Heli ditch North Sea G-REDL: NOT condolences
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Unfortunately when it comes to HUMs limits the intial values set by the manufacturer can be miles out and only through operational experience will accurate limit values be found.
That doesnt mean testing to destruction but a gradual increase in limits from very low intial values. The idea is that the limits are accurate components coming back to the shop for failing vibe levels are actually buggered not 90% expensive false alarms.
Only then can you have faith in HUMs.
B412 BHVMS is a good example, Bell had lots of knowledge of vibration levels from the Canadian Griffon HUMs system but as it was military technology they werent allowed to use it for setting civilian values.
SO we all went through a few years, still going a little bit, of pulling stuff off that was pefectly fine.
HUMs is only one tool and its not to be followed with blind faith.
Its all about the interpretation.
That doesnt mean testing to destruction but a gradual increase in limits from very low intial values. The idea is that the limits are accurate components coming back to the shop for failing vibe levels are actually buggered not 90% expensive false alarms.
Only then can you have faith in HUMs.
B412 BHVMS is a good example, Bell had lots of knowledge of vibration levels from the Canadian Griffon HUMs system but as it was military technology they werent allowed to use it for setting civilian values.
SO we all went through a few years, still going a little bit, of pulling stuff off that was pefectly fine.
HUMs is only one tool and its not to be followed with blind faith.
Its all about the interpretation.
Again it appears the MGB was in alert so that to me tells me that the thresholds were set correctly
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Yeah good point WAH.
I do think its all about the way you interpret the data, not every tiny increase in vibration is worthy of a component replacment.
HUMs is not the be-all and end-all people think.
I do think its all about the way you interpret the data, not every tiny increase in vibration is worthy of a component replacment.
HUMs is not the be-all and end-all people think.
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I also totally disagree with the above posters comments regarding the HUMs guys and their decisions.
Before we start blame storming maybe we should collect all the facts.
Before we start blame storming maybe we should collect all the facts.
I perhaps should wait until all the facts are reported on before making judgement. I just feel that with prior warning to this,then the families of the deceased are not going to be taking this lightly.
I do not think i have stated anything factually incorrect, this is an open forum where we are discussing what happened.
Hums is not the be all and end all, perhaps not, but in this case, it's the only tool available for detection of MGB faults. The pilot would not have spotted it on his walk round, nor would the engineer have spotted it on his pre flight inspection.
I'll let others have their say now, i've made my point.
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Back off guys - some of you are jumping to premature conclusions again.
Although there was a chip detected some 25 hours before the event, and enhanced HUMS monitoring set up, there is nothing in the report to indicate that this is conected to the catastrophic failure. It may be but equally it may not.
Let's wait until the experts come up with the goods.
Although there was a chip detected some 25 hours before the event, and enhanced HUMS monitoring set up, there is nothing in the report to indicate that this is conected to the catastrophic failure. It may be but equally it may not.
Let's wait until the experts come up with the goods.
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IHUMS again.
The IHUMS information is only as good as the interpretation thereof. It is obvious that while a chip can increase vibration levels a crack may show nothing until it has changed the characteristics of the involved component.
In the case of the 330J in Borneo there was no IHUMS involved. That gearbox has a magnetic plug not a chip detector so the pilot had no warning of an increase in debris. A Soap sample has been taken just before the failure and did not trigger any warning although there were chips of scale floating in the sample. They do not show in the analysis.
The mechanics of that failure were that a barrel roller in one of the epicyclic gear bearing was shedding chips and did so until the reduction in diameter released the roller which went upwards them came down between the planetery gears and the ring gear resulting in the ring gear cracking and departing.resulting in the accident.
In the case of the 330J in Borneo there was no IHUMS involved. That gearbox has a magnetic plug not a chip detector so the pilot had no warning of an increase in debris. A Soap sample has been taken just before the failure and did not trigger any warning although there were chips of scale floating in the sample. They do not show in the analysis.
The mechanics of that failure were that a barrel roller in one of the epicyclic gear bearing was shedding chips and did so until the reduction in diameter released the roller which went upwards them came down between the planetery gears and the ring gear resulting in the ring gear cracking and departing.resulting in the accident.
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A little restraint.
WAH,
I read the quote, below, from the report slightly differently to you (I believe).
I believe the HUMS data that the report is referring to is that from moments prior to the failure. Data from the accident flight. Data recovered from the wreckage. Not information available prior to the flight.
If this is the case, there has been no evidence, yet, of any HUMS trend alerting the engineering staff to any abnormalities.
Correct me if I am mistaken.
I read the quote, below, from the report slightly differently to you (I believe).
The investigation has so far revealed that the MGB had suffered from a major failure within the epicyclic module. This is supported by HUMS data; however, this is not yet fully understood.
If this is the case, there has been no evidence, yet, of any HUMS trend alerting the engineering staff to any abnormalities.
Correct me if I am mistaken.
Wah - the report states -
"The HUMS data for the day’s operation, including the accident flight, has also been recovered"
This information would not have been available to any HUMS team prior to the accident and it may well be that the 'supporting evidence' was from this period!
How about editing your comments to remove the slanders?
TeeS
"The HUMS data for the day’s operation, including the accident flight, has also been recovered"
This information would not have been available to any HUMS team prior to the accident and it may well be that the 'supporting evidence' was from this period!
How about editing your comments to remove the slanders?
TeeS
Flyt3est et al
The point is that its having to set thresholds that is the problem - no-one knows where the threshold should be until after the disaster. Indeed some failure modes can cause some parameters to decrease, not increase.
This is the relevance of the Neural Networking analysis trial being carried out by the CAA & GE (was Smiths). I will have to do my homework to find out just where we are with that one, but the concept is that the output data from the HUMS is fed into a neural network - a piece of software that mimics the human brain in the way it works. You teach it what a normal, healthy gearbox looks like, then it will identify a bad gearbox without having to be told thresholds etc. I would liken it to how a doctor could look at a person's face and see that they were ill. If you tried to describe mathmatically what he was seeing, it would be very difficult, but because he has learned what a healthy person looks like, he knows what an unhealthy person looks like.
I read an interim report on the technique, it seems to be very sucessful.
Regarding the design of the 332L / L2 and 225, the gearboxes are very similar in layout and design. I think the 332L and L2 are identical, with just some increased case hardening of the gears for the L2. For the 225, the external casing of the epicyclic stages looks completely different, presumably this being to make the casing stronger to deal with the greatly increased torque (cruise power on a 225 is more than both engines of a 332L running at max contingency). I am not sure how different the internals are, obviously they are beefed up somehow. Bearing in mind the above, its interesting to note that the ASB / AD relates to the L2 and 225, but not to the L.
They L2 has a manual chip detector for the epicyclic stages, the 225 has a remote-reading electrical one. We have had very few transmission chip problems with the 225 compared to the 332L, the 225's weak spot seems to be the mast bearing - we have had to change some heads due to chip detection there.
Regarding the engine turbine burst, this cannot be surprising since the overspeed shutdown mechanism specifically disables overspeed shutdown on the second engine, once the first has detected an overspeed. It was designed on the premise that an overspeed would be caused by disconnection of the engine drive shaft or freewheel, not a complete disconnection of the head from the transmission affecting both engines. I never liked this concept because I thought that double engine failure was prefereable to uncontained rise in N2 or Nr.
The 225 has inherent overspeed shutdown for both engines - its blade shedding concept which sheds the N2 blades in a controlled way (they are contained within the engine shielding) at around 135%N2, so this burst would not have occurred on the 225, but once the head is lost, that would be rather academic.
HC
The point is that its having to set thresholds that is the problem - no-one knows where the threshold should be until after the disaster. Indeed some failure modes can cause some parameters to decrease, not increase.
This is the relevance of the Neural Networking analysis trial being carried out by the CAA & GE (was Smiths). I will have to do my homework to find out just where we are with that one, but the concept is that the output data from the HUMS is fed into a neural network - a piece of software that mimics the human brain in the way it works. You teach it what a normal, healthy gearbox looks like, then it will identify a bad gearbox without having to be told thresholds etc. I would liken it to how a doctor could look at a person's face and see that they were ill. If you tried to describe mathmatically what he was seeing, it would be very difficult, but because he has learned what a healthy person looks like, he knows what an unhealthy person looks like.
I read an interim report on the technique, it seems to be very sucessful.
Regarding the design of the 332L / L2 and 225, the gearboxes are very similar in layout and design. I think the 332L and L2 are identical, with just some increased case hardening of the gears for the L2. For the 225, the external casing of the epicyclic stages looks completely different, presumably this being to make the casing stronger to deal with the greatly increased torque (cruise power on a 225 is more than both engines of a 332L running at max contingency). I am not sure how different the internals are, obviously they are beefed up somehow. Bearing in mind the above, its interesting to note that the ASB / AD relates to the L2 and 225, but not to the L.
They L2 has a manual chip detector for the epicyclic stages, the 225 has a remote-reading electrical one. We have had very few transmission chip problems with the 225 compared to the 332L, the 225's weak spot seems to be the mast bearing - we have had to change some heads due to chip detection there.
Regarding the engine turbine burst, this cannot be surprising since the overspeed shutdown mechanism specifically disables overspeed shutdown on the second engine, once the first has detected an overspeed. It was designed on the premise that an overspeed would be caused by disconnection of the engine drive shaft or freewheel, not a complete disconnection of the head from the transmission affecting both engines. I never liked this concept because I thought that double engine failure was prefereable to uncontained rise in N2 or Nr.
The 225 has inherent overspeed shutdown for both engines - its blade shedding concept which sheds the N2 blades in a controlled way (they are contained within the engine shielding) at around 135%N2, so this burst would not have occurred on the 225, but once the head is lost, that would be rather academic.
HC
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TeeS
I suppose it depends on your interpretation of the quote.
I read it as the epicyclic was already under close monitoring and had been a week prior to the accident. However i do conceed that this may be totally unrelated to why the gearbox failed. I find it unlikely, but it is possible. I say that, as any sort of defect would be picked up by the system in some way shape or form. If a gear crack is developing, HUMS will pick up on this, the noise signature would change, the amount of energy in the data would change, the meshing tones and frequencies would change. Gearboxes dont just fail out of the blue, something has to cause it too. (i.e something has developed over x amount of time)
If it was indicating the same fault that downed this AC beforehand then i stand by my comment that HUMS was doing what it was supposed to be doing. If it was something totally unrelated then HUMS needs developing further.
Jetboxer....The aircraft was under 25FH close monitoring and daily chip det inspections so engineering were aware of a potential MGB defect prior to flight. (We do not know if its related though) That is factual now.
My conclusion, all i can gather from this is that EC/HUMS dpt were aware of a potential problem and in hindsight they should have ordered a boroscope inspection. (Yes it's easy to say that now, but they are the experts) I do not think any individual is to blame, nor am i pointing the finger. I do realise that a boroscope inspection everytime a MGB exceedance is generated is totally unrealistic, but in this case if that advice was passed on to engineering i would like to think that this accident would never have happened.
I dont know if im being too harsh here or not, apologies if i am offending anyone. I suppose i am just thinking out loud.
I suppose it depends on your interpretation of the quote.
I read it as the epicyclic was already under close monitoring and had been a week prior to the accident. However i do conceed that this may be totally unrelated to why the gearbox failed. I find it unlikely, but it is possible. I say that, as any sort of defect would be picked up by the system in some way shape or form. If a gear crack is developing, HUMS will pick up on this, the noise signature would change, the amount of energy in the data would change, the meshing tones and frequencies would change. Gearboxes dont just fail out of the blue, something has to cause it too. (i.e something has developed over x amount of time)
If it was indicating the same fault that downed this AC beforehand then i stand by my comment that HUMS was doing what it was supposed to be doing. If it was something totally unrelated then HUMS needs developing further.
Jetboxer....The aircraft was under 25FH close monitoring and daily chip det inspections so engineering were aware of a potential MGB defect prior to flight. (We do not know if its related though) That is factual now.
My conclusion, all i can gather from this is that EC/HUMS dpt were aware of a potential problem and in hindsight they should have ordered a boroscope inspection. (Yes it's easy to say that now, but they are the experts) I do not think any individual is to blame, nor am i pointing the finger. I do realise that a boroscope inspection everytime a MGB exceedance is generated is totally unrealistic, but in this case if that advice was passed on to engineering i would like to think that this accident would never have happened.
I dont know if im being too harsh here or not, apologies if i am offending anyone. I suppose i am just thinking out loud.
Last edited by WAH; 11th Apr 2009 at 09:02.
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I have read the report and it is truly horrific!!
Surely the time has come to "Ramp Up" the HUMS system and integrate a warning system driven by it, into the cockpit.
As AAIB say the failure is supported by the HUMS data (I assume for that flight leading up to the accident) such a system may well have produced a significantly different outcome.
Surely the time has come to "Ramp Up" the HUMS system and integrate a warning system driven by it, into the cockpit.
As AAIB say the failure is supported by the HUMS data (I assume for that flight leading up to the accident) such a system may well have produced a significantly different outcome.
HC,
I'm not sure if I understand, don't you have a MGB CHIP light in the 332L2 ?
Regards
Aser
They L2 has a manual chip detector for the epicyclic stages, the 225 has a remote-reading electrical one.
Regards
Aser
Aser, sorry if I was not clear. The 332L2 has an MGB chip light, but its detector is in the sump at the bottom. There is (at least) one other chip detector higher up, designed to catch any chips coming from the epicyclic stage, but that is not electrical - you have to take it out to visually check for chips.
On the 225, there are 3 chip detectors, one in the sump, one for the epicyclic and one for the mast bearing, all 3 are remote reading and can bring on the MGB Chip light.
HC
On the 225, there are 3 chip detectors, one in the sump, one for the epicyclic and one for the mast bearing, all 3 are remote reading and can bring on the MGB Chip light.
HC
Double Bogey
Effective epicyclic gear diagnostics has not been developed and proofed in an operational role. The last report I saw from the CAA was that HUMS missed or false detected 70% of epicyclic faults. This would result in three falses for every true. With this rate coupled with cockpit alarms the SAR guys would stay busy.
As HC a lot of research is ongoing to improve this, but their is a long way to go. A recent test where an epicyclic fault was introduced in a bench test clearly showed the fault. However, when put on an aircraft the rotor vibration hid the signature. It is amusing that this data was shared with a major HUMS supplier along with data from a good transmission. The vendor was not told which was which. In their final report they showed conclusively that their advanced diagnostics could reliably detect the fault. The only problem is they identified the good transmission as having the fault.
The Sultan
Effective epicyclic gear diagnostics has not been developed and proofed in an operational role. The last report I saw from the CAA was that HUMS missed or false detected 70% of epicyclic faults. This would result in three falses for every true. With this rate coupled with cockpit alarms the SAR guys would stay busy.
As HC a lot of research is ongoing to improve this, but their is a long way to go. A recent test where an epicyclic fault was introduced in a bench test clearly showed the fault. However, when put on an aircraft the rotor vibration hid the signature. It is amusing that this data was shared with a major HUMS supplier along with data from a good transmission. The vendor was not told which was which. In their final report they showed conclusively that their advanced diagnostics could reliably detect the fault. The only problem is they identified the good transmission as having the fault.
The Sultan
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Chip Detector-Electrical signal sent to CWP when chips are present
Mag Plug- Non electrical, collects ferrous material, must be removed for inspection.
Are we to say that the L2 has no chip detector for the Epicyclic gears only a mag plug..
Mag Plug- Non electrical, collects ferrous material, must be removed for inspection.
Are we to say that the L2 has no chip detector for the Epicyclic gears only a mag plug..
Thanks rapman, your terminlogy is what I should have used in the first place!
No, there is no Chip detector for the epicyclic on the L or L2, only a mag plug. However, its all in the same oil system so chips from the epicyclic can still make their way to the chip detector in the sump. It would be interesting to know to what extent the mag plug captures epicyclic chips, thus denying them their rightful place on the sump chip detector.
However, in the case of a crack in an epicyclic gear etc, its unlikely that much debris is released until the part actually fails, so the absence of a chip detector is not necessarily a factor in this accident - but it might have given them a light 2 seconds before catastrophy!
HC
No, there is no Chip detector for the epicyclic on the L or L2, only a mag plug. However, its all in the same oil system so chips from the epicyclic can still make their way to the chip detector in the sump. It would be interesting to know to what extent the mag plug captures epicyclic chips, thus denying them their rightful place on the sump chip detector.
However, in the case of a crack in an epicyclic gear etc, its unlikely that much debris is released until the part actually fails, so the absence of a chip detector is not necessarily a factor in this accident - but it might have given them a light 2 seconds before catastrophy!
HC
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Lots of talk here about HUMS not detecting the problem that led to this catastrophy.
The AS356 gearbox has had occurances of cracks being found in the epicyclic gearing, during overhaul. There was no history of 'chips' as a result of the cracking.
The manufacturer implemented a 50 hr boroscope inspection of the gearing.
My point is that HUMS is an additional tool/ aid to early fault detection, and not the all seeing, answer to everything that some people think it is.
The AS356 gearbox has had occurances of cracks being found in the epicyclic gearing, during overhaul. There was no history of 'chips' as a result of the cracking.
The manufacturer implemented a 50 hr boroscope inspection of the gearing.
My point is that HUMS is an additional tool/ aid to early fault detection, and not the all seeing, answer to everything that some people think it is.
I would suggest that a detailed borescope inspection of an epicyclic geartrain in situ is almost impossible. Attempting to keep track of the numerous gears all rotating at the same time would be nigh on impossible and as to the bearings most would have no access and would be effectively invisible.
It might pick up a large crack in a ring gear carrier or similar but finding anything else would be by chance.
It might pick up a large crack in a ring gear carrier or similar but finding anything else would be by chance.
Special 25.
I agree. As a long time offshore SLF, my (our) nightmare too. Your calculation was the same as mine. Shivers down my spine.
Well done AAIB for the speed of release of this initial information.
And to those who so quickly jump on to HUMS etc. & perhaps by implication may be criticising the engineers' judgment, I suggest you please read HeliComparators note - and those of others such as Sultan who have expertise in this area - very carefully.
I too wish we could be perfect in our predictions. It's not possible. Sometimes we err.. And if all that happens is that serviceable machines are constantly withdrawn for checks that find nothing, eventually we have the "boy who cried wolf" position. The engineers are in an unenviable situation at times.
And every time we extend the capability of equipment we expose more & slightly different potential risks. This gearbox is an uprated version, broadly speaking, of a previous one. Slightly different and hopefully better techniques for manufacture and maintenance are developed. Sometimes we find unexpected outcomes and have to revise our methods. So we inch forward. Sometimes at much greater cost than we'd wish.
I agree. As a long time offshore SLF, my (our) nightmare too. Your calculation was the same as mine. Shivers down my spine.
Well done AAIB for the speed of release of this initial information.
And to those who so quickly jump on to HUMS etc. & perhaps by implication may be criticising the engineers' judgment, I suggest you please read HeliComparators note - and those of others such as Sultan who have expertise in this area - very carefully.
I too wish we could be perfect in our predictions. It's not possible. Sometimes we err.. And if all that happens is that serviceable machines are constantly withdrawn for checks that find nothing, eventually we have the "boy who cried wolf" position. The engineers are in an unenviable situation at times.
And every time we extend the capability of equipment we expose more & slightly different potential risks. This gearbox is an uprated version, broadly speaking, of a previous one. Slightly different and hopefully better techniques for manufacture and maintenance are developed. Sometimes we find unexpected outcomes and have to revise our methods. So we inch forward. Sometimes at much greater cost than we'd wish.
Last edited by biscuit74; 11th Apr 2009 at 10:48.
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You are right, and to be more specific in the case of 365 g/boxes, the cracks were in the carrier.
What I was trying to say was that some faults aren't necessarily detectable (or very difficult to detect) with HUMS and that other methods are sometimes needed for fault detection.
What I was trying to say was that some faults aren't necessarily detectable (or very difficult to detect) with HUMS and that other methods are sometimes needed for fault detection.
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Eric..The Eurohums groundstation would pinpoint where in the epicyclic to focus your attention in regards to a boroscope inspection on the actual gearbox. Epicyclic is a big area, you can pinpoint it down to ring, planets, sun gears etc from what the HUMS is pointing at. You wouldn't just go in blind looking for a crack.
It's incorrect stating that it would be next to useless as i myself have found damage to internal gearing through boroscope inspections, 2 IGB's 1 MGB and 3 TGB's in the 10 or so years ive been doing this job. None had HUMS fitted, just inspections of magnetic probes.
As to chip detector/mag probes, they are not going to pick up gear cracks, very little debris if any will be given off.
It's incorrect stating that it would be next to useless as i myself have found damage to internal gearing through boroscope inspections, 2 IGB's 1 MGB and 3 TGB's in the 10 or so years ive been doing this job. None had HUMS fitted, just inspections of magnetic probes.
As to chip detector/mag probes, they are not going to pick up gear cracks, very little debris if any will be given off.