Heli ditch North Sea G-REDL: NOT condolences
G-REDL Accident Report Published
The long awaited accident report concerning G-REDL is published today.
Air Accidents Investigation: Formal Report 2/2011 published
The document itself is located at http://www.aaib.gov.uk/cms_resources...1%20G-REDL.pdf
Air Accidents Investigation: Formal Report 2/2011 published
The document itself is located at http://www.aaib.gov.uk/cms_resources...1%20G-REDL.pdf
Why the silence?
The Conclusions section raises some very interesting questions re certification and design assumptions.
Some topics that jumped out at me had to do with the location and operation of the Magnetic Chip Detectors....and the EC Engineering assumption that gearbox failures would always produce spalling.
Also interesting is the note about the decision to end Oil Sampling and analysis of the samples back in the mid-80's....or did I misunderstand what that reference was all about?
It is interesting that the FAR's get referenced....for a EC product.
The Conclusions section raises some very interesting questions re certification and design assumptions.
Some topics that jumped out at me had to do with the location and operation of the Magnetic Chip Detectors....and the EC Engineering assumption that gearbox failures would always produce spalling.
Also interesting is the note about the decision to end Oil Sampling and analysis of the samples back in the mid-80's....or did I misunderstand what that reference was all about?
It is interesting that the FAR's get referenced....for a EC product.
Yes does seem a bit quiet. I suppose one would not like to be in the engineering team that were dealing with the chip issue, perhaps there is some desire not to give them a harder time than they will already be having.
The reason for the FAR references I suppose is that FAR29 and CS29 are virtually identical so no point in trying to change one without the other. There are of course a few FAA certified EC225s (but no L2s as far as I know) around.
One thing that did not get a mention (unless I missed it) is the language issue. EC of course have very much better English than we do French, however I routinely see misinterpretations on both sides. One only has to read EC's documentation to realise that it is not written by someone who has English as their first language.
At present all the documentation is written by the specialists in French, then translated into English, perhaps by someone who is a translator, not a technical specialist. Thereby creating the potential for misinterpretation etc. Interestingly we did receive a visit from the EC guy in charge of Flight Manuals a year or so ago - he said that the longer term plan was to have all the documents written originally in English, then translated into French as required. Sounds like a good plan but then he was English, so not sure how easy it will be for him to achieve that!
The language issue is surely good grounds for having solid and written dialogue with the manufacturer on critical issues. Yes as I said the written word can be ambiguous but it is surely better than the spoken word!
If fingers are pointed at the individual engineers concerned, that will be unfortunate because just like so many aviation accidents, the root cause is not the momentary action by the indivdual, rather the failure by the company's managers to ensure robust procedures and systems are in place with adequate monitoring for compliance.
The reason for the FAR references I suppose is that FAR29 and CS29 are virtually identical so no point in trying to change one without the other. There are of course a few FAA certified EC225s (but no L2s as far as I know) around.
One thing that did not get a mention (unless I missed it) is the language issue. EC of course have very much better English than we do French, however I routinely see misinterpretations on both sides. One only has to read EC's documentation to realise that it is not written by someone who has English as their first language.
At present all the documentation is written by the specialists in French, then translated into English, perhaps by someone who is a translator, not a technical specialist. Thereby creating the potential for misinterpretation etc. Interestingly we did receive a visit from the EC guy in charge of Flight Manuals a year or so ago - he said that the longer term plan was to have all the documents written originally in English, then translated into French as required. Sounds like a good plan but then he was English, so not sure how easy it will be for him to achieve that!
The language issue is surely good grounds for having solid and written dialogue with the manufacturer on critical issues. Yes as I said the written word can be ambiguous but it is surely better than the spoken word!
If fingers are pointed at the individual engineers concerned, that will be unfortunate because just like so many aviation accidents, the root cause is not the momentary action by the indivdual, rather the failure by the company's managers to ensure robust procedures and systems are in place with adequate monitoring for compliance.
ISTR the "similar accident" referred to in the report was SA330J 9M-SSC in Miri where the language barrier blurred the interpretation of debris amounting to 50mm2 was an issue - ie/ was it 5 x 10mm or 50 x 50mm.
HC,
It not just French/English conversions where language lets us down but also in English/English (in this case English/American).
My use of the word "Engineer" is in the American usage....as in Design Engineers and not the British usage which in American is "Mechanic"....those that use hand tools etc...in the maintenance and repair of things.
I particularly note the issues raised about the assumptions all serious MGB failures would produce "spalling", that there would be adequate oil flow to move any foreign matter in the oil within reach of magnetic sensors, and the decision that oil sampling was no longer necessary.
We are talking about possibly the most important component of a helicopter when we talk of Main Gear Boxes....yet we are confronted with decisions and designs that fail to meet all contingencies.
How many times we heard that kind of comment about the S-92 MGB design?
All I am looking for here is the exact same kind of analysis by those that are so critical of the Sikorsky S-92 and its Engineers. No one gave a damn about their feelings....why should we treat the French Engineers any differently?
The discussion is all about the design, its flaws, and how to correct them and the certification system so we have "safe" aircraft to fly.
IMHO.....until the Gearboxes in question are re-designed to correct their shortcoming in detecting such failures...identify all foreign objects in the oil the come from gear/bearing/race wear and cracking....and oil analysis is re-instituted as a standard practice...and less reliance on HUMs as the main Gearbox Health measure is a standard engineering practice, I would suggest the industry has not adequately addressed the situation.
Now HC....care to discuss the technical issues raised in the Report?
In your view...what do they tell us about the current state of EC MGB design and safety monitoring of the gearboxes by flight crews, maintenance crews, and the Manufactuer?
Where do we go from here to improve the situation?
As to worrying about the feelings of people....how about we get concerned about the feelings of the next of kin of those that get killed in a second occurrence of this kind of failure.....or will it be the third occurrence if we accept the Report's findings?
It not just French/English conversions where language lets us down but also in English/English (in this case English/American).
My use of the word "Engineer" is in the American usage....as in Design Engineers and not the British usage which in American is "Mechanic"....those that use hand tools etc...in the maintenance and repair of things.
I particularly note the issues raised about the assumptions all serious MGB failures would produce "spalling", that there would be adequate oil flow to move any foreign matter in the oil within reach of magnetic sensors, and the decision that oil sampling was no longer necessary.
We are talking about possibly the most important component of a helicopter when we talk of Main Gear Boxes....yet we are confronted with decisions and designs that fail to meet all contingencies.
How many times we heard that kind of comment about the S-92 MGB design?
All I am looking for here is the exact same kind of analysis by those that are so critical of the Sikorsky S-92 and its Engineers. No one gave a damn about their feelings....why should we treat the French Engineers any differently?
The discussion is all about the design, its flaws, and how to correct them and the certification system so we have "safe" aircraft to fly.
IMHO.....until the Gearboxes in question are re-designed to correct their shortcoming in detecting such failures...identify all foreign objects in the oil the come from gear/bearing/race wear and cracking....and oil analysis is re-instituted as a standard practice...and less reliance on HUMs as the main Gearbox Health measure is a standard engineering practice, I would suggest the industry has not adequately addressed the situation.
Now HC....care to discuss the technical issues raised in the Report?
In your view...what do they tell us about the current state of EC MGB design and safety monitoring of the gearboxes by flight crews, maintenance crews, and the Manufactuer?
Where do we go from here to improve the situation?
As to worrying about the feelings of people....how about we get concerned about the feelings of the next of kin of those that get killed in a second occurrence of this kind of failure.....or will it be the third occurrence if we accept the Report's findings?
From the point of view of a pilot and not an engineer:
I was heartened by the quality of the AAIB's forensic investigation that permitted establishment the point of failure to (part of) a specific second stage planetary gear in the epicyclic module. Further that the chip, collected on the 25th March, was from a 'second stage planet gear' - concluded, not just from its metallurgical composition but from the marks made during 'honing' process - was astonishing.
I was also astounded by the further analysis of that chip which appeared to indicate that the source of the stress leading to the fatigue cracks was some foreign agent whose indent could still be seen. Although not making any conclusive statements, there was a hint that the introduction of this foreign object could have resulted from the previous conical housing/rotor head replacement on the 1st March.
I agree with HC that the assignment of blame is futile but a system where such an important precursor is permitted to be discarded because the conclusion was reached that "the particle was a piece of 'scale'" - i.e. that it was believed to be "...silver or cadmium plating, which, in accordance with MTC 20.08.01.601, was '...unimportant...'" should be a wake-up call for management everywhere. Perhaps such areas of critical judgement should have far tighter control mechanisms if so many actions and consequences are likely to stem from them.
We have previously discussed the placement of the magnets on the epicyclic module separator plates (since rectified) - which might have captured debris from the stressed gear and thus prevented them from reaching the chip detector. Surely another failure of FMECA in the design process.
The reference to FARs (as opposed to JARs or EASA CS) is because the 332 was certificated to FARs (JARs were not in yet in place) and, specifically to amendment 24 of FARs. As you should be aware, the certification basis for an aircraft is frozen at a particular amendment version - which is why the FAA regulatory library has to provide all previous version on their regulatory web site.
Like others, I was disappointed with the failure of HUMS to provide adequate warning of failure (which confirmed experience from previous planetary gear failures in the S61); further, that it might never be an effective detector of bearing wear/failure in the epicyclic module (even with AAD). Clearly, we have to move back to debris monitoring using old and proven methods such as SOAP and other oil analysis techniques as is recommended in the report.
All-in-all a fascinating report and one which, if it is extensively studied, will add to the knowledge of helicopter professionals everywhere.
Jim
I was heartened by the quality of the AAIB's forensic investigation that permitted establishment the point of failure to (part of) a specific second stage planetary gear in the epicyclic module. Further that the chip, collected on the 25th March, was from a 'second stage planet gear' - concluded, not just from its metallurgical composition but from the marks made during 'honing' process - was astonishing.
I was also astounded by the further analysis of that chip which appeared to indicate that the source of the stress leading to the fatigue cracks was some foreign agent whose indent could still be seen. Although not making any conclusive statements, there was a hint that the introduction of this foreign object could have resulted from the previous conical housing/rotor head replacement on the 1st March.
I agree with HC that the assignment of blame is futile but a system where such an important precursor is permitted to be discarded because the conclusion was reached that "the particle was a piece of 'scale'" - i.e. that it was believed to be "...silver or cadmium plating, which, in accordance with MTC 20.08.01.601, was '...unimportant...'" should be a wake-up call for management everywhere. Perhaps such areas of critical judgement should have far tighter control mechanisms if so many actions and consequences are likely to stem from them.
We have previously discussed the placement of the magnets on the epicyclic module separator plates (since rectified) - which might have captured debris from the stressed gear and thus prevented them from reaching the chip detector. Surely another failure of FMECA in the design process.
The reference to FARs (as opposed to JARs or EASA CS) is because the 332 was certificated to FARs (JARs were not in yet in place) and, specifically to amendment 24 of FARs. As you should be aware, the certification basis for an aircraft is frozen at a particular amendment version - which is why the FAA regulatory library has to provide all previous version on their regulatory web site.
Like others, I was disappointed with the failure of HUMS to provide adequate warning of failure (which confirmed experience from previous planetary gear failures in the S61); further, that it might never be an effective detector of bearing wear/failure in the epicyclic module (even with AAD). Clearly, we have to move back to debris monitoring using old and proven methods such as SOAP and other oil analysis techniques as is recommended in the report.
All-in-all a fascinating report and one which, if it is extensively studied, will add to the knowledge of helicopter professionals everywhere.
Jim
Last edited by JimL; 25th Nov 2011 at 14:26.
Assignment of blame is not the objective.....finding an effective solution to the problems is the exact goal.
Did we rely on advanced technology, caste off proven techniques in the process, and by doing so....set ourselves up for failure?
As you correctly note JimL....the investigation begs questions whose answers are very important if they can be determined.
Every time we open up something as complex as a Gearbox....or dissasemble an aircraft's components to "inspect"....do we not introduce the possibility of contamination that can lead to failures shortly after the "satisfactory" inspection of the component?
Where do we draw the line between "inspecting" and "monitoring" to ensure a component remains "safe and serviceable" until TBO?
Did we rely on advanced technology, caste off proven techniques in the process, and by doing so....set ourselves up for failure?
As you correctly note JimL....the investigation begs questions whose answers are very important if they can be determined.
Every time we open up something as complex as a Gearbox....or dissasemble an aircraft's components to "inspect"....do we not introduce the possibility of contamination that can lead to failures shortly after the "satisfactory" inspection of the component?
Where do we draw the line between "inspecting" and "monitoring" to ensure a component remains "safe and serviceable" until TBO?
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Ref - AAIB Report page 130 Aircraft Maintenance Manual AS332 MK 2 Sub task 60-00-00-212-001 Para 3.1.1 Examination of the magnetic element.
If particles are found open the epicyclic module
Remove the epicyclic module.as per 63-22-00-021
Use a magnet to recover all the particles which you find on the magnets (3) of the collector.
There is no evidence in the report that this maintenance instruction was followed.
Whilst it is true to say “ The ring of magnets installed on the AS332 L2 and EC225 main rotor gearboxes reduced the probability of detecting released debris from the epicyclic module,” having had a detection the next maintenance action intended by the Design Engineers to assure airworthiness does not appear for what ever reason to have been taken.
It is notable that the report 2.7.3 Maintenance actions states “Had the magnetic particle recovered from the epicyclic chip detector been found in isolation, it is probable that the appropriate maintenance manual tasks would have been applied without consulting the helicopter manufacturer.” and “Since the accident the operator has completed a review of its maintenance practices and procedures and has measures in place to ensure that reference is made to the approved documentation when completing maintenance tasks.”
The AAIB report is an excellent illustration of how what at the time could be claimed to be unforseen is now blindingly obvious.
If particles are found open the epicyclic module
Remove the epicyclic module.as per 63-22-00-021
Use a magnet to recover all the particles which you find on the magnets (3) of the collector.
There is no evidence in the report that this maintenance instruction was followed.
Whilst it is true to say “ The ring of magnets installed on the AS332 L2 and EC225 main rotor gearboxes reduced the probability of detecting released debris from the epicyclic module,” having had a detection the next maintenance action intended by the Design Engineers to assure airworthiness does not appear for what ever reason to have been taken.
It is notable that the report 2.7.3 Maintenance actions states “Had the magnetic particle recovered from the epicyclic chip detector been found in isolation, it is probable that the appropriate maintenance manual tasks would have been applied without consulting the helicopter manufacturer.” and “Since the accident the operator has completed a review of its maintenance practices and procedures and has measures in place to ensure that reference is made to the approved documentation when completing maintenance tasks.”
The AAIB report is an excellent illustration of how what at the time could be claimed to be unforseen is now blindingly obvious.
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I was disappointed with the failure of HUMS to provide adequate warning of failure
Hi SAS
My sympathy for the maintenance guys was aimed at those working for Bond, not for EC.
Regarding your defensiveness of the 92 there are a couple of differences between it and the L2 - the first being that unlike the 92 they were not marketed as being the safest heli on the planet (ie expectations of the 92 were high) and that gearbox has been flying a lot longer than the 92's one with only 1 failure so far. That failure probably wouldn't have happened if the manufacturer's procedure had been applied as intended.
In its relatively short career, the 92 has killed a few and scared a whole lot more with a number of "near misses" whilst the similar number of 225s in service haven't much scared anyone (yet!).
On the subject of detecting transmission failures, HUMS and debris monitoring are complimentary techniques each with their areas of effectiveness. Because the planet gears are passing around the sun gear as well as rotating, the energy of their vibration signature, even after signal averaging, seems to be too weak for current technology.
I do feel that HUMS technology has stagnated a bit, even though AAD is a great advance it is only applied to the existing HUMS analysis and that has not really advanced since its inception as far as I can tell. The long standing flaws in the software mentioned in the report are an indication that there is insufficient investment in the technology (in terms of both time and money) which is unfortunately so typical of that sort of thing these days - once the push to get new stuff in place dies down a bit, compliance with such "good ideas" is checked by people who don't really understand the sharp end of it and just want to tick the box to keep the client/Authority happy. The full value and potential is never realised.
HUMS will never be perfect, nor will debris monitoring (and SOAP only works for microscopic-sized debris, not chips) and neither will transmission. Of course transmissions don't have to be perfect, the certification rules don't require zero failure rate, just one low enough so that those designing and certifying it will have likely retired before the first failure occurs!
My sympathy for the maintenance guys was aimed at those working for Bond, not for EC.
Regarding your defensiveness of the 92 there are a couple of differences between it and the L2 - the first being that unlike the 92 they were not marketed as being the safest heli on the planet (ie expectations of the 92 were high) and that gearbox has been flying a lot longer than the 92's one with only 1 failure so far. That failure probably wouldn't have happened if the manufacturer's procedure had been applied as intended.
In its relatively short career, the 92 has killed a few and scared a whole lot more with a number of "near misses" whilst the similar number of 225s in service haven't much scared anyone (yet!).
On the subject of detecting transmission failures, HUMS and debris monitoring are complimentary techniques each with their areas of effectiveness. Because the planet gears are passing around the sun gear as well as rotating, the energy of their vibration signature, even after signal averaging, seems to be too weak for current technology.
I do feel that HUMS technology has stagnated a bit, even though AAD is a great advance it is only applied to the existing HUMS analysis and that has not really advanced since its inception as far as I can tell. The long standing flaws in the software mentioned in the report are an indication that there is insufficient investment in the technology (in terms of both time and money) which is unfortunately so typical of that sort of thing these days - once the push to get new stuff in place dies down a bit, compliance with such "good ideas" is checked by people who don't really understand the sharp end of it and just want to tick the box to keep the client/Authority happy. The full value and potential is never realised.
HUMS will never be perfect, nor will debris monitoring (and SOAP only works for microscopic-sized debris, not chips) and neither will transmission. Of course transmissions don't have to be perfect, the certification rules don't require zero failure rate, just one low enough so that those designing and certifying it will have likely retired before the first failure occurs!
Just thought I would expand on the SOAP thing. SOAP is not the solution:
SOAP uses spectrography to detect microscopic metallic particles such as are produced by general wear, plain bearing degradation etc
In the REDL case, the problem seems to be that a chunk of roller bearing raceway surface broke away due to spalling. This left a pit which was the likely start of a fatigue crack. Neither the chunk, or the fatigue crack, released any microscopic wear particles, so there would be nothing for SOAP to detect.
Therefore SOAP or its ilk would not have shown any problem prior to the accident, and in this respect unfortunately the otherwise good AAIB report is misleading (aka incorrect).
Chip detection is the only current means to detect this type of failure. It did of course work correctly, but unfortunately the implications of the chip were not correctly grasped by the maintenance guys and the accident happened. Silver and cadmium are non-magnetic so it seems pretty unlikely to me that they would be stuck to magnetic chip detector!
SOAP uses spectrography to detect microscopic metallic particles such as are produced by general wear, plain bearing degradation etc
In the REDL case, the problem seems to be that a chunk of roller bearing raceway surface broke away due to spalling. This left a pit which was the likely start of a fatigue crack. Neither the chunk, or the fatigue crack, released any microscopic wear particles, so there would be nothing for SOAP to detect.
Therefore SOAP or its ilk would not have shown any problem prior to the accident, and in this respect unfortunately the otherwise good AAIB report is misleading (aka incorrect).
Chip detection is the only current means to detect this type of failure. It did of course work correctly, but unfortunately the implications of the chip were not correctly grasped by the maintenance guys and the accident happened. Silver and cadmium are non-magnetic so it seems pretty unlikely to me that they would be stuck to magnetic chip detector!
transmissions don't have to be perfect, the certification rules don't require zero failure rate, just one low enough so that those designing and certifying it will have likely retired before the first failure occurs
Any ritual suicides anywhere we haven't heard of?
At best they retire with a full pension....worse case they stay on to commit more murders....then retire with that fat pension.
While SOAP may not have shown any pre cursor to this accident, it is very good general practice and should be routinely re introduced.
Brian Abraham also raises a point which jumped out at me when I read the report. The 1,022 HZ noise, not being type typical may, with hindsight have been a possible indicator. Should operators and manufacturers consider adding audio to one of the regularly monitored HUMS parameters?
Brian Abraham also raises a point which jumped out at me when I read the report. The 1,022 HZ noise, not being type typical may, with hindsight have been a possible indicator. Should operators and manufacturers consider adding audio to one of the regularly monitored HUMS parameters?
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While I would not want to stifle any well intentioned creative thinking about possible indicators like acoustic signatures, they perhaps border on being a red herring here.
The elephant in the room is the AS332L2 was certified with magnets to catch epicyclic debris such as spalling, and the Maintenance Manual (see AAIB report App C2) called for them to be examined after any chip indication on the epicyclic mag plug. The mag plug and the magnets were specifically fitted to give early warning of failures.
According to AAIB they were never examined on G-REDL, on 25 March 2009 or subsequently.
The elephant in the room is the AS332L2 was certified with magnets to catch epicyclic debris such as spalling, and the Maintenance Manual (see AAIB report App C2) called for them to be examined after any chip indication on the epicyclic mag plug. The mag plug and the magnets were specifically fitted to give early warning of failures.
According to AAIB they were never examined on G-REDL, on 25 March 2009 or subsequently.
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In a similar vein to SASless's post above, a Puma had just taken off from Pau to go mountain flying a few years ago, the crewman decided that there was an unusual noise coming from down the tail. The rest of the crew couldnt hear it but took the crewman at his word and returned to base. Eng removed the mag plug (from IIRC the intermediate g/b) and it looked like a golf ball of chips and dust; and a very short way from letting go completely. It's not usually the noise that's important, but the change in noise.
Or indeed SASless's post below now. What is going on with the posting order recently?
Or indeed SASless's post below now. What is going on with the posting order recently?
Last edited by obnoxio f*ckwit; 26th Nov 2011 at 11:14. Reason: Utterly random posting orders!
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Arnie Madsen, I agree. On one occasion where I was in a situation that could have resulted in multiply deaths (only one died as a result) my expletive to emergency services was exactly the same, the only difference I remember was that I used it about eleven times in a three minute exchange with an operator who was askign mostly irrelevant questions, thats stress for you.
My ealrier comment about multiple incidetns with this aircraft, just to acknowledge that yes, I am not aware the cougar incident was a different aircraft as was one other.
My ealrier comment about multiple incidetns with this aircraft, just to acknowledge that yes, I am not aware the cougar incident was a different aircraft as was one other.
Sound does very much play a role in Gearboxes or any other bearing function.
How many of us (who drive the same car or light truck for multiple hundreds of thousands of miles) have not at some point heard the "rum...rum...rum...rum" sound of a bad wheel bearing or universal joint?
While flying a corporate MD-500E....I began to complain the MGB just did not sound right....it had changed noticeably to me over the time I had been flying it. For the longest time the CP and Mechanic/Engineer treated me like I was merely going mad. I finally forced the issue and refused to fly the machine unless we pulled the gearbox and did a split case inspection.
Good thing we did....it was in the process of losing some bolts that retained one of the gears. One bolt had worked completely loose and found its way into the sump....without fouling anything, no spalling, no chip detector...just a change in the sound of the gearbox and I am sure a change in its vibration pattern but without HUMs who would have known.
The question I would have on adding a "sound channel" to HUMs....how would one arrive at a "normal standard" for comparison over time? What would be a "significant variation" that would trigger a maintenance action?
How many of us (who drive the same car or light truck for multiple hundreds of thousands of miles) have not at some point heard the "rum...rum...rum...rum" sound of a bad wheel bearing or universal joint?
While flying a corporate MD-500E....I began to complain the MGB just did not sound right....it had changed noticeably to me over the time I had been flying it. For the longest time the CP and Mechanic/Engineer treated me like I was merely going mad. I finally forced the issue and refused to fly the machine unless we pulled the gearbox and did a split case inspection.
Good thing we did....it was in the process of losing some bolts that retained one of the gears. One bolt had worked completely loose and found its way into the sump....without fouling anything, no spalling, no chip detector...just a change in the sound of the gearbox and I am sure a change in its vibration pattern but without HUMs who would have known.
The question I would have on adding a "sound channel" to HUMs....how would one arrive at a "normal standard" for comparison over time? What would be a "significant variation" that would trigger a maintenance action?
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Geoff
You ask
According to the AAIB:
The AMM however was available and required the epicyclic module to be opened and inspected (squib's 'elephant').
You ask
Does anyone know if EC had a Tech Rep for the 332 in UK/ADN and if so was he consulted? Sometimes a good Tech Rep will be a mine of information that can help hard pushed and shift-working maintenance personnel to react appropriately to the first signs of a major defect.
a manufacturer’s technical representative is based in Aberdeen to provide direct support to local operators. This representative was not available in Aberdeen on 25 March 2009 due to a pre-planned commitment.
Last edited by zalt; 26th Nov 2011 at 14:34. Reason: to add clarity at a time when posts are being filed out of sequence
II / SAS
I very much doubt that the 1022 hz tone was relevant. That frequency corresponds to a rotational speed of 61,320 rpm or for say a gear with 30 teeth, gear meshing frequency at 2044 rpm which I think is much faster than the upper epicyclic planets are going. It is more likely to be an avionics cooling fan or even electrical (not sound) interference picked up on the microphone leads.
Interesting but one could spend one's life chasing sounds that are not related to the transmission.
I very much doubt that the 1022 hz tone was relevant. That frequency corresponds to a rotational speed of 61,320 rpm or for say a gear with 30 teeth, gear meshing frequency at 2044 rpm which I think is much faster than the upper epicyclic planets are going. It is more likely to be an avionics cooling fan or even electrical (not sound) interference picked up on the microphone leads.
Interesting but one could spend one's life chasing sounds that are not related to the transmission.