Main Gear Boxes and The Grand Lottery
You can't design out the MRGB but maybe you can reduce the effects of a failure.
Look at how the gearbox works in the 500 and I believe the Apache.
Most gearboxes support the main rotor and have flying controls bolted to them. A major failure can lead to loss of rotor and/or control.
In the 500 design the gearbox is underslung and in theory could fall out completely with no effect on the flying controls and the abilty of the aircraft to autorotate.
For large aircraft this would be a radical redesign, resulting in either less internal space or a taller aircraft. Not impossible though just different.
Look at how the gearbox works in the 500 and I believe the Apache.
Most gearboxes support the main rotor and have flying controls bolted to them. A major failure can lead to loss of rotor and/or control.
In the 500 design the gearbox is underslung and in theory could fall out completely with no effect on the flying controls and the abilty of the aircraft to autorotate.
For large aircraft this would be a radical redesign, resulting in either less internal space or a taller aircraft. Not impossible though just different.
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As I recall the engineer has a number of tools at his disposal, from the basic chip detector and the Mk one eye ball, periodic micro analyse of oil (every 50 hours as I remember) analysis of any partials picked up by the mag plug, and filter. Trending of the results of the 50 hr regular check. On the 61 this was extremely important as the white metal bearings started to shed indicating a potential problem.
This simple system of periodic sampling and trend analyses gave a good indication of gearbox condition. Now add the hums and vibration analyses system performance and add these trending results to the ones above and hopefully a good monitoring system both by engineering chaps and computer and what do we end up with??
Seemingly we go back to those old days of sudden and unexplained catastrophic failure of a major link in the drive train. The investigation will provide some answers and will recommend a series of quick responses. The authorities will huff and puff and seemingly repeat past history, by actually doing very little, and in another 20 years we will be adding to a similar thread in pp with nice to do stuff and how it could all have been avoided ‘’IF’.
Waiting to be pilloried, but someone had to state the obvious.
Outhouse.
This simple system of periodic sampling and trend analyses gave a good indication of gearbox condition. Now add the hums and vibration analyses system performance and add these trending results to the ones above and hopefully a good monitoring system both by engineering chaps and computer and what do we end up with??
Seemingly we go back to those old days of sudden and unexplained catastrophic failure of a major link in the drive train. The investigation will provide some answers and will recommend a series of quick responses. The authorities will huff and puff and seemingly repeat past history, by actually doing very little, and in another 20 years we will be adding to a similar thread in pp with nice to do stuff and how it could all have been avoided ‘’IF’.
Waiting to be pilloried, but someone had to state the obvious.
Outhouse.
Last edited by outhouse; 13th Apr 2009 at 16:34.
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Thanks for those two posts Ericferret and Outhouse as they sum up the deficiencies in this matter. Neither of you will be pilloried by me. The mechanical outcome of the transmission failure in this case is almost identical to the outcome on Chinook FC. The AAIB have already reported on that and made recommendations to stop a recurrence. But a loss of rotor head has occurred again with the loss of all on board. There are at least too problems here, the failure of the gearbox and the resultant failure of the rotor head and control of the A/C. The latter should not follow the failure of the former and must be engineered out. I appreciate that in this case it was inevitable. (It must have been considered at the design stage and as nothing was done must have been ignored) Yes pilots must know what is going on in their gearboxes, yes engineers must have the tools to predict failure before it occurs and yes pilots should be able to autorotate following a transmission failure. A lot of work will need to follow and a lot of money spent to remove this risk from helicopter travel, we now know what happened, we now need to get after it. What caused it to happen I leave to the AAIB, but their findings and recommendations must eliminate a recurrence. ( and please do not use the word "ensure" in your recommendations as it is not sufficiently specific, please tell us exactly what needs to be done.) Radical re-design? Yes if that is what it takes. Would this known fatal failure mechanism be allowed to persist in any any other industry? No!
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Mmmm ...
Eric .....
You mention the 500 ... it too had some gearbox issues in the past ... vaguely remember an AD which required removing the Xmsn oil level sight glass and checking all the main gear attach bolts for loss of torque .... AND I found one (bolt) which had failed and was hanging by its lockwire!
Stopped taking transmissions for granted after that ( I think about 1977/8 IIRC).
Eric .....
You mention the 500 ... it too had some gearbox issues in the past ... vaguely remember an AD which required removing the Xmsn oil level sight glass and checking all the main gear attach bolts for loss of torque .... AND I found one (bolt) which had failed and was hanging by its lockwire!
Stopped taking transmissions for granted after that ( I think about 1977/8 IIRC).
Below the Glidepath - not correcting
Sobering, yes ... but also somewhat to be expecetd given that the acft our fixed wing counterparts are flying have several more decades of practical cumulative development experience - about 100 years for FW vs about 60 years for RW (give or take a few). And, a similar but even more exaggerated experience gap in terms of cumulative units produced. The next 40 years will likely see considerable advances in most aspects of helicopter safety and reliability.
It takes little imagination to see that any design, engineering, or mechanical failure in that environment is followed by virtually instant and catastrophic loss of the aircraft. In contrast high speed rotable failures in fixed wing aircraft occur at the powerplants, so although that brings its own range of problems, containment technology and FOD resistance usually allow for a more favorable outcome when comparing the loss of a engine fan blade with a main rotor blade.
Just trying to think a bit.
If a bearing fails, or a gear starts to fail, making bigger parts come loose thats to big to hit the magnetic plug/chip detection system (like parts of cogs or so)...if these parts jam between the gears...could that cause the epicyclic gear to brake ? ..and/or possibly make the MGB suffer a sudden decrease in speed and/or sudden stop ?
My thought is that it is not sure that the origin of the fault was in the epicyclic ? If so, there shouldnt have helped if HUMS could detect cracks in the epicyclic gear ?
If a bearing fails, or a gear starts to fail, making bigger parts come loose thats to big to hit the magnetic plug/chip detection system (like parts of cogs or so)...if these parts jam between the gears...could that cause the epicyclic gear to brake ? ..and/or possibly make the MGB suffer a sudden decrease in speed and/or sudden stop ?
My thought is that it is not sure that the origin of the fault was in the epicyclic ? If so, there shouldnt have helped if HUMS could detect cracks in the epicyclic gear ?
A pilot should be checking his deferred defect log and if any MGB HUMS related defects are noted he should be demanding to see the trend graphs for his own piece of mind (and question them). No matter how good an engineers pre flight or pilots walk around, We are not able to detect the state of play inside the most crucial part of the machine through normal visual inspection. Only HUMS can do so and if there is a rising trend above a threshold that raises concern then a pilot should be quite within his rights to refuse that aircraft.
2nd, if the HUMS events (if there are any) are within the thresholds, and are being monitored in accordance with the AMM/AMP, and therefore manufacturer's?/Maintenance Organisation's approved protocols, why would you expect the pilot to overide this? Would you apply the same logic to Track and Balance figures, engine HSS vibrations, SOAP samples?
SPINWING
The 500 main box has indeed had it's share of internal woes over the years.
So even better is a design that (in theory!!!) allows the gearbox to be a redundant item still giving you autorotation under control.
The 500 main box has indeed had it's share of internal woes over the years.
So even better is a design that (in theory!!!) allows the gearbox to be a redundant item still giving you autorotation under control.
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AGARICUS BISPORUS I never worked for BAH or BIH at Aberdeen. However I was lead to believe that your version of events is correct.It could be just hearsay but the engineer who told me worked on the Chinnooks for several years.
I reread the AIB report recently and was surprised that it conflicted with what I believed was the story behind the gearbox mod.
I reread the AIB report recently and was surprised that it conflicted with what I believed was the story behind the gearbox mod.
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AAKEE A Hiller 12E suffered the type of failure you discuss in the 70's. Operated by Bond Helicopters in a previous incarnation I believe. It can be found on the AAIB website in the archive section. Sorry can't remember the year.
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A pilot should be checking his deferred defect log and if any MGB HUMS related defects are noted he should be demanding to see the trend graphs for his own piece of mind (and question them). No matter how good an engineers pre flight or pilots walk around, We are not able to detect the state of play inside the most crucial part of the machine through normal visual inspection. Only HUMS can do so and if there is a rising trend above a threshold that raises concern then a pilot should be quite within his rights to refuse that aircraft.
The implications of this is that the Flight Crew do not at present have the ability to manage the totalily of risk when applied to the actual flight requirement, as you would do say for an deferred defect that might affect the type of flight you need to carry out.
Whether this is viable/sensible for the future is a far wider debate.
What may be more appropriate (and I have no expertise at all in this area) is whether sufficient HUMs data was being picked up immediatley prior to the failure that, if it had been made avialable to the crew, they could have taken immediate intervention to prevent the full catastrophic conclusion that we have just witnessed.
Should/can certain types/levels of HUMS data be provided to the crew in the form of a warning/alert system that, allied to a specified in-flight abnormal/emergency procedure, would prevent such an incident reaching its ultimate conclusion.
Are there any experts in the HUMS field out there who think such a system is viable and in particular, is the databank of alerts sufficiently developed to provide appropriate thresholds for warning and intervention?
DB
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ERIC FERRET * OILANDGASMAN
Having just read your posts again I think you both have made a very very, valid points. The design concept of makiing the MGB "multitasking" certainly leaves the helicopter utterly susceptable to catastrophic failure and one could argue, that in doing all these jobs (load bearing in the air and on the ground) it is probably subjected to a whole bunch of unecessary stresses.
It is starnge that this concept has remained a feature for many helicopters yet the safety advantages, as you guys have mentioned, allied to the reduction in downtime (as the MGB could be removed without having to remove head and blades) would make you think it should have been thought about before.
Any engineering wizzards out there who could explain this anomaly???
Having just read your posts again I think you both have made a very very, valid points. The design concept of makiing the MGB "multitasking" certainly leaves the helicopter utterly susceptable to catastrophic failure and one could argue, that in doing all these jobs (load bearing in the air and on the ground) it is probably subjected to a whole bunch of unecessary stresses.
It is starnge that this concept has remained a feature for many helicopters yet the safety advantages, as you guys have mentioned, allied to the reduction in downtime (as the MGB could be removed without having to remove head and blades) would make you think it should have been thought about before.
Any engineering wizzards out there who could explain this anomaly???
It is probably in bad form to quote oneself however:
Jim
As Flyt3est has indicated, the HHMAG (a group which contained the world-wide great and good of the HUMS community - what a pity the advent of EASA caused its demise) discussed cockpit indication at length.
Because the issue was so complex and in view of the lack (at that time) of the required level of reliability, and the simplicity of the systems (events only - and even those provided on a heuristic basis) - a working group was tasked to consider it in detail. This group made a finding that, for the time being, cockpit indications would only complicate the life of the pilot - not simplify it. It also reiterated that the aim of HUMS was to spot developing faults, clusters and trends in time for them to be addressed on the ground and before they could impact upon a flight.
In common with others, I still have my doubts that cockpit indications provide the answer. Perhaps the problem is believing a primary indication that appears irrational and cannot be confirmed by some other means. It could be that, in these days of digital systems and software control, more effort is required in the provision of algorithms that provide (and indicate) secondary confirmation based upon alternative diagnosis paths.
Because the issue was so complex and in view of the lack (at that time) of the required level of reliability, and the simplicity of the systems (events only - and even those provided on a heuristic basis) - a working group was tasked to consider it in detail. This group made a finding that, for the time being, cockpit indications would only complicate the life of the pilot - not simplify it. It also reiterated that the aim of HUMS was to spot developing faults, clusters and trends in time for them to be addressed on the ground and before they could impact upon a flight.
In common with others, I still have my doubts that cockpit indications provide the answer. Perhaps the problem is believing a primary indication that appears irrational and cannot be confirmed by some other means. It could be that, in these days of digital systems and software control, more effort is required in the provision of algorithms that provide (and indicate) secondary confirmation based upon alternative diagnosis paths.
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Double Bogey, Eric Ferret, et al
Those are interesting points that are raised, however we need to bear in mind that no matter how thorough and detailed a manufacturing, inspection and maintainance process is there is still a REMOTE possibility that a catastrophic failure of this nature could have happened suddenly without a noticeable timeous trend on the HUMS or without making chips/metal.
These are machines that at the end of the day are subject to sometimes tragic random failures. I am not trying to start a fight here, merely making a point of view that IMO needs to be remembered.
As for the alternate load bearing structure of the Apache rotor system, bear in mind that it may not be feasable from a design point of view if you want to carry passengers to do it like that on the modern civil helicopter although it would make my job a lot easier for swapping out an MGB if it were.
Those are interesting points that are raised, however we need to bear in mind that no matter how thorough and detailed a manufacturing, inspection and maintainance process is there is still a REMOTE possibility that a catastrophic failure of this nature could have happened suddenly without a noticeable timeous trend on the HUMS or without making chips/metal.
These are machines that at the end of the day are subject to sometimes tragic random failures. I am not trying to start a fight here, merely making a point of view that IMO needs to be remembered.
As for the alternate load bearing structure of the Apache rotor system, bear in mind that it may not be feasable from a design point of view if you want to carry passengers to do it like that on the modern civil helicopter although it would make my job a lot easier for swapping out an MGB if it were.
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JimL, I take the point that there are/have been some real "brains" working the issue of HUMS indications. Having said that, a starting point for pumas would be:
COCKPIT THRESHOLD ALERT for all MGB, TGB, IGB and ECU HOT-END bearing pickups. These being the weak points in the design concept.
I would consider being oblidged to land at the nearest safe site a considerable advantage rather than a complication!!!
Like I said before (and on the other thread) the jury is effectively still out on this one and until we know the EXACT circumstance of the Operator's response to the data they may or may not have had available prior releasing the aircraft to service we cannot know if the the requirement for an instant indication is valid let alone viable.
However, if it is the case that the HUMs only began squeaking during the fatal flight then there surley must be an argument for letting the crew know what is going on in the MGB during flight as the implications in this case would be that rapid and catastrophic deterioration can occur within the space of one flight. I have always hoped that the HUMS would have speaked long before the failure propogates to its full conclusion.
DB
COCKPIT THRESHOLD ALERT for all MGB, TGB, IGB and ECU HOT-END bearing pickups. These being the weak points in the design concept.
I would consider being oblidged to land at the nearest safe site a considerable advantage rather than a complication!!!
Like I said before (and on the other thread) the jury is effectively still out on this one and until we know the EXACT circumstance of the Operator's response to the data they may or may not have had available prior releasing the aircraft to service we cannot know if the the requirement for an instant indication is valid let alone viable.
However, if it is the case that the HUMs only began squeaking during the fatal flight then there surley must be an argument for letting the crew know what is going on in the MGB during flight as the implications in this case would be that rapid and catastrophic deterioration can occur within the space of one flight. I have always hoped that the HUMS would have speaked long before the failure propogates to its full conclusion.
DB
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As an old fart and one who has like 
the posters in this thread enjoyed the discussions re the new and developing technology available to engineers in the quest of safety in the critical areas mainly the rotor drive system. I really don’t want to continue to be pessimistic but do understand the limitations to any fault monitoring system. As has been shown HUMS is a basic trending system, it will generate alerts; identify trending and progressive deterioration of bearings, tooth mesh, gear condition and changing general harmonics within selective parts of the drive train. The effectiveness of the system is down to the system design, computer programme and the ground operator monitoring the system.
Question, can this system in its present mode of operation pick out and alert the pilot/engineer to a possible instant failure, guess the answer is NO. (Based on the limited info available)
Question 2, is the pilot in the loop regarding the data and results of the HUMS system while in flight, answer NO.
Question 3, should he be? How could this be achieved and how could this be included in his ECL action response.
O
the posters in this thread enjoyed the discussions re the new and developing technology available to engineers in the quest of safety in the critical areas mainly the rotor drive system. I really don’t want to continue to be pessimistic but do understand the limitations to any fault monitoring system. As has been shown HUMS is a basic trending system, it will generate alerts; identify trending and progressive deterioration of bearings, tooth mesh, gear condition and changing general harmonics within selective parts of the drive train. The effectiveness of the system is down to the system design, computer programme and the ground operator monitoring the system.Question, can this system in its present mode of operation pick out and alert the pilot/engineer to a possible instant failure, guess the answer is NO. (Based on the limited info available)
Question 2, is the pilot in the loop regarding the data and results of the HUMS system while in flight, answer NO.
Question 3, should he be? How could this be achieved and how could this be included in his ECL action response.
O
As for the alternate load bearing structure of the Apache rotor system, bear in mind that it may not be feasable from a design point of view if you want to carry passengers to do it like that on the modern civil helicopter although it would make my job a lot easier for swapping out an MGB if it were.
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It takes little imagination to see that any design, engineering, or mechanical failure in that environment is followed by virtually instant and catastrophic loss of the aircraft.
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Yes we can...
I wouldn't be surprised to hear that the designers at EC and Sikorsky and maybe elsewhere are rising to the challenge of creating a gearbox design that will deliver the kind of failure modes that do not result in catastrophic failure. Multiple load paths, controls protected (probably electrically actuated), and a guarantee of autorotation even if the whole lot goes pop. I'm sure it can be done it requires some motivation and 25 years after HARP I think it's time to do something that will deliver that paradigm shift needed to move helicopter airworthiness another rung up the ladder.
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