Main Gear Boxes and The Grand Lottery
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Spinwing, the BAH Chinook crash was caused by a transmission failure.
Briefly a big ring-gear (about 2 feet across) is fitted to the vertical mainshaft with a lot of bolts to take power from the horizontal high-speed shaft. Some, that is some, of these bolts were found to be losing torque slightly over the x000hr inspection period. A hugely complex and costly fitting of oversized bolts with higher torque capability was instigated.
Primarily due to a change in the washers used under these new bigger bolts (a lubrication channel was deleted) salt particles got into the thread area and started a corrosion pit which propagated a circumfrential stress fracture in the ring gear which finally failed. The 'box was fitted with a comprehensive chip detector system. Chip detectors cannot detect a stress fracture until metal starts spalling off, by which time it's probably far, far too late.
Post modified in deference to the remarks in the post below, though very little of the original was apparently incorrect.
Addendum...
At the time it was widely & popularly believed within the BAH community that the modifications were unnecessary, that the BA and Boeing engineers had developed a perfectly acceptable solution to the torque loss that did not involve oversizing bolts (and thus the incorporation of the non-scalloped shim. This, iirc, did involve Loctite) and that this mod had been forced against their better judgement by the CAA despite strident objections. Further, it was stated categorically within my hearing by people who should have known that one feature of the scalloped (grooved) shim was that it allowed lube oil to flush salt contaminants out of the joint, thus removing a potential cause of corrosion. The new-mod solid shims could not do this, and the view was that this lack of flushing almost certainly promoted, if not caused, tha corrosion pit that started the process.
Cetainly not a forseeable error chain, but a good example of the complexity of such events.
Briefly a big ring-gear (about 2 feet across) is fitted to the vertical mainshaft with a lot of bolts to take power from the horizontal high-speed shaft. Some, that is some, of these bolts were found to be losing torque slightly over the x000hr inspection period. A hugely complex and costly fitting of oversized bolts with higher torque capability was instigated.
Primarily due to a change in the washers used under these new bigger bolts (a lubrication channel was deleted) salt particles got into the thread area and started a corrosion pit which propagated a circumfrential stress fracture in the ring gear which finally failed. The 'box was fitted with a comprehensive chip detector system. Chip detectors cannot detect a stress fracture until metal starts spalling off, by which time it's probably far, far too late.
Post modified in deference to the remarks in the post below, though very little of the original was apparently incorrect.
Addendum...
At the time it was widely & popularly believed within the BAH community that the modifications were unnecessary, that the BA and Boeing engineers had developed a perfectly acceptable solution to the torque loss that did not involve oversizing bolts (and thus the incorporation of the non-scalloped shim. This, iirc, did involve Loctite) and that this mod had been forced against their better judgement by the CAA despite strident objections. Further, it was stated categorically within my hearing by people who should have known that one feature of the scalloped (grooved) shim was that it allowed lube oil to flush salt contaminants out of the joint, thus removing a potential cause of corrosion. The new-mod solid shims could not do this, and the view was that this lack of flushing almost certainly promoted, if not caused, tha corrosion pit that started the process.
Cetainly not a forseeable error chain, but a good example of the complexity of such events.
Last edited by Agaricus bisporus; 12th Apr 2009 at 13:25.
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AAIB tell a different story: Air Accidents Investigation: 2/1988 G-BWFC
AAIB tell a different story: Air Accidents Investigation: 2/1988 G-BWFC
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New technolody - old mentality
With the new helicopters like S92 EC225 AW139 we fly the latest technology, full glass cockpit, fully computerised, fully monitered (CVR,FDR, HUMS), and basic magnetic plugs ... 
In flight we only have access to basic informations like temp and pressure and chip warning.
What would be the cost to bring all those datas centralised and available in flight with a proper monitoring.
As an example in the 139 I can know my hydraulic level at any time in flight, the level is monitered, but any significant change in a short time (leak) won't be noticed until the level is low. We always now too late
If something goes wrong in the MGB IGB or TGB any significant change in vibration level and increasing in a short time should be displayed on the screen and not only "recorded".
Don't tell me it's difficult to do, what computers can't do that ?
When you find yourself in one of those extremely remote probabilities, the few minutes or even seconds advance warning may change everything.
Manufacturers have to think differently when they design new aircrafts.
3rd millenium technology with last centenary thinking and design
In flight we only have access to basic informations like temp and pressure and chip warning.
What would be the cost to bring all those datas centralised and available in flight with a proper monitoring.
As an example in the 139 I can know my hydraulic level at any time in flight, the level is monitered, but any significant change in a short time (leak) won't be noticed until the level is low. We always now too late
If something goes wrong in the MGB IGB or TGB any significant change in vibration level and increasing in a short time should be displayed on the screen and not only "recorded".
Don't tell me it's difficult to do, what computers can't do that ?
When you find yourself in one of those extremely remote probabilities, the few minutes or even seconds advance warning may change everything.
Manufacturers have to think differently when they design new aircrafts.
3rd millenium technology with last centenary thinking and design
Thread Starter
You engineers out there correct me if I am wrong.....you certainly have in the past!
Do not chip detectors tend to find "bearing" bits rather than "gear" bits?
Gear wear is more likely to produce fewer but larger chunks as bits of gear are broken off thus fewer bits are produced and are of larger sizes than bearing bits.
Chip detectors don't do squat for finding cracked gears.
Do not chip detectors tend to find "bearing" bits rather than "gear" bits?
Gear wear is more likely to produce fewer but larger chunks as bits of gear are broken off thus fewer bits are produced and are of larger sizes than bearing bits.
Chip detectors don't do squat for finding cracked gears.
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Gearbox again
Hi SASless you are quite correct. although the chip detectors/mag plugs collect any ferrous material floating about in the oil. You may recall that a couple of years ago I found an entire gear tooth from a combining gear box in Port Harcourt that showed up only as an intermittent flash on the chip detector warning light.
I was asked to post this on another thread but it really belongs to this one - it comes from the HARP report of 1984:
These gearboxes are remarkable examples of the art of the mechanical engineer. Although simplicity is a good starting point in the conceptual stage, complexity is inevitable, and it is not surprising that each designer has found his own solutions. The panel has seen spur gear trains, epicyclics, bevels; it has seen normal straight teeth, 'conformal' tooth profiles; it has seen ball bearings, rollers, tapered rollers, shafts with separate ball races and shafts with rollers running direct on them; we have seen aluminium and magnesium alloy cases. The only common element noted was in the gear material, a traditional low carbon nickel-chrome case hardening steel, either carburised or nitrided, and invariably made of high purity vacuum melted stock.
Notwithstanding the exceptional facilities for gear production we were shown, and the great attention to quality control being paid we are bound to record that in our view it will never be possible to eliminate completely the possibility of failure of some part of so complex a mechanism in service, between specified inspection or overhaul periods.
Although minor surface damage to a gear or bearing is unlikely to be instantly catastrophic, and should be detectable, and although gearboxes are tested to run for a period such as 30 minutes with the oil supply failed, the possibility of a major failure within the box is ever present.
Since with present knowledge a main rotor cannot be duplicated although a tail rotor gearbox might (conceivably a helicopter may survive the failure of the latter) we now come up against the fundamental difference between normal aircraft and rotorcraft -the inability to guard against a possible defect by duplication.
The Panel believes that, while gearbox development and testing should be as stringent as practicable, other means must be introduced to monitor the condition or "health" of the gearbox, preferably in flight. Elementary condition monitoring is already practiced (e.g. oil chip detectors), but much more attention should be paid to this. We make our detail recommendations on this subject later in para. 8.5.
There have been failures of transmission systems taking the drive to the tail rotor, or coupling twin rotor heads. We believe it may be possible to achieve a degree of redundancy here, or at any rate 'damage tolerance'. On the other hand it may be simpler to have some other method of cancelling main rotor torque to use in an emergency (bleed gas jet, rudder surface •••••• )
There is no need to repeat anything in this quote but my underlines emphasize a number of salient points in the text. In original post I also indicated "The text in paragraph 8.5 (referred to above) contains recommendations for 'condition monitoring'; methods include: chip detectors; vibration monitoring; thermal detection or imaging in flight; oil sampling; telemetry (i.e. instantaneous download of HUMS data); and usage monitoring."
The exchange of information between 'The Sultan' and 'flyt3est' was interesting because they both have extensive (design) knowledge of HUMS - and both admit to weaknesses in the system of monitoring of the epicyclic modules. I tend to agree with Sultan, and the message contained in HARP report, that there any number of tools - each of which has its part to play. There is also the issue briefly touched upon in the HARP report that the role of monitoring tools is to deal with unexpected conditions within the inspection and overhaul periods. The setting of such periods, although important, becomes less critical when monitoring is effective – but it must be shown to be effective.
With respect to the S92 accident, no amount of monitoring (using the tools mentioned above) would have prevented that accident - but we have already extensively dealt with that issue in the S92 thread.
Just as we have inspection and overhaul periods in continuing airworthiness, we also need to have confidence that monitoring and early detection of incipient faults will provide sufficient warning to avoid unecessary stress/actions from the pilot in flight. We have also recently reset our tolerence levels to faults extremely low following these two accidents (and hence other threads reporting diversions). We must not put pilots in a position where cockpit indications result in additional hazards; having said that, Sultan's point about fuzz burning and further monitoring is valid.
Unlike the S92, the L2 accident appears to have come out of the blue. It is likely that the precursors to this accident, although subtle, were there; if they were and are pinpointed by the AAIB and EC (as I am sure they will be), the interim policy outlined in the AD provided by EC and EASA will be augmented/replaced by other procedures.
Helicomparator has a valid point that there has been little research into improvement of the HUMS monitoring, and specifically HUMS interpretation, for some time. The fact is that the teams which initially promoted HUMS have exhausted themselves with efforts in ensuring HUMS is implemented and, more importantly, that it is endorsed by the manufacturers (as enthusiastically as it has by Sikorsky). Another factor is that a number of the personalities who were the driving force behind the HUMS initiative have left the scene – some in retirement. HHMAG has also been disbanded since legal competence for airworthiness passed to EASA (and has not been replaced).
In spite of the success of the work of key personalities of the HHMAG and the resultant acceptance of HUMS, it should be noted that although VHM (for larger helicopters) is an existing requirement in the North Sea States and a Recommended practice in ICAO (and work was in progress to include requirement in the JARs by 2012 – which had been accepted by all manufacturers), there is no sign of a requirement in the new EASA OPS Proposal. When these regulations trump the national ones of the UK, Holland, Norway and Ireland, the requirement will be no more.
Why, is that? Is a question we might ask!
Jim
The most complex element of a helicopter, after perhaps the engine, is the transmission gearbox, taking the drive at high speed from one, two or more engines generally in a horizontal plane, and gearing it down by 80 or 100 to 1, or so, to a single vertical shaft carrying the rotor head, with another drive going rearward to a tail rotor (or another rotor head system), the engines having free Wheels, and accessory drives being taken off for generators and oil pumps.
These gearboxes are remarkable examples of the art of the mechanical engineer. Although simplicity is a good starting point in the conceptual stage, complexity is inevitable, and it is not surprising that each designer has found his own solutions. The panel has seen spur gear trains, epicyclics, bevels; it has seen normal straight teeth, 'conformal' tooth profiles; it has seen ball bearings, rollers, tapered rollers, shafts with separate ball races and shafts with rollers running direct on them; we have seen aluminium and magnesium alloy cases. The only common element noted was in the gear material, a traditional low carbon nickel-chrome case hardening steel, either carburised or nitrided, and invariably made of high purity vacuum melted stock.
Notwithstanding the exceptional facilities for gear production we were shown, and the great attention to quality control being paid we are bound to record that in our view it will never be possible to eliminate completely the possibility of failure of some part of so complex a mechanism in service, between specified inspection or overhaul periods.
Although minor surface damage to a gear or bearing is unlikely to be instantly catastrophic, and should be detectable, and although gearboxes are tested to run for a period such as 30 minutes with the oil supply failed, the possibility of a major failure within the box is ever present.
Since with present knowledge a main rotor cannot be duplicated although a tail rotor gearbox might (conceivably a helicopter may survive the failure of the latter) we now come up against the fundamental difference between normal aircraft and rotorcraft -the inability to guard against a possible defect by duplication.
The Panel believes that, while gearbox development and testing should be as stringent as practicable, other means must be introduced to monitor the condition or "health" of the gearbox, preferably in flight. Elementary condition monitoring is already practiced (e.g. oil chip detectors), but much more attention should be paid to this. We make our detail recommendations on this subject later in para. 8.5.
There have been failures of transmission systems taking the drive to the tail rotor, or coupling twin rotor heads. We believe it may be possible to achieve a degree of redundancy here, or at any rate 'damage tolerance'. On the other hand it may be simpler to have some other method of cancelling main rotor torque to use in an emergency (bleed gas jet, rudder surface •••••• )
The exchange of information between 'The Sultan' and 'flyt3est' was interesting because they both have extensive (design) knowledge of HUMS - and both admit to weaknesses in the system of monitoring of the epicyclic modules. I tend to agree with Sultan, and the message contained in HARP report, that there any number of tools - each of which has its part to play. There is also the issue briefly touched upon in the HARP report that the role of monitoring tools is to deal with unexpected conditions within the inspection and overhaul periods. The setting of such periods, although important, becomes less critical when monitoring is effective – but it must be shown to be effective.
With respect to the S92 accident, no amount of monitoring (using the tools mentioned above) would have prevented that accident - but we have already extensively dealt with that issue in the S92 thread.
Just as we have inspection and overhaul periods in continuing airworthiness, we also need to have confidence that monitoring and early detection of incipient faults will provide sufficient warning to avoid unecessary stress/actions from the pilot in flight. We have also recently reset our tolerence levels to faults extremely low following these two accidents (and hence other threads reporting diversions). We must not put pilots in a position where cockpit indications result in additional hazards; having said that, Sultan's point about fuzz burning and further monitoring is valid.
Unlike the S92, the L2 accident appears to have come out of the blue. It is likely that the precursors to this accident, although subtle, were there; if they were and are pinpointed by the AAIB and EC (as I am sure they will be), the interim policy outlined in the AD provided by EC and EASA will be augmented/replaced by other procedures.
Helicomparator has a valid point that there has been little research into improvement of the HUMS monitoring, and specifically HUMS interpretation, for some time. The fact is that the teams which initially promoted HUMS have exhausted themselves with efforts in ensuring HUMS is implemented and, more importantly, that it is endorsed by the manufacturers (as enthusiastically as it has by Sikorsky). Another factor is that a number of the personalities who were the driving force behind the HUMS initiative have left the scene – some in retirement. HHMAG has also been disbanded since legal competence for airworthiness passed to EASA (and has not been replaced).
In spite of the success of the work of key personalities of the HHMAG and the resultant acceptance of HUMS, it should be noted that although VHM (for larger helicopters) is an existing requirement in the North Sea States and a Recommended practice in ICAO (and work was in progress to include requirement in the JARs by 2012 – which had been accepted by all manufacturers), there is no sign of a requirement in the new EASA OPS Proposal. When these regulations trump the national ones of the UK, Holland, Norway and Ireland, the requirement will be no more.
Why, is that? Is a question we might ask!
Jim
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With most of the chip detectors I come by, they worked only on ferrous materials, and of size approximately 100µm and more. As even the electric ones work on principle of collecting chips with magnetic field, and then shorting an electric circuit inside with that said chip. If the chip is not ferrous (coatings, light alloys, polymers etc.) or too small - it will not indicate.
Better chip monitoring systems include spectrography and lubricant flow monitoring... known in aviation industry by common name: "expensive".
Unfortunately in a scenario of protective coating failure it can lead to relatively fast destruction of the gear teeth, although producing small debris - around 20-40µm - for such case only constant gear monitoring system that includes magnetic field monitoring could pick such failure in time... although this require Hall sensors sets on each and every gear, making it very expensive.
It is possible to design an "intelligent" gearbox that will sense any type of failure, and warn crew about it, although I'm not sure anyone would like to pay 3 or more times more for a helicopter than one with "dumb" design.
Better chip monitoring systems include spectrography and lubricant flow monitoring... known in aviation industry by common name: "expensive".
Unfortunately in a scenario of protective coating failure it can lead to relatively fast destruction of the gear teeth, although producing small debris - around 20-40µm - for such case only constant gear monitoring system that includes magnetic field monitoring could pick such failure in time... although this require Hall sensors sets on each and every gear, making it very expensive.
It is possible to design an "intelligent" gearbox that will sense any type of failure, and warn crew about it, although I'm not sure anyone would like to pay 3 or more times more for a helicopter than one with "dumb" design.
The thread kind of drifted a bit. SASless originally asked:
It's probably fate.
In reading the accident report of the BV-234 G-BWFC in 1986, it became clear that the manufacturer had been having issues with that forward transmission since the introduction of the civilian model 234 in 1980. (From that we can surmise that Boeing-Vertol was having the same issues with the CH-47, but were less evident to the general public.) They were working on it; they thought they had a solution.
Nevertheless, the failure of G-BWFC's forward transmission happened so quickly and unexpectedly that the crew had no time to analyze and do something about it. A similar fate seems to have befallen the crew of the Puma that just crashed in the North Sea.
No amount of testing and computer-generated data can predict every failure. A manufacturer can run a transmission in a test rig for hours and hours, but can it accurately simulate all of the inflight and harmonic vibrations and loads that the airframe either generates or is subject to? Hardly.
Helicopters keep finding new and/or different ways of killing us. It occurs to me that no matter how "safe" the manufacturers tell us these products are, those of us who fly these wacky machines on a routine basis are still very much test-pilots, much more so than our fixed-wing counterparts.
That is a sobering thought.
Is it "Fate"....a simple roll of the dice or are we assisted by the system letting us down?
In reading the accident report of the BV-234 G-BWFC in 1986, it became clear that the manufacturer had been having issues with that forward transmission since the introduction of the civilian model 234 in 1980. (From that we can surmise that Boeing-Vertol was having the same issues with the CH-47, but were less evident to the general public.) They were working on it; they thought they had a solution.
Nevertheless, the failure of G-BWFC's forward transmission happened so quickly and unexpectedly that the crew had no time to analyze and do something about it. A similar fate seems to have befallen the crew of the Puma that just crashed in the North Sea.
No amount of testing and computer-generated data can predict every failure. A manufacturer can run a transmission in a test rig for hours and hours, but can it accurately simulate all of the inflight and harmonic vibrations and loads that the airframe either generates or is subject to? Hardly.
Helicopters keep finding new and/or different ways of killing us. It occurs to me that no matter how "safe" the manufacturers tell us these products are, those of us who fly these wacky machines on a routine basis are still very much test-pilots, much more so than our fixed-wing counterparts.
That is a sobering thought.
FH1100 Pilot,
The purpose of collecting data on flights is not to simulate but to measure; and with that measurement comes the ability to spot emerging anomalies (clusters and trends) and predict when failure is about to occur.
The great thing about neural nets is that they support machine learning and do not need the setting of thresholds; eventually, after establishing what normality is, and without human intervention, they can spot abnormality and therefore the potential anomaly.
Perhaps what some of us are bemoaning, is that we saw a demonstration of at least one of these programs (the GE software running over the Bristow data-store) a couple of years ago. In fact we also saw the same software running over the HOMP data. What this can spot is something like a pilot who constantly approaches faster than the rest of the pilots - not fast enough so that it triggers an alert (because thresholds have to be set high enough to avoid nuisance alerts) but a group of data points that sit in a cluster and so become an abnormal pattern.
There will always be faulty parts (bad material, poor machining, incorrect assembly etc) and they will conspire to break outside the normal inspection pattern or before overhaul. The breaking of such parts will always take the occurrence outside the 'extremely remote'. What collection of data and monitoring does is to allow us to eliminate premature failure by spotting the precursors.
In the same way, HOMP (FDM) permits us to identify behaviour patterns that, when isolated are not themselves dangerous but, when put in the mix with other elements might be the final link in the causal chain of the accident.
What most of us really want, is to take the human out of the heavy process that is post flight data analysis; much better that software systems do that for us and then alert when the abnormal pattern is observed - they are much better at this than we are. Leave the humans to the standard intervention (nuts, bolts, mag-plugs, inspection) - they are really good at that.
The problem I have with the arithmetic of a probability of 'extremely remote' is that it has to encompass the knowledge of continuing airworthiness (when do I target my inspection, when do I do my overhaul); hence the target figure is preserved because the faulty element, when found, is removed from the calculation. For a practical example look to the introduction of the EC155 to Nigeria; there never was going to be an engine failure because no engine sat in the aircraft for more than 200 hrs. It is setting these intervention intervals that is the real skill and the one which permits a very small figure like 'extremely remote' to exist.
In an extremely complex system, we can mitigate the errors made in the establishment of such intervals only by monitoring. It was this very point that was the basis of the quote from the HARP report.
Jim
The purpose of collecting data on flights is not to simulate but to measure; and with that measurement comes the ability to spot emerging anomalies (clusters and trends) and predict when failure is about to occur.
The great thing about neural nets is that they support machine learning and do not need the setting of thresholds; eventually, after establishing what normality is, and without human intervention, they can spot abnormality and therefore the potential anomaly.
Perhaps what some of us are bemoaning, is that we saw a demonstration of at least one of these programs (the GE software running over the Bristow data-store) a couple of years ago. In fact we also saw the same software running over the HOMP data. What this can spot is something like a pilot who constantly approaches faster than the rest of the pilots - not fast enough so that it triggers an alert (because thresholds have to be set high enough to avoid nuisance alerts) but a group of data points that sit in a cluster and so become an abnormal pattern.
There will always be faulty parts (bad material, poor machining, incorrect assembly etc) and they will conspire to break outside the normal inspection pattern or before overhaul. The breaking of such parts will always take the occurrence outside the 'extremely remote'. What collection of data and monitoring does is to allow us to eliminate premature failure by spotting the precursors.
In the same way, HOMP (FDM) permits us to identify behaviour patterns that, when isolated are not themselves dangerous but, when put in the mix with other elements might be the final link in the causal chain of the accident.
What most of us really want, is to take the human out of the heavy process that is post flight data analysis; much better that software systems do that for us and then alert when the abnormal pattern is observed - they are much better at this than we are. Leave the humans to the standard intervention (nuts, bolts, mag-plugs, inspection) - they are really good at that.
The problem I have with the arithmetic of a probability of 'extremely remote' is that it has to encompass the knowledge of continuing airworthiness (when do I target my inspection, when do I do my overhaul); hence the target figure is preserved because the faulty element, when found, is removed from the calculation. For a practical example look to the introduction of the EC155 to Nigeria; there never was going to be an engine failure because no engine sat in the aircraft for more than 200 hrs. It is setting these intervention intervals that is the real skill and the one which permits a very small figure like 'extremely remote' to exist.
In an extremely complex system, we can mitigate the errors made in the establishment of such intervals only by monitoring. It was this very point that was the basis of the quote from the HARP report.
Jim
Last edited by JimL; 12th Apr 2009 at 17:46.
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Someone had mentioned that titanium is a fragile metal. I’m not a metallurgist by any means but it seems that every Blackhawk crash that I have seen, the rotorhead is wholly intact. By the way, the entire rotorhead is made from titanium. I’m only making an observation, not a conclusion.
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Titanium alloys may have low strength - lower then Aluminium ones, although only when the % of titanium is higher than 98% - those almost pure Titanium alloys are used only in medicine, as are extremely resistant to corrosion. Not useful for aviation industry though - here are usually alloys with less than 90% Titanium, with the rest being Aluminium, Vanadium, Chromium etc. Those alloys are very tough, light, and both corrosion and heat resistant. The only reason why aircrafts are not made entirely of Titanium alloys (Black Bird and X-15 excluded) is because of high cost of the TiO ore, and manufacturing - as it is not very efficient - milling is very expensive because of high material strength, welding was not possible on the wider scale until 1990s, and casting is problematic because of other materials usage, and time.
I played with few things made of titanium alloys, and those were almost indestructible. Also take a look at the wreckage pictures of F-14s - main central wing spar that is also a main fuel tank and the whole aircraft hang from it - is always intact. I can't imagine that Titanium parts designed with aviation industry safety factors could be broken under normal operation loads, although as we all know it happened few times already, but I don't believe the material selection had anything to do with it.
I played with few things made of titanium alloys, and those were almost indestructible. Also take a look at the wreckage pictures of F-14s - main central wing spar that is also a main fuel tank and the whole aircraft hang from it - is always intact. I can't imagine that Titanium parts designed with aviation industry safety factors could be broken under normal operation loads, although as we all know it happened few times already, but I don't believe the material selection had anything to do with it.
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Sometimes I think manufacturers are stuck in very old ways of solving problems. Gearboxes being one of them.
It would obviously be fully feasible to design a rotor system without a metal-against-metal-soaked-in-oil grinding gearbox - either with belts or by having a bigger diameter turbine stage and direct drive the rotor.
Maybe manufacturers of big helicopters should take a look at how far the RC heli's rotor systems have improved over the years.
It would obviously be fully feasible to design a rotor system without a metal-against-metal-soaked-in-oil grinding gearbox - either with belts or by having a bigger diameter turbine stage and direct drive the rotor.
Maybe manufacturers of big helicopters should take a look at how far the RC heli's rotor systems have improved over the years.
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For a practical example look to the introduction of the EC155 to Nigeria; there never was going to be an engine failure because no engine sat in the aircraft for more than 200 hrs.
Nigeria is not the only place the EC155 was introduced and it has been more successful (with much longer engine 'sitting' times) in those places. So there was still the probability of an engine failure with that aircraft/ engine type, just not in Nigeria.
Thread Starter
What is the technical reason that prevents putting a fluid level sensor in a gearbox sump and connecting it to at least a warning light or contents gauge and warning light?
I have never heard of one on a helicopter transmission ever thus there must be a good reason why would not work.
I have never heard of one on a helicopter transmission ever thus there must be a good reason why would not work.
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I think the reason theres no xmsn fluid indicator is the oil is really getting thrown around in there and it may give false readings. If youve ever watched a sight glass in a gearbox with the rotors turning youll know what i mean.
Thats the problem with using a sump as a reservoir
Theres also the problem of blocked jets and oil starved bearings which would not be picked up by a fluid level.
Thats the problem with using a sump as a reservoir
Theres also the problem of blocked jets and oil starved bearings which would not be picked up by a fluid level.
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those of us who fly these wacky machines on a routine basis are still very much test-pilots, much more so than our fixed-wing counterparts.
That is a sobering thought.
That is a sobering thought.
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I personally feel that given the types of people who frequent our industry, with their engineering know- how, HUMS knowledge, pilots and those with experience and influence in regulatory spheres, the biggest crime here would be to fail to seize the current situation and use it as a catalyst to kick new life into flight safety initiatives.
My biggest fear is that this will not happen, and in 12 months, this whole tragic episode will be forgotten, and we will not have learned and improved to the best of our collective abilities. That would be a failure.
My biggest fear is that this will not happen, and in 12 months, this whole tragic episode will be forgotten, and we will not have learned and improved to the best of our collective abilities. That would be a failure.
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JimL, your post #27 has that piece of information which has caused some measure of debate. I've got the scars to show for it!!!! 
I know context is everything, but its little wonder people may have erroneous beliefs.
I know context is everything, but its little wonder people may have erroneous beliefs.
Although minor surface damage to a gear or bearing is unlikely to be instantly catastrophic, and should be detectable, and although gearboxes are tested to run for a period such as 30 minutes with the oil supply failed, the possibility of a major failure within the box is ever present.
Brian,
Perhaps some clarification is necessary here; at the time that this report was produced - i.e November 1984, the members of the HARP (a committee set up by the Airworthiness Requirements Board (ARB)) would have been aware of the contents of the proposed amendment to FAR 29 but would have had no direct interest because Certification in the UK was to an alternative set of requirements - i.e. BCARS. (Without checking, I would make the assumption that this requirement was contained in BCARS. It was also not clear at that time that the UK would eventually be forced to abandon BCARS in favour of FARs (in the form of harmonised JARs).)
Perhaps a more interesting fact is that in the proposed NPRM, the text was slightly different.
By the time the requirement was finalised as amendment 29-26 in 1988, the rule contained the conditioning text we now see. Normally one can examine the explanatory text of the final rule and establish why the text has been subsequently amended. In the case of the text of 29.927(c), the explanatory text is as follows:
The 'law of unintended consequences' in action?
By putting in this clarification, there is no intent to reopen the discussion on the 30 minute run-dry capability (and the reason why it was not underlined in my previous post) only to clarify the extract from the HARP report.
Jim
Perhaps some clarification is necessary here; at the time that this report was produced - i.e November 1984, the members of the HARP (a committee set up by the Airworthiness Requirements Board (ARB)) would have been aware of the contents of the proposed amendment to FAR 29 but would have had no direct interest because Certification in the UK was to an alternative set of requirements - i.e. BCARS. (Without checking, I would make the assumption that this requirement was contained in BCARS. It was also not clear at that time that the UK would eventually be forced to abandon BCARS in favour of FARs (in the form of harmonised JARs).)
Perhaps a more interesting fact is that in the proposed NPRM, the text was slightly different.
3-46. By amending Sec. 29.927 by revising paragraphs (c), (d) (introductory text), (d)(2), and by adding new paragraph (f) to read as follows:
Sec. 29.927 Additional tests.
* * * * *
(c) Lubrication system failure. For lubrication systems, the function of which is required for operation of the rotor drive system, the following apply:
(1) Category A. It must be shown by tests that each rotor drive system, where the probable failure of any element could result in the loss of lubricant, is capable of continued operation, although not necessarily without damage, for a period of at least 30 minutes at a torque and rotational speed prescribed by the applicant for continued flight, after indication to the flightcrew of the loss of lubricant.
Sec. 29.927 Additional tests.
* * * * *
(c) Lubrication system failure. For lubrication systems, the function of which is required for operation of the rotor drive system, the following apply:
(1) Category A. It must be shown by tests that each rotor drive system, where the probable failure of any element could result in the loss of lubricant, is capable of continued operation, although not necessarily without damage, for a period of at least 30 minutes at a torque and rotational speed prescribed by the applicant for continued flight, after indication to the flightcrew of the loss of lubricant.
Proposal 3-46. The notice proposed several amendments to Sec. 29.927. Paragraph (c) is changed by revising and extending the rotor drive system lubrication failure test requirements for Category A rotorcraft and by clarifying the corresponding test requirements for Category B rotorcraft. Category A aircraft must have significant continued flight capability after a failure in order to optimize eventual landing opportunities. However, indefinite flight following the lubrication system failure is not expected. The changes to the Category B rotorcraft drive system lubrication failure test requirements are largely for clarification and are not substantive.
A commenter notes that paragraph (c), as proposed, could be interpreted to preclude credit for auxiliary lubrication systems or to require consideration of lubricant failures to self-lubricated bearings. This was not intended, and the wording of paragraphs (c)(1) and (c)(2) has been revised to eliminate this possible ambiguity.
A commenter notes that paragraph (c), as proposed, could be interpreted to preclude credit for auxiliary lubrication systems or to require consideration of lubricant failures to self-lubricated bearings. This was not intended, and the wording of paragraphs (c)(1) and (c)(2) has been revised to eliminate this possible ambiguity.
By putting in this clarification, there is no intent to reopen the discussion on the 30 minute run-dry capability (and the reason why it was not underlined in my previous post) only to clarify the extract from the HARP report.
Jim
Last edited by JimL; 13th Apr 2009 at 07:53. Reason: Correction of Grammar
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Sultan wrote:
To detect a crack the only concept I can see as practicable would be a delta resistance detection system of some kind. (thermal or magnetic resonance imaging being cited as too costly above). Strain and stress guages have been around for a long time. In simple terms I envisage a system that runs a current through the gearbox from entry to exit and gives a resistance level readout which would have a known behavioural pattern for load/torque/temp settings. A crack or significant fatigue (chip loss)would hopefully give a warning fractionally in advance of chip /temp/pressure warning systems.
Does anyone know of such a system?
The added benefit of such a system is that it may be applied in non magnetic material systems.. as long as they conduct a current. (Ceramics can be made conductive or not as required.. I believe.
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.
Does anyone know of such a system?
The added benefit of such a system is that it may be applied in non magnetic material systems.. as long as they conduct a current. (Ceramics can be made conductive or not as required.. I believe.
