Lack of lubrication certification for helicopter gearboxes

[Thomas coupling]

https://www.easa.europa.eu/system/files/dfu/NPA%202017-07.pdf

Has this been discussed already?

[Lonewolf_50]

If you can, might want to fix the typo in your title. (For a moment I had to think about what ubrication might be). Thanks for the link, I'll need to ponder a bit before any comment.

[Thomas coupling]

Cheers me dears. Oil see to it ferzakerly

[Lonewolf_50]

You asked for comments.
Comments provided.
The only significant drawbacks of this proposal are economic. This results from the increased cost of certification testing, and potential for increased weight of the gearbox and lubrication system, dependant upon the design solution chosen by the applicant. But the impact to CS-29 helicopter manufacturers is considered to be negligible Because those who wrote the paper won't have to do it. (Yeah, I saw the table at the bottom, and all I can offer is :rolleyes:.

Regarding paragraph 2.4: would anybody care to offer up a concise version of "where does 30 minutes come from?" (as opposed to 20, or 15, or 10, etc) to bring us up to date? I realize that this has been done to death in various S-92 / Blackhawk / Puma / 225 / 332 threads and bun fights. Reestablishing that old baseline early in this discussion might be helpful. (I see that I may not get my comments in by 31 July, but that's OK, nobody listens to me anyway)

3.2.1. b. makes some points on Failure mode analysis. Like what I see there.

Regarding para 6(a)(1) .. Oh, wait, now it's a 36 minute test. Where does that number come from? (See also 6(e) (1) (iii))

Secondary indications: yeah. Keep moving forward on that thought.

Here's an idea: make the requirement 25 minutes, and substantiate that. Not sure what's so magical about 30 minutes, or 36 minutes. Pick a different number, perhaps? 4.1.2. Who is affected This NPA will affect designers and operators of large rotorcraft as well as certifying authorities. The four largest helicopter manufacturers affected by the changes proposed in this NPA are Airbus Helicopters, Bell Helicopter, Leonardo Helicopters, and Sikorsky Aircraft Corporation. And every operator of same. (this section explored some financial issues) 4.5.3. Social impact There are no relevant social impacts for either option. Really? If there weren't a social impact, you'd not have been tasked to come up with this.

[JohnDixson]

Lonewolf, re the 30 minute requirement. I believe this first appeared in the UTTAS Material Need document which then became the UTTAS Request For Proposal in terms of the design/test requirements for the aircraft. There was an Army board headed up by a Colonel Bud Patnode which was charged with coming up with the requirements. I talked with him a few times during the fly-off competition, but never about the 30 minute rule. His stories about the internal fights about the skids vs wheels decision were " entertaining ". That group started their work around 1968 I believe, so it is probably safe to say that UH-1 experience in Vietnam influenced the 30 minute number. I do recall that a UH-1 down south ( I was in the Central Highlands 65-66 ) had a transmission oil problem, tried to stretch it and the following aircraft saw his rotor slow down and they went in. There may have been others. Someone with access to the Army Aviation Systems Command, Flight Standards Div records should be able to answer your question definitively.

[megan]

John, in our neck of the woods (IV Corp) scuttlebutt was a dry transmission was good for either 30 or 60 minutes in the Huey, forget which. Reputedly from a Bell rep. Had occasion to try it out (no pressure) when the filter O ring let go and flew for some period to get to a FSB (<15 minutes, memory as to how long not that good). Occurred during the Cambodia push when spares were impossible to get, hence maintenance reuse of O ring during filter change. Maintenance arrived and I refused to fly it home, they topped it up and flew home. Can tell you a T-53 will run for long enough to take off and get you out of trouble with no oil as well.

[JohnDixson]

Megan, the story re the machine ( 1965 or 66 ) was that it was a few minutes. I found the following link for a later incident, which quoted 2 minutes:

https://www.vhpa.org/KIA/incident/700217101ACD.HTM

[Lonewolf_50]

John, I may be able to find someone who can dig that up. Depends on who retired recently.

The lore in the SH-2F (passed down to us newbies in the early 80's) was that one crew lost the oil in the combining gear box and headed back to their little frigate, at the NATOPS recommended power setting. About 30 minutes later they landed and shut down, so there was one data point that it might work for 30 minutes based on an event. But NATOPS was still all about "get it out of the sky" as the rule.

[riff_raff]

That EASA notice was interesting reading. Most of the proposed changes to certification of gearbox designs for loss of lubrication operation seem appropriate, but there are a few areas that could be improved.

The most important thing to consider with rotorcraft transmission systems is that the primary function of oil is for cooling the internal components. The amount of oil flow required for "lubrication" is quite small in comparison to the oil flow required for cooling. So the most effective approach to improving loss of lube performance are design changes that improve heat transfer away from critical areas in the gearbox, such as bearing and gear mesh contacts. Or using the recently developed advanced materials for gears, bearings, housings,etc. that can tolerate higher temps.

Another change to loss of lube qual testing would be requiring two or three gearboxes to be tested. And the gearbox test articles used should have been run so they represent the average condition of a gearbox in service.

Finally, there were a couple proposed requirements that seemed a bit optimistic. For example, the Class 3 failure criteria (Imminent Failure) on p.18 states "the efficiency of the gearbox may be reduced by up to 10%.". A decent new build MRGB probably has an input/output efficiency of ~96%. So a gearbox on the verge of imminent failure after the loss of lube test cycle must still demonstrate an input/output efficiency of >86%. I don't think this particular requirement is realistic.

[RVDT]

In more recent times the 30 minutes was a piece of PR on a certain model that still does not comply to this day.

Oh hang on a minute ................thats what this thread is actually about. :ouch:

[Lonewolf_50]

riff, are you referring to isotropic superfinishing for gears in the transmissions?

[zalt]

In more recent times the 30 minutes was a piece of PR on a certain model that still does not comply to this day.

Oh hang on a minute ................thats what this thread is actually about. :ouch:

You must be mistaken!

When asked
"From memory, if pilot action is required "within several seconds" of a system failure, such as a MGB oil pressure loss, isn't that non compliant with Part 29?"

The PM responded on PPRuNe (http://www.pprune.org/rotorheads/191379-ec225.html#post1454917)
"And the S-92's oil protection system protected the transmission to the FAR/JAR with such aplomb that we shut the test down at 3 hours, not 30 minutes, with the system still running along (but admittedly pretty close to its end). That is 2 1/2 hours longer than required. The pilot must activate the system, but has several seconds to do so after clear indications, and the checklist does not say "land immediately."

So don't spread any lies:=
Oh wait, turms out that in NL terms 3 hours becomes 10 minutes.
http://esq.h-cdn.co/assets/cm/15/05/54cb2a231bbf4_-_reconstructed-helicopter-0909-lg-9784320.jpg

[riff_raff]

riff, are you referring to isotropic superfinishing for gears in the transmissions?

ISF of all gear and bearing hydrodynamic contact surfaces is great way to improve performance under marginal lubrication conditions. It adds some cost but otherwise should have minimal impact on the existing design. Here's an article on the subject (http://www.gearsolutions.com/article/detail/5810/scuffing-resistance-of-isotropic-superfinished-precision-gears). This is a quote from the third paragraph: "The results of the current testing reveals that isotropic superfinished SAE9310 specimens show at least a 40°F higher lubricant temperature at the point of scuffing compared to as-ground baseline gears. Based on these results and the previous studies, it was concluded that this isotropic superfinishing technology should be incorporated in all future aerospace gear designs."

In my post I also mentioned recently developed materials that provide significantly increased high temperature capability. In particular there is C64 steel alloy (http://www.questek.com/ferrium-c64.html) (AMS 6509) for carburized gears. Since the early 80's X-53 steel alloy (AMS 6308) was the best available material for carburized gears in terms of high-temp capability, with a tempering limit of ~400degF. In comparison, C64 steel alloy has a tempering limit of ~925degF. The tempering temperature of a steel alloy is important because if it is exceeded in service the material will begin to de-temper and lose strength. For rolling element bearings, there is M50NiL (AMS 6278) which is a carburizing grade alloy that can replace the existing standard high-temp material M50 (AMS 6491), which is a through hardening alloy. While M50NiL has similar high-temp capability as M50, it also provides better fracture toughness and lower friction characteristics at boundary contact conditions. Once again, minimizing friction under marginal lubrication conditions is critical for keeping component temperatures down.

To minimize friction in rolling element bearings under marginal lubrication conditions, a recent development is applying amorphous carbon coatings (http://evolution.skf.com/us/hard-coating-for-heavy/) (DLC) to the rollers/balls. This adds some cost, but otherwise should have minimal impact on an existing design. The only potential issue is how to detect debris generated by this coating.

[LRP]

Megan, the story re the machine ( 1965 or 66 ) was that it was a few minutes. I found the following link for a later incident, which quoted 2 minutes:

https://www.vhpa.org/KIA/incident/700217101ACD.HTM

A friend of mine was operating a restricted category UH-1H at the Grand Canyon about 6 years ago. Lost transmission oil pressure at about 500 agl, the rotor stopped turning at about 10 ft., he was seriously injured as a result. Don't know how long it ran dry, but it wasn't very long.

NTSB said it was a transmission failure from a lack of lubrication.

[SASless]

Zalt,

Did the Cougar Crew comply with the Checklist?

How many times did the Co-Pilot tell the Captain the Checklist's last entry said to Land/Ditch?

Was there a 30 Minute Run Dry Time required by Certification?

Did the Aircraft receive Certification?

Have you read the Cougar Report in detail?


Under Section 4.2 ACTION REQUIRED, The Accident Report lays out the basic problem with all of this 30 Minute Run Dry Time issue. The Authorities amended the Requirement to allow the use of the definition "Extremely Remote" and by doing so undermined their own Certification Standard.

The FAA also allowed the use of the Pilot Activated Isolation system and in the Cougar case that was of no use as the entire contents would have leaked out even had the System been activated due to the Filter being the cause of the leak.

http://www.tsb.gc.ca/eng/rapports-reports/aviation/2009/a09a0016/a09a0016.pdf

[zalt]

SASless

I'm loathed to 'answer' questions you have your own answers to! But yes I have read the accident report in great detail.

You might not have realised it but you have conceded that if Sikorsky had actually designed the S-92A to run for 30 minutes after a loss of lubrication, the crash would not have occurred as they would have reached Cape Spear.

This latest EASA NPA does not change that at the time of certification a test with a loss of lubrication (not just a bit of the oil) was already required (see TSB footnote 84).

Despite your question below about what the FO said, in fact the FULL CVR transcript was not published by TSB so we can't examine the whole discussion between the crew and also with their ops centre. Perhaps Sikorsky's sale of the 3 float bag configuration to an operator flying over the Grand Banks may have prayed on their minds. In March in the Atlantic with 3 float bags a capsize and some fatalities was very possible.

The Canadian press had a field day when it emerged that the original marketing material used in Canada HAD claimed the MGB could run for 30 minutes after an oil loss. The TSB also have a section in their report about social media. I wonder which site they were thinking of:confused:oh: Perhaps that material and the Program Managers claim of 3 hours was an influence in delaying a decision despite the RFM?

Perhaps three S-92A incidents just the previous year with oil system anomalies that did not involve oil loss may have also clouded their judgement.

A Norwegian accident report 4 1/2 YEARS AFTER the Cougar accident considered the Sikorsky checklists on oil loss were STILL confusing http://www.aibn.no/Aviation/Published-reports/2015-11-eng

I must correct myself. The S-92A gearbox that failed on 6 August 2002 (see TSB page 100) when the test was done the normal way followed by the rest of the industry failed after 11 minutes not 10 as I implied below. A long way less than 30! Its only after that that Sikorsky "revisited" the requirements as the TSB put it and focused on what would not be "extremely remote".

The Cougar accident shows that one failure mode claimed to be "extremely remote" was not. But the oil loss in Canada was not the first such oil loss on the S-92A. The previous year one occurred in Australia. That should have rung loud alarm bells that a key certification claim was false. If we apply the logic that some apply to other helicopters and their gearboxes, the S-92A should have been grounded until the root cause of that failure to meet certification requirements was fully understood....

[RVDT]

Not to read too much into it - Leonardo claim that the 139 has proven to be good for 60 minutes and the 189 for 50 minutes on certification tests.

So it's not as if it cannot be achieved. They obviously put a bunch of work into it.

[espresso drinker]

Zalt, I couldn't agree more with your post above. You can see from my previous posts that this is a subject of great interest to me. How could an aircraft continue to fly after it had actively demonstrated that the extremely remote 'loop hole', under which it had been certified, was actually not actually extremely remote after all. And, worse still continue to fly without immediate modifications, after the earlier incident, leading to a complete loss of an aircraft and multiple fatalities.

It is also extremely worrying that the Canadian navy are getting a militarized version of this same aircraft. How extremely remote is extremely remote when people are firing all sorts of s**t at you. A couple of 0.50" rounds through your MGB casing over water and what happens next?

[FC80]

A couple of 0.50" rounds through your MGB casing over water and what happens next?

If a total loss of oil occurs, you comply with the RFM and land immediately.

[RVDT]

If a total loss of oil occurs, you comply with the RFM and land immediately.

With the added proviso that in the case of the 92 it should be less than how many minutes?

Given that as far as we are aware from the poor statistical sample of about 2 occurrences - one in a test cell and one for real -
both achieved ~ 11 minutes. Lets put a margin in for error of 50% which is generous and round it down - 5 minutes.
Also not forgetting that on the Cougar flight most of the 11 minutes would have been power off or low power in a rapid descent from altitude.
As the failure was primarily in the TR output you could probably have a failure regardless of the power level.

Another issue on the Cougar incident was the pre-occupation with the fact that the oil temperature remained normal. This is a fallacy that has been perpetuated for years that low or no oil will give you high oil temperature. Theory and experience would prove the exact opposite or it has no correlation. Yet it is still preached.

These guys were looking at an "extremely remote" event and making up for it as they went.

The 92 has had quite a few "extremely remote" events and incidents where "cry wolf" was patched up and lulled a lot of people into the wrong frame of mind.

"Extremely remote" has been redefined if you are looking for an example as it clearly does not meet the alternative.

[FC80]

Like you say, 'less than 10 minutes' seems to be about as precise as we know.

I completely agree that the lack of an emergency lube system is unacceptable.

In my experience it is absolutely drilled into 92 pilots now that MGB oil px below 5 PSI is a land immediately situation - regardless of any other indications.

[SASless]

It would seem a careful review of the Cougar Accident Report is needed.

For a start....read up about the descent profile that was flown and the time line.

Correlate that with the CVR data that was released.

Then try to fathom what was not released involving the discussion with the Cougar Base....which until released will have to remain subject to conjecture.

Ask yourself why it was not released.....and ponder what might have been said that a lot of people do not want made public.

Anyone care to enlighten us as to what the content and detail of that still private discussion was?

We thrashed this about years ago shortly after the tragedy and later upon release of the Accident Report.

Mis-stating the facts does not serve any good purpose.


As to SA "selling a three Bag System".....who bought it? If it was not the right Kit for the Tasking offshore Newfoundland....then why was the aircraft being operated with it installed?

Prior failures that did not result in Oil Loss....how is that pertinent to an event where Oil was in fact lost?

If the Norwegians, almost five Years later find the ECL re Oil Loss are still "confusing" then who why has that Operator and others not resolved the ambiguity or problems with those Procedures? Don't most Operators write their own Checklists and Procedures....then they must be approved by the Authority they operate under?

The FAA Certification Requirements were met....or so it would appear as the Aircraft was certified and put into and remains in service.

We all can agree those Standards are not as they can and should be....and should be improved and made to improve the safety of the Aircraft and Occupants.

There were design faults with the 92 just as there have been with every aircraft ever built. Sadly, far too often we learn of them in very tragic circumstances.

Pointing the finger in a single direction does not afford much progress in improving the situation. I can see many facets to the MGB designs we see in modern Helicopters but don't see as being "only" the manufacturer's load to carry all by themselves.

It involves everyone in the process to look for problems....in design, manufacturing, testing, certification, and maintenance. It is a long Chain with many Links.....and as we know....one Link fails...the Chain fails.

Just as in the 225 situation with the MGB....we all can accept a proper design should be adequate to prevent two "Extremely Remote" events of aircraft shedding whole Rotor Systems but yet we see the results of exactly two such events in a fairly short time.

For sure....we need to reconsider this "Extremely Remote" concept.

Is it really possible to design a Helicopter that is failure proof?

[espresso drinker]

SASless, it's probably not possible to design a helicopter that is failure proof (that's another subject). BUT it is possible to design one with a 30 minute, or more, DRY RUN TIME! They are out there and are flying and have been for decades. :{

[SASless]

......and the ones that cannot....what about them?

What about the aircraft with MGB's that are not able to contain a gear failure and afford a safe landing?

How many single point failures do we tolerate that can result in a catastrophic result?

Some of you are banging a drum about run dry and are ignoring another glaring problem right before your eyes.

Why 30 Minutes....why not 60 minutes....120 minutes?

What is magic about 30 minutes?

This gets back to my point about Certification Standards....and the fact they are not made in a vacuum by the various Authorities. Certification testing is not done in a vacuum....thus we should not be having these arguments following some tragedy where we see unforeseen but preventable disasters.....or disasters that go unexplained.

Open your eyes folks....it is not a SA, Leonardo, AH, Bell problem....it is an Industry Problem that affects all of us.

Shall we go back through all the new aircraft from the past few years and remind ourselves of the problems each of them faced after being put into Operation?

The 92 with the Oil Filter and Transmission mounts, the 225 with the Epicycle Module, the 139 Tail Booms, the 109 with its problems, the 205 with its problems, the Chinook....let he who be without Sin here....throw that Stone.

No....there shall never be a fault free aircraft....but we should hope to see them as failure tolerant as possible.

Two good Case Studies can be the 92 and the 225 MGB's if we want.

Why not take a very critical look at each....and see the lessons that can be learned....they take us down separate but similar tracks and lets see where the problems started in each of them.

Each hoped to be State of the Art Machines....and each in its own way is. Each has had problems.

Each started in different Cultures....one American and the other European and they are two very different environments....what role does that play in all of this?

[espresso drinker]

SASless, your very well written argument is accepted. And I agree that typically no new aircraft is without it's problems and the certification process, together with all the other controls, needs to be robust enough to ensure that all potential problems are resolved before the aircraft gains certification. If any issues subsequently come to life after entering service then the certification of that aircraft must surely be reviewed and suspended if necessary.

Then as an operator don't buy it until it's been flying for a few thousand hours (look up the bathtub reliability curve!) :O

[megan]

needs to be robust enough to ensure that all potential problems are resolved before the aircraft gains certificationThe certification process cannot uncover all the issues that may arise once the aircraft is put into the rigours of commercial use. A couple of examples.

About to enter downwind at home base following an offshore flight in a 212, when a high freq manifested itself, lasted about ten seconds, and terminated with a bang. Everything normal in the cockpit, and normal landing and shutdown followed. Big puddle of oil underneath on the pad coming from the hell hole. Oil in transmission sight glass zilch. Rotor brake disc was found lying on the deck and the bevel gear had dropped into the trans and flung about taking chunks out of gear wheels various. Ultimate cause I don't know, but I'm not about to suggest it signified a great problem for the 212 type. Stuff happens, though I'm mutchly glad it didn't happen offshore and having to ditch in mega seas in the middle of winter.

http://www.adf-gallery.com.au/gallery/albums/Wessex-N7-215/AMOF_N7_215a.jpghttp://www.adf-gallery.com.au/gallery/albums/Wessex-N7-215/IMG_1102.sized.jpg

One of our pilots landed a 205 on fixed floats in the ocean to pick up the survivors from the above Wessex. Aircraft was returning home at low altitude and slow speed with a high freq, when it suddenly rolled inverted and crashed. Two pilots survived, crewman and pax perished. Cause? Gearbox spat a wheel out through the case, which in turn removed one of the jacks. Only ever flew the H-34 version, but once again I'm not about to suggest the aircraft had a history of main G/B issues. Stuff happens.

[SASless]

The Bell MGB design use a long shaft that affords the ability of the Gear Box to provide a Load Path that works to transfer loading when gears begin to break apart. I am curious about the basic differences between the new designs as compared to the older designs in that regard.

I wonder what the statistics are on MGB failures in Legacy Aircraft are as compared to "new" design Gearboxes.....particularly in Catastrophic failures that result in the loss of the aircraft and its occupants.


http://www.huey.co.uk/images/history/techsheet6.jpg



http://aerossurance.com/wp-content/uploads/2016/05/ec225-mgb.jpg

[Lonewolf_50]

@SASless: the Seahawk/Blackhawk/S-70 main rotor shaft goes all the way through the transmission "box" from the bottom (where the Jesus nut is) out through the top to the head, though the shaft extension is what connects it to the hub. I am pretty sure the S-92 is the same since the gbx (and much else) were S-70/Blackhawk derivative. There are a variety of things that can go badly.


Well over a decade ago a carrier assembly had a serious failure inside a Blackhawk box, which was a major factor in the Navy, the Army (and IIRC the Air Force) getting together with Sikorsky and agreeing on an improvement/redesign of the carrier assembly. So far so good.

Gearboxes are hard things to get right, to be sure. (I think the guys at Sikorsky are reliving that reality again as they work their way through the details on the CH-53K).

[riff_raff]

The certification process cannot uncover all the issues that may arise once the aircraft is put into the rigours of commercial use.......Only ever flew the H-34 version, but once again I'm not about to suggest the aircraft had a history of main G/B issues. Stuff happens.

megan,

Your point is excellent. The purpose of qualification testing, including the run-dry test procedure, is to validate all analysis used to certify the design. There is a specific type of certification analysis performed called Failure Mode, Effects and Criticality Analysis (FMECA) that is intended to ensure no credible failure mode will produce a catastrophic event. The FMECA evaluates every component and system from design thru manufacturing and throughout its service life. Of course, the analysis does not consider unforeseen conditions, as you noted above.

The 1982 CH-47 crash in Mannheim is a perfect example of how an unanticipated event involving the lube system of a main rotor gearbox resulted in a catastrophic failure. What makes this case even more unusual is that it was caused by loss of flow at a single oil jet, while the rest of the gearbox lube system continued to function normally. The crew had no idea what was causing the problem, and the final catastrophic structural failure occurred just a couple minutes after the crew became aware of a problem. The picture below shows the rear main gearbox separated from the airframe.

It is quite possible that the aircraft had been flying with the blocked oil jet for a substantial number of hours. Unfortunately, this particular loss-of-lube failure mode was never considered in the FMEA. So no steps were taken to prevent it from happening, or even alert the crew it was occurring so they could take appropriate action.

After it was determined that the root cause of this loss-of-lube failure was the small oil jet orifice becoming obstructed by walnut shell debris trapped inside the gearbox housing oil galleries from grit blast cleaning during an overhaul, the overhaul procedures were modified and the design of oil jets were revised. It is now standard industry gearbox design practice to include a "last chance" filter screen on every lube oil jet to prevent debris from obstructing the small diameter metering orifice holes.

http://parasavoie.50ans.free.fr/chinook/crash_fichiers/74-22292_one_point_five_seconds_before_impact.jpg

[megan]

riff_raff, I remember doing a US Army accident course in '71 and mention was made of an engine oil leak in the Chinook which travelled up the gearbox drive tunnel and acted as a blow torch on the gearbox, with the same results as your photo. How do you go about predicting an event such as that? An unenviable job attempting to dot all the i's and cross all the t's of what can occur.small oil jet orifice becoming obstructed by walnut shell debris trapped inside the gearbox housing oil galleries from grit blast cleaning during an overhaul, the overhaul procedures were modified and the design of oil jets were revised. It is now standard industry gearbox design practice to include a "last chance" filter screen on every lube oil jet to prevent debris from obstructing the small diameter metering orifice holesEverything old becomes new again, in a way. The P-47 in WWII European theatre were having an inordinate number of engine failures. Cause was sand from the casting process not being successfully cleaned out.

[SASless]

Candy Bar Wrappers were being found in Chinook Transmission Sumps....Boeing discovered the boxes in for overhaul were stored next to an employee break area and the wrappers were being thrown into them instead of Trash Cans.

Once the Transmission halves were moved away from the Break Area....the problem was solved.


Then there was the Bell Short Shaft on the Huey....that could be installed wrong way around.

Lots of Murphy issues have cropped up over the years.

[riff_raff]

Then there was the Bell Short Shaft on the Huey....that could be installed wrong way around.......
Lots of Murphy issues have cropped up over the years.

I have worked on a few military aircraft programs over the years, and one thing I've noted is that the military leadership takes the safety of their pilots and crews very seriously. On one hand, they are usually forgiving of problems that were hard to foresee. But on the other hand, they have zero tolerance for problems that should have been obvious and preventable, or due to simple carelessness.

The US DoD does a good job of documenting "lessons learned" from every accident, and they make the reports readily available, so there is no excuse for anyone making the same mistake again. One very important factor in preventing problems is an engineering staff with lots of relevant experience. But there was a long period where relatively few new rotorcraft gearboxes were designed in the US, so much of the collective experience was lost.

[etudiant]

The Russians build some very impressive gearboxes for their large helicopters.
Is there anything different about their design approach that could be usefully exploited by others?

[SASless]

Within a page or two...you get a very quick answer to your question.

I did not read further as it is early and I am just getting into my first Cup of Coffee!


https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910005833.pdf

[Lonewolf_50]

@SASless, that's a great article. Thanks for the link. Good reading, but also requiring coffee.

[etudiant]

Within a page or two...you get a very quick answer to your question.

I did not read further as it is early and I am just getting into my first Cup of Coffee!


https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910005833.pdf

Thank you, a very interesting document.
It suggests that Russian helicopters don't much use planetary gear arrangements in their transmissions.
Why not is a question that requires a level of knowledge and expertise that only an industry professional could muster, if they are willing to offer it.

[megan]

Prompted by SAS's last post I got to thinking about what gearboxes internals involve. Hats off to the engineers/designers/manufacturers. Shakespeare in "As You Like It" wrote "And thereby hangs a tale", in the case of gearboxes, thereby hangs a tail - yours and mine.

https://www.geartechnology.com/issues/0115x/Involute.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860005142.pdf
http://www.dtic.mil/dtic/tr/fulltext/u2/a257727.pdf
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADP000703

[Lonewolf_50]

Megan, when flying Sea Sprites and Seahawks, the three things I always worried about were the blades, the head, and the main transmission. (The tail you can survive if it goes ... though a tail failure did kill a guy I know ... )
Some things, if they break, mean that you are dead.
I won't say that's why I drink, but I will say it's a contributor to the overall libation. :)

[SASless]

There were times when flying single pilot over wide expanses of saltwater in good weather with not much to do but "think"....I sometimes pondered the notion of what would it be like to shuck a Blade.

What really had me wondering was what Clock Angle the Blade would depart... and would you be able to see it disappearing from view as it went!

[megan]

I sometimes pondered the notion of what would it be like to shuck a BladeAt times thought the same SAS, thinking of good friend Jerry Hardy.

[SASless]

At times thought the same SAS, thinking of good friend Jerry Hardy.



Too true....he was such a good guy.....frequently think of him!

[riff_raff]

......What about the aircraft with MGB's that are not able to contain a gear failure and afford a safe landing? How many single point failures do we tolerate that can result in a catastrophic result?
No...there shall never be a fault free aircraft....but we should hope to see them as failure tolerant as possible.

SASless-

The discussion of legacy Russian helicopter main gearbox designs jogged my memory. Consider the massive (8,000 lb) Mi-26 main gearbox design shown below. Instead of epicyclic stage(s) for the final drive, it uses a torque-split configuration (http://avia.pro/sites/default/files/pictures/katastrofi3/glavnyy_reduktor_vertoleta_1.png) with 16 small diameter pinions driving a stacked pair of large diameter external bull gears. One major reason for using this configuration is to allow more room for a large diameter rotor shaft.

Regarding your question shown above asking about the capability to "contain a gear failure" so that it does not fully disable the gearbox, I thought you might find the following of interest. There was a paper published by the AHS back in 2008 about a trade study conducted by Sikorsky comparing an advanced epicyclic configuration to a split-torque configuration (somewhat similar to the Mi-26 MRGB) for the new CH-53K main gearbox. One conclusion from the study was that the split-torque configuration was less susceptible to damage from FOD being trapped between the output stage gears. Here's a quote from page 8 of the paper:

"Third, materials such as FOD (foreign Object debris) and wear debris could either be trapped inside or drop into a multiple-stage planetary system due to the planetary motion of those pinions between the ring gears the sun gears, which more likely could cause damage to those rotating parts.
The split-torque MGB design configuration, on the other hand, eliminates the above concerns……….Besides, the trapping of FOD in the split-torque MGB is less likely since all herringbone pinions stay outside the bull gear."


The approach Sikorsky engineers are discussing is slightly different than what I believe you were thinking of, but the goal is similar. That is preventing debris produced by a failed component from causing further damage. Sikorsky chose to use a split-torque configuration for the new CH-53K instead of the epicyclic configuration used on the previous H-53 models, Based on this paper, it seems apparent that Sikorsky engineers were considering the type of problem you brought up back in 2008.

The paper is worth reading if you can find a copy. It's titled, "Trade study on different design configurations of the CH-53K main gearbox".


http://www.aviastar.org/foto/gallery/mil/mi-26_5.jpg

[Concentric]

Finally, there were a couple proposed requirements that seemed a bit optimistic. For example, the Class 3 failure criteria (Imminent Failure) on p.18 states "the efficiency of the gearbox may be reduced by up to 10%.". A decent new build MRGB probably has an input/output efficiency of ~96%. So a gearbox on the verge of imminent failure after the loss of lube test cycle must still demonstrate an input/output efficiency of >86%. I don't think this particular requirement is realistic.
Riff, you might find this short presentation (http://aerossurance.com/wp-content/uploads/2016/01/2.4_Tamborini_Design-and-Development-Strategies-for-Main-Gear-Box.pdf) (download) from Leonardo interesting. If I am reading it right, that quoted 96.5% seems to refer to the MGB running after loss of lubricant (LoL) though it is not clear where in the timeline of the test that was recorded. Could that possibly be after 50 minutes LoL?

[riff_raff]

Concentric, it says "an overall transmission efficiency up to 96.5% at min cruise power (LoL test condition)".

A couple things to consider with this claim. What losses are included in that overall efficiency number? Were the oil pumps not operational? At what point during the LoL test was that peak efficiency number recorded? Having little oil flow within the gearbox housing would result in reduced gear windage losses and lower viscous losses in the rolling element bearings, at least during the very early stage of the testing.

One unusual thing about the overall efficiency of rotorcraft main gearboxes is the effect load has. Most MRGB losses are two types. Speed dependent and load dependent. For example, rolling element bearings have both viscous (oil churning) losses which are primarily speed dependent, and contact losses (sliding, skidding, etc) which are more load dependent. If the MRGB is operated at max speed and low load, the speed dependent losses remain high and only the load dependent losses drop. And a rotorcraft MRGB driven by turboshaft engines typically operates within a very narrow speed range close to maximum, with the input gear stages turning at high speeds.

With this in mind, it becomes apparent that if you compare two MRGBs operating at the same speed, but one at high load and one at low load, the one at high load should have better overall efficiency. If the 96.5% efficiency at cruise power number in that presentation was recorded at the end of a LoL test, that would be very impressive. And would indicate that the gearbox suffered very little damage during the test.