary Superiority
By the way, if anybody reads your first citation, please also read the comments of the author (scroll to the end way past the links with the curvaceous girls). There the author admits that he miscalculated when he stated in his initial paragraph that the 'usual' stats where off by a factor of 66!
Illusionary Superiority
By the way, if anybody reads your first citation, please also read the comments of the author (scroll to the end way past the links with the curvaceous girls). There the author admits that he miscalculated when he stated in his initial paragraph that the 'usual' stats where off by a factor of 66!
There is a book called Freakonomics
In this book it tells the Big Picture.
Hour for Hour in an Aircraft seat compared to a Car seat you are more likely to die. Statistics don't lie.
I don't know of to many people who spend more time in an Aircraft compared to a Car.
I have personally been in 2 Aircraft that had to do forced Landings.
Apart from a Flat tyre I have never had to pull over in a car because of mechanical problems, and I spend far more time in a car than an Aircraft.
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if duplicating were actually a good idea then surely duplicating the gearbox would make sense ?
each engine could have it's drive split and drive into each gearbox
each gearbox could independently drive the rotor mast
there would of course be a ton of freewheel units and run-dry oil pumps so every eventuality would be covered
no dafter than carrying 2 engines
the failure rate would be 1x10^-64 so at least in theory nothing can go wrong
each engine could have it's drive split and drive into each gearbox
each gearbox could independently drive the rotor mast
there would of course be a ton of freewheel units and run-dry oil pumps so every eventuality would be covered
no dafter than carrying 2 engines
the failure rate would be 1x10^-64 so at least in theory nothing can go wrong
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I don't know of any MRGB designs that use C64 gears. However, the AW609 MRGB does use X-53 gears, primarily for improved loss of lube capability. The main reason that C64 is not currently used is because it was just developed a couple years ago. Bell studied upgrading existing gearboxes with C64 gears under the recent FARDS program.
Upgrading existing MRGBs with C64 gears in certain locations where potential for scoring is high under loss of lube operation (such as spiral bevel or output stage planet meshes) would be a very cost effective approach. While the C64 (AMS 6509) raw material is currently around 2-3 times the $/lb cost of 9310 (AMS 6265), it does provide some advantages in manufacturing processing. One important advantage is that C64 is designed to use a much cleaner gas quench instead of an oil quench.
Riff_Raff,
There are a lot of Pilots sitting at their computer....looking at the screen....and wondering what language you are speaking!
If you and those like me are going to have commonality of definition....you shall have to go back to four letter words and arm waving!
I got lost back in the time I was expected to know all the interworking of a cereal bowl compass in order to use an RMI for figuring out what direction I was going.
There are a lot of Pilots sitting at their computer....looking at the screen....and wondering what language you are speaking!
If you and those like me are going to have commonality of definition....you shall have to go back to four letter words and arm waving!
I got lost back in the time I was expected to know all the interworking of a cereal bowl compass in order to use an RMI for figuring out what direction I was going.
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Riff_Raff, There are a lot of Pilots sitting at their computer....looking at the screen....and wondering what language you are speaking! If you and those like me are going to have commonality of definition....you shall have to go back to four letter words and arm waving!
I got lost back in the time I was expected to know all the interworking of a cereal bowl compass in order to use an RMI for figuring out what direction I was going.
I got lost back in the time I was expected to know all the interworking of a cereal bowl compass in order to use an RMI for figuring out what direction I was going.
If it's any help, the simple version of what I posted is as follows. There is an advanced type of alloy gear steel called C64 that has recently been developed which can safely operate at temperatures ~500degF higher than steel gear alloys used in current gearboxes. Using this advanced steel alloy to upgrade certain gears of existing main rotor transmissions could likely provide a significant increase in loss of lube performance, at a reasonable cost.
Does that explanation make sense to you?
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It sounds like the new materials being developed and becoming available will push the capabilities of new rotorcraft into areas where older types will struggle to compete, with evolutionary upgrades no match for fresh design (except perhaps where those types can demonstrate a long service history with very few if any major accidents).
Where will that leave aircraft such as the H225 and AS332L2? Were it not for two accidents, both apparently caused initially by fatigue fracturing of a planet gear of ‘current spec’ 16NCD13 material, we might not be discussing the merits or feasibility of changing this material and the H225 might have a healthier future. Was the planet gear material in the SA330 or AS332 up to L1 variant the same as in the L2 and EC225, I wonder?
In visualising what may have gone wrong in these 2 similar accidents in the moments following planet gear fracture, it has struck me that AH really put all their eggs in one basket depending on these gears not to fail.
My interpretation of the G-REDL accident and report 2/2011 is that it wasn’t simply the failure of one gear that brought the aircraft down. In photo figure 21, the other 7 gears look pretty serviceable and should have been able to accommodate the small increase in load share if that was the only design consideration.
It wasn’t simply that the ring gear split at the 5 o’clock position either, as seen in Figure 20. There would probably have been enough of the remaining attachment of the epicyclic casing at that moment to keep the mast in place and some measure of pitch control possible.
However, the way that successive bolting around the epicyclic casing would subsequently fail as trapped debris worked its way around the module, would lead to the lower bearing in the conical housing becoming displaced. Thereafter the loss of control and breakup sequence would have been very quick.
This thread discussion has wandered interestingly to a side issue of “run-dry” capability and, learning of some posters past military experiences, I began to understand there are other reasons apart from the condition of the North Sea why you might not want to make an immediate landing, for loss of oil or other cause.
It is not unusual in military aircraft to provide, at the cost of some increased weight, some ‘armoured’ protection for critical components against hostile action. Even civilian turbine engine casings will be designed to contain a certain degree of blade shedding.
So could not AH have provided some means of preventing larger items of debris from a fractured planet gear from becoming entrained in the gear meshing? I know there is not a whole lot of room between planets but I wonder if some sort of guarding built into the carrier design might delay the break up of the planet gear by bending forces; retain debris including rollers from escaping into the other workings of the gearbox; and keep debris from elsewhere out from the planet itself. The critical objective would be to keep debris out of the sun gear (especially) and the ring gear meshing. Planet gear tooth damage might be tolerant to an extent.
If this could buy just a few minutes to make an emergency descent and landing I think the extra weight would be worth it. Indeed, I am surprised that certification does not demand it.
To be fair to AH, the recommendations in AAIB report 2/2011 were to review the design of the planet gear, not the consequences that actually bring down the aircraft. If you don’t ask the right question you probably will not get the right answer.
If guarding of the planet gears is beyond engineering capability then could not the bottom of the conical housing be provided with independent location, in principal not unlike the upper lift housing? Some sort of external brackets between MGB and conical housing, 3 as a minimum and possibly close to M/R servos to limit deflection of a burst ring gear from breaking or jamming a servo?
If my questions sound a bit dumb it is because I am not an aviation ‘insider’. My contribution to aviation is mostly as moveable ballast, so if instructed to I will respectfully go and sit at the back and buckle up!
Where will that leave aircraft such as the H225 and AS332L2? Were it not for two accidents, both apparently caused initially by fatigue fracturing of a planet gear of ‘current spec’ 16NCD13 material, we might not be discussing the merits or feasibility of changing this material and the H225 might have a healthier future. Was the planet gear material in the SA330 or AS332 up to L1 variant the same as in the L2 and EC225, I wonder?
In visualising what may have gone wrong in these 2 similar accidents in the moments following planet gear fracture, it has struck me that AH really put all their eggs in one basket depending on these gears not to fail.
My interpretation of the G-REDL accident and report 2/2011 is that it wasn’t simply the failure of one gear that brought the aircraft down. In photo figure 21, the other 7 gears look pretty serviceable and should have been able to accommodate the small increase in load share if that was the only design consideration.
It wasn’t simply that the ring gear split at the 5 o’clock position either, as seen in Figure 20. There would probably have been enough of the remaining attachment of the epicyclic casing at that moment to keep the mast in place and some measure of pitch control possible.
However, the way that successive bolting around the epicyclic casing would subsequently fail as trapped debris worked its way around the module, would lead to the lower bearing in the conical housing becoming displaced. Thereafter the loss of control and breakup sequence would have been very quick.
This thread discussion has wandered interestingly to a side issue of “run-dry” capability and, learning of some posters past military experiences, I began to understand there are other reasons apart from the condition of the North Sea why you might not want to make an immediate landing, for loss of oil or other cause.
It is not unusual in military aircraft to provide, at the cost of some increased weight, some ‘armoured’ protection for critical components against hostile action. Even civilian turbine engine casings will be designed to contain a certain degree of blade shedding.
So could not AH have provided some means of preventing larger items of debris from a fractured planet gear from becoming entrained in the gear meshing? I know there is not a whole lot of room between planets but I wonder if some sort of guarding built into the carrier design might delay the break up of the planet gear by bending forces; retain debris including rollers from escaping into the other workings of the gearbox; and keep debris from elsewhere out from the planet itself. The critical objective would be to keep debris out of the sun gear (especially) and the ring gear meshing. Planet gear tooth damage might be tolerant to an extent.
If this could buy just a few minutes to make an emergency descent and landing I think the extra weight would be worth it. Indeed, I am surprised that certification does not demand it.
To be fair to AH, the recommendations in AAIB report 2/2011 were to review the design of the planet gear, not the consequences that actually bring down the aircraft. If you don’t ask the right question you probably will not get the right answer.
If guarding of the planet gears is beyond engineering capability then could not the bottom of the conical housing be provided with independent location, in principal not unlike the upper lift housing? Some sort of external brackets between MGB and conical housing, 3 as a minimum and possibly close to M/R servos to limit deflection of a burst ring gear from breaking or jamming a servo?
If my questions sound a bit dumb it is because I am not an aviation ‘insider’. My contribution to aviation is mostly as moveable ballast, so if instructed to I will respectfully go and sit at the back and buckle up!
Last edited by Concentric; 11th Jul 2016 at 09:55. Reason: paragraph spacing
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If my memory serves me well the MD 900 series employ a design that shares the flight and rotational loads through two separate load paths. Is this so? and if it is, why don't we employ that standard across the board.
If the certification standards included this requirement we might see some more robust designs that are damage tolerant when critical components fail.
G.
If the certification standards included this requirement we might see some more robust designs that are damage tolerant when critical components fail.
G.
SASless: I understood riff, but it helps that I was an engineering major so the terms aren't foreign to me. 
The cost differential riff mentioned is a non-trivial issue when considering whether to upgrade / replace. Any manufacturer for a civil or military contract has to account for lead times for exotic / specialty alloys, and the cost versus customer requirements (and regulatory requirements) -- a balancing act that goes into any final design or upgrade decision. Part of the reason I asked is due to a different topic related to gear wear resistance (super finishing) that I understand Boeing uses on some gears in the Apache ... but we are going off topic so I'll stop there. (Either of those choices are open to AH as the final evaluation of their gears is put into print).
@Concentric: an interesting line of inquiry in your post, but for most operators, the key metric is that as components wear an indication/warning of wear or impending failure gives them a chance to change a component before failure. I am not sure that a design "to deal with it once the gears come apart" is what the industry wants. The "hey, this one's beginning to wear/lose material" trigger gets the box off the aircraft before these critical parts fail.
The cost differential riff mentioned is a non-trivial issue when considering whether to upgrade / replace. Any manufacturer for a civil or military contract has to account for lead times for exotic / specialty alloys, and the cost versus customer requirements (and regulatory requirements) -- a balancing act that goes into any final design or upgrade decision. Part of the reason I asked is due to a different topic related to gear wear resistance (super finishing) that I understand Boeing uses on some gears in the Apache ... but we are going off topic so I'll stop there. (Either of those choices are open to AH as the final evaluation of their gears is put into print).
@Concentric: an interesting line of inquiry in your post, but for most operators, the key metric is that as components wear an indication/warning of wear or impending failure gives them a chance to change a component before failure. I am not sure that a design "to deal with it once the gears come apart" is what the industry wants. The "hey, this one's beginning to wear/lose material" trigger gets the box off the aircraft before these critical parts fail.
Last edited by Lonewolf_50; 11th Jul 2016 at 13:14.
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Dual load-paths
Yes, correct Geoff - 2 x separate load-paths. As Eric says, 'old' technology, but proven, and yes, also, read across from the Hughes 500. Keep it simple (K.I.S.S) ~ VFR
If my memory serves me well the MD 900 series employ a design that shares the flight and rotational loads through two separate load paths. Is this so? and if it is, why don't we employ that standard across the board.
If the certification standards included this requirement we might see some more robust designs that are damage tolerant when critical components fail.
G.
If the certification standards included this requirement we might see some more robust designs that are damage tolerant when critical components fail.
G.
Humans ofter overestimate their own level of control over things, personally I prefer robust design over overconfidence in monitoring systems as a general principle. That doesn't mean that I think monitoring systems shouldn't exist, it's just not wise to think one has all eventualities covered - because one never has.