S-92 gearbox crack

[SpareParts]

Dont know if anyone has heard of this yet

Crack found in gearbox of Cougar Helicopters Sikorsky S-92 in Halifax - Yahoo! Canada News (http://ca.news.yahoo.com/s/capress/091102/national/chopper_gearbox_crack)

-SPAREPARTS

[Ian Corrigible]

Ugh. So much for "It's a North Sea problem only..." :ouch:

I/C

[funkymonkey77]

bad news, anyone know why they r cracking?

[maxwelg2]

bad news, anyone know why they r cracking?

That's the million-dollar question that SKY is supposed to be answering. Some questions that came to my mind were (I'm sure someone on this forum can provide comments):

What specific alloy is the MGB housing manufactured from?

Is there a link between this issue and the previous AD from 2007 regarding the main transmission bolt failure?

Is there an issue with dis-similar material thermal expansion properties changing the effective torque on the mounting feet during flight operations?

Is there a background vibration issue or excessive shear force present on the mounting bolts/feet?

Is there any correlation within the HUMS data for the affected S-92a units to date that could be registered as a specific threshold signature, or are there no raw sensors close to the mountings?

Has this issue been seen before on previous helo designs, and how was it engineered out?

Now we have 2 critical areas of this MGB design that are suspect to be sub-standard, namely the filter housing studs when they were made from titanium, and now the main transmission mounting feet. Hopefully SKY will produce some answers soon before the S-92a reputation is even further destroyed...:rolleyes:

Fly safe

Max

[212man]

Is there any correlation within the HUMS data for the affected S-92a units to date that could be registered as a specific threshold signature, or are there no raw sensors close to the mountings?

That possibility, along with many others, is being actively investigated.

[Brian Abraham]

Has this issue been seen before on previous helo designs, and how was it engineered out?
Had occasion to ground the worlds fleet of Bell 206's when what seemed to be a crack was found on a main gear box mounting lug. Turned out to be merely a crack in the surface finish but with the facilities available on a ship at sea we did not have the means to establish that fact.

[NorthSeaTiger]

Would be interesting to know TSN of the affected boxes and at what power settings the affected machines are being flown.

NST

[HeliComparator]

Apparently we have had a cracked lug with a gearbox in single-digit hours. Seems very odd, something has changed either a dodgy batch of castings or increased movement in the airframe for some reason. I don't think Sikorsky understand it yet.

HC

[Beaucoup Movement]

I agree HC, It does seem to be happening on a particular airframe every time. (certainly in our company). At least we can rule out the fact that it's a "north sea problem only".

I currently fly the 92, Am I happy about this - no, Am I happy Sikorsky are on top of it all - yes. I also trust our engineers. Interesting how It's been happening recently & not since it's introduction.

Let's hope they can finally sort the problem soon,

BM

[Hullaballoo]

Wow. The engineers may be on top of this, but gear boxes shouldn't crack. Ever. I thought they were considered to be primary structure, like a wing spar.

And cracking after single-digit hours??? That would scare the :mad: out of me.

[riff_raff]

hullaballoo,

"The engineers may be on top of this, but gear boxes shouldn't crack"

You are spot on. An MRGB housing that carries rotor loads is a crit 1 piece of structure. As such, the casting must undergo class 1, grade A or B, 100% radiographic inspection at the foundry, with very tightly controlled acceptance criteria. And the part is also analyzed with a very conservative FoS to boot. Under these tightly controlled QA and analysis procedures, a casting structural failure would be highly unlikely.

If the failure occurred at/near a structural attachment point, I would suspect that the root cause is more likely due to an improperly installed fastener that came loose and created excessive loads on the remaining attach points.

Of course, this is all pure speculation on my part. So take it for what it's worth.

Regards,
riff_raff

[choppersky]

Newfoundland helicopter grounded because of cracks in gearbox housing

Published on January 28th, 2010 ST. JOHN'S, N.L. - A Newfoundland-based helicopter used to ferry workers to offshore rigs has been grounded because of a hairline crack in the aircraft's main gearbox housing.
The crack was found Tuesday in a mounting foot that is used to attach the housing to the chopper's airframe, a spokesman for Cougar Helicopters Inc. confirmed Thursday.
The helicopter, a Sikorsky S-92A based in St. John's, is the same type of aircraft that crashed off the coast of Newfoundland last March, killing 17 of the 18 people aboard.
Cougar Helicopters also operated that aircraft.
Christian Kittleson, a spokesman for parent company VIH Aviation in Victoria, said it's the second time a Cougar helicopter has been grounded because of a crack in a mounting foot.
The entire gearbox housing on a Halifax-based S-92A Sikorsky was replaced in November, he said.
The housing on the Newfoundland aircraft should be replaced by Friday, he added.
Kittleson said the mounting feet are inspected after every flight in response to a alert service bulletin issued last year by Sikorsky.
"We expected that it would happen again," Kittleson said in an interview. "It was a matter of time."
The problem is considered a "benign issue" that does not compromise the integrity of the aircraft, he said.
On Tuesday, the European Aviation Safety Agency issued a airworthiness directive that requires replacement of another part on the S-92A.
The agency said the choppers' filter bowl assembly in the main rotor gearbox has to be replaced with a newer model.
Kittleson said the company made the changes last month.
The Transportation Safety Board is still investigating the cause of last year's crash in the North Atlantic, about 60 kilometres east of St. John's. However, it has been confirmed that the chopper pilots reported a loss of oil pressure in the gearbox before losing control of the aircraft.
The Canadian agency has said studs broke on the filter assembly, resulting in the loss of a large quantity of oil.
Less than two weeks after the crash of Flight 491, the U.S. Federal Aviation Administration grounded S-92As worldwide with a directive requiring replacement of the studs.
In November, The Canadian Press obtained documents from the European agency that revealed the gearbox of a S-92A had failed a test that required it to run for 30 minutes without oil.
But Sikorsky has said it has proven to aviation authorities that the chances of an oil leak from the gearbox is extremely unlikely and that the installation of a bypass valve resolves the only identifiable cause of a main gearbox leak.
Earlier this month, the families of 15 passengers who died in the crash, as well as the sole survivor of the tragedy, reached a settlement in their lawsuit against Sikorsky.

Newfoundland helicopter grounded because of cracks in gearbox housing - Society - Canada - World - The Moose Jaw Times Herald (http://www.mjtimes.sk.ca/Canada---World/Society/2010-01-28/article-585934/Newfoundland-helicopter-grounded-because-of-cracks-in-gearbox-housing/1)

[maxwelg2]

Kittleson said the mounting feet are inspected after every flight in response to a alert service bulletin issued last year by Sikorsky.
"We expected that it would happen again," Kittleson said in an interview. "It was a matter of time."
The problem is considered a "benign issue" that does not compromise the integrity of the aircraft, he said.

Not a very "benign" issue for us PAX up here, confidence in this helo is yet again at a low point. Explanations and engineering solutions required...

Come on guys, get this issue sorted and put to bed once and for all. This is getting tiresome.

Fly safe

Max

[immaengineer]

Any updates on this Gearbox problem/solution?

[Grunt92]

There are a few changes recently, the latest being the reduction to the overhaul life being reduced to 1400 hours and just this last week being further reduced to 1000 hrs.
The instalation procedures for fitting the MGB have also been revised, the feet are no longer put down using sealeant UNDER the foot and they are now metal to metal contact with the airframe. With this reduction to the overhaul life I only hope Sikorsky can keep up with supplying gearboxes on time, otherwise our clients will get rather displeased with the otherwise very good S92A, when will the S92B come out.???

[Fareastdriver]

they are now metal to metal contact with the airframe

In a offshore envirionment with a salty atmosphere I would expect this to cause dissimilar metal corrosion between the gearbox feet and the airframe.

Ask Boeing Vertol what happens in a BV234 when you have dissimilar metal in the front gearbox.

[industry insider]

Hi Grunt

The life reduction is to the casting only and is temporary. The next stage is to supply all operators with a casting with additional material around the foot. This will be an interim solution. All the cracks are on one bolt on the rear foot only. The long term solution is to use the casting from the Canadian MHP gearbox which is rated to 29k lbs plus MTOM and which will have a 4000 hour life.

All retrofit parts for all of the S-92 product improvements are supplied free to operators, even back to the first production aircraft S/N 92-006 as well as being incorporated into current production ships.

Sikorsky does not apparently want any customers to have an inferior mod state which is why the company will continue to supply retrofit product improvement parts at no cost to the customer.

[immaengineer]

So the interm fix is to not use Lube? and eventually use the "MHP" Gearbox? Will that really fix the problems?

[Lonewolf_50]

So the interm fix is to not use Lube? and eventually use the "MHP" Gearbox? Will that really fix the problems?
Do you understand the difference between lubrication and sealant? An engineer ought to. ;) Look at what he wrote ...
The instalation procedures for fitting the MGB have also been revised, the feet are no longer put down using sealeant UNDER the foot and they are now metal to metal contact with the airframe.

May I ask you for some clarification?

I'll offer a guess that the "metal to metal" actually has, on both the transmission deck and the gearbox, conversion coating, sealant-coating (for the gearbox, along the lines of Rockhard/Sermetel/whatever the flavor of the month is), a primer, and then paint. (If MGB isn't Mag, is that layer of coating necessary?) Or, are the feet not painted, just primed?

That isn't quite "metal to metal," is it? ;) Maybe I am being nitpicky here.

That said, an operator's concerns about vibration and time in the salt environment are reasonably well voiced. I will guess also that sealant is applied in a healthy bead around the feet where they attach to the transmission deck. (Keep that moisture out, I say!)

Not asking you to reveal any proprietary info, just hoping to fill in a few gaps by what I assume (perhaps incorrectly) would be standard practice based on older Sikorsky aircraft.
With this reduction to the overhaul life I only hope Sikorsky can keep up with supplying gearboxes on time, otherwise our clients will get rather displeased with the otherwise very good S92A, when will the S92B come out.
I'd guess that Sikorsky folks know that from the inside better than folks on the outside. :)

@maxwelg2: you pose excellent questions, which hopefully the engineering team at Sikorsky will have answers for.

[squib66]

Hope - Hope seems to be something that Sikorsky don't supply to their customers as standard...

Looking back at the recent posts I see no mention of the SECOND complete foot detachment in flight. Has that really been kept under wraps?

Should this thread not be merged?

[immaengineer]

I was being sarcastic when i used the term "Lube". To me that isn't a certifiable fix with all the vibration that'd be transfered from "metal to metal" The Gearbox's are definitely coated, but whose to say the coating won't crack over time and allow moisture in? etc.

[Lonewolf_50]

The Gearbox's are definitely coated, but whose to say the coating won't crack over time and allow moisture in? etc.

If you are an operator, visually inspecting your gear boxes (be it daily, between flights, or weekly) to detect flaws in the coatings (and subsequent touchup) is what many people call "good maintenance practices." Likewise, sometimes a flaw/crack in a coating leads you to inspect for cracks in the base metal.

Yes indeed, as you point out, coatings can be compromised over time.

The second part of my post was questions to Grunt92, rather than yourself, but perhaps I failed to make that clear. Apologies for any confusion. :sad:

[Soave_Pilot]

Hopefully I won't step a foot on this a/c! Some scary stuff.:sad: Let's all stick to S-76's.:ok:

[Horror box]

Hopefully I won't step a foot on this a/c! Some scary stuff. Let's all stick to S-76's.

Now that is a joke - surely! The 76, whilst in its time an acceptable machine, it certainly has a less than stellar record all-round.

[Horror box]

as per usual - the truth is not getting in the way of some good old scare-mongering!
- The S92 has a known issue with cracking in one of the mounting feet for the MGB
- It is only one of the four feet in which this is occurring, and is occurring in the same area each time.
- Whilst not optimal, natural design redundancy means that the remaining three struts are more than capable of holding the structure indefinitely.
- New maintenance procedures require inspection of all mounting feet before every single flight and this is stated and signed in the DMR.
- If a crack is found, the aircraft is grounded immediately.
- Comprehensive study is being carried out by SAC in cooperation with the operators to find all possible causes.
- MGB life has been significantly reduced until the problem is rectified at the cost of SAC.
- Flight procedures have included reducing cruise torque to reduce vibration levels, and reduce the overall stress on the gearbox mounting, until the cause can be conformed and fixed.

No I don't work for SAC, but as most will realise I do fly the S92, and am satisfied with the level of management of the issue, as are I hope most who work with machine.

[maxwelg2]

Lonewolf, thanks for the positive feedback, I've been patiently waiting for news on the uprated MGB release to civilian industry. Reduction of overhaul periods is a contingency, not a solution. What metric is used to determine the numbers of hours, hopefully not the same one that was used to calculate 1250 hours for filter housing stud change out...was FMECA used?

So far no conclusive answers to my questions, still waiting for SAC to be man enough to publicly address these issues and instill confidence in the S-92, most likely not going to happen until a lot a trouble-free flying hours are clocked up to prove that they are actively managing the still present design issues.

What still amazes me is that this helo is allowed to operate in hostile environments such as offshore personnel transfer without 30-minute run dry capability when IMHO the FAR29 clause is not being met due to the number of failures to date (2) as I count the Broome incident as a failure as well regardless of the conjecture on the root cause failure mode i.e. the filter housing should not have created a leakage path due to sub-standard design/material selection. Explain that one to me guys...

Safe flying

Max

[riff_raff]

maxwelg2,

The MRGB housing casting cracks on the S-92 are a problem that should probably not have occurred if all of the FAA regulations were followed. FAR 29.621 defines a structural MRGB casting as "critical", which means it should have an additional "casting factor" of 1.25 applied on top of the standard 1.5 FoS used for structural analysis. So a proper structural analysis of this part should have had plenty of room for error.

FAR Part 29 Sec. 29.621 effective as of 11/28/1997 (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/F130A948FC97E46185256613006BA9BC?OpenDocument)

Additionally, "critical" class castings are required to undergo 100% radiographic and dye penetrant inspection under FAR 29.621, so there should be no castings put in service with any existing surface/subsurface structural flaws.

Good manufacturing practice also would dictate that critical areas in a structural casting (like the attachment feet in the S-92 housing) would be areas where tensile test coupons are periodically cut from sample castings and checked for mechanical properties. This ensures that foundry processes used to produce the casting are being properly controlled.

As for in-service cracking, some high strength aluminum alloys can experience stress corrosion cracking, especially under conditions of sustained load and exposure to marine atmospheres. But most MRGB housings are cast from 356 or 357 aluminum alloys, which have good corrosion resistance and thus are normally resistant to stress corrosion cracking.

Regards,
riff_raff

[industry insider]

I believe that the MGB housing is magnesium not aluminium.

[immaengineer]

Yea, It's Mag, and it goes through all of those inspections you mentioned as well...

[rotormatic]

New proposed FAA Airworthiness Directive:



List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Safety.

The Proposed Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration proposes to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701.

Sec. 39.13 [Amended]

2. Section 39.13 is amended by adding a new airworthiness directive to read as follows:

Sikorsky Aircraft Corporation: Docket No. FAA-2010-0720; Directorate Identifier 2010-SW-050-AD.

Applicability: Model S-92A helicopters, with main gearbox housing, part number 92351-15110-042, -043, -044, or -045, installed, certificated in any category.

Compliance: Required as indicated, unless done previously.

To prevent failure of the main gearbox housing mounting foot pad, loss of the main gearbox, and subsequent loss of control of the helicopter, do the following:

(a) Within 60 days, revise the airworthiness limitations section of the Instructions for Continued Airworthiness by reducing the life limits of the affected main gearbox housing from 2700 hours time-in- service (TIS) to 1000 hours TIS.

(b) After revising the life limit in accordance with paragraph (a) of this AD, before further flight, replace any main gearbox housing that exceeds the life limit of 1000 hours TIS.

(c) To request a different method of compliance or a different compliance time for this AD, follow the procedures in 14 CFR 39.19. Contact the Manager, Boston Aircraft Certification Office, FAA, Attn: Michael Schwetz, Aviation Safety Engineer, 12 New England Executive Park, Burlington, MA 01803, telephone (781) 238-7761, fax (781) 238-7170, for information about previously approved alternative methods of compliance.

(d) The Joint Aircraft System/Component (JASC) Code is 6320: Main Rotor Gearbox.

[500e]

I know there is inspection every flight ! ( New maintenance procedures require inspection of all mounting feet before every single flight and this is stated and signed in the DMR.) Does every landing\take off qualify as a flight? & how does 1000 hours fit in, as there is a post saying crack in single hours.

[Scotsheli]

I believe the AD is aimed at preventing a circumferential crack which could result in the complete severing of a RH foot. SAC I believe have been working to reduce the total times on the cases in service already.

The visual inspection (whilst also effective at detecting the above - one would hope!) is not to be confused with the intent of the AD - this inspection is aimed at detecting the hairline crack in the outer edge of the foot which originates from the bolt hole; this remains a 10 hour inspection I believe.

Scots

[industry insider]

The inspection intervals were validated by fatigue testing on an MGB testing rig using a production MGB casting with a deliberately severed foot. The test rig simulates the flight characteristics of the aircraft. The test showed the other feet MGB capable of exceeding the 10 hour flight time inspection by 7 times.

[500e]

I I
I would Suggest that the casing was tested to arrive at the 2700 hours
As riff raff quoted
Good manufacturing practice also would dictate that critical areas in a structural casting (like the attachment feet in the S-92 housing) would be areas where tensile test coupons are periodically cut from sample castings and checked for mechanical properties. This ensures that foundry processes used to produce the casting are being properly controlled.
There seems to be a flaw in either design \ material, or the loads \ vibrations analysis are way off, either way why should the 3 leg test be any better? as the original calcs seem to be adrift.

[industry insider]

Sven, no cracks started appearing until 2009 and then only on aircraft where MGB and or bolts had been replaced outside the factory environment. Until then, no foot cracks appeared indicating that there was nothing amiss with the load paths used on original testing.

[widgeon]

500E , in my experience of other GB the life limited items on the Gearbox seldom include the housing . In most cases a full inspection and some minor reworks may be required to the housings at the first cycle, normally the parts that are replaced at each overhaul are seals and bearings. Correct me if I am wrong but the S92 was sold for it's low DOC based on the fact that all major components were on condition , was the 2700 hr TBO there from the start ?.

[maxwelg2]

...no cracks started appearing until 2009 and then only on aircraft where MGB and or bolts had been replaced outside the factory environment. Until then, no foot cracks appeared indicating that there was nothing amiss with the load paths used on original testing.

Interesting point, 2700 TIS = 337 days @ 8 hours/day. S92 has been in service in Newfoundland since circa 2006. So even with non-stop daytime operations the MGBs would have been changed out at least 3 times prior to the cracking issue appearing. I'm assuming that the removed MGBs were sent back for overhaul and re-conditioning, which would then result in the re-use of the same housings, or are they scrapped?

MGB housing cracks started appearing post-MGB change-out by operators rather than SAC. Surely there is no difference in the procedure that SAC use versus the operator?

Was there any change to QA/QC on the MGB housings and/or supplier?

Perhaps something else is changing during "run-in" of the A/C e.g. alignment of the mounting points leading to additional stress points not seen in the MGB test rig?

How about thermal expansion effect on chassis versus MGB material, related to ambient and MGB operating temperatures? From memory I recall the original cracking was only seen in offshore operational areas, is there a link to the ambient operating conditions, and do SAC use an environmental simulator area when using the MGB test rig to temperature cycle as well as emulate TIS?

I'm sure that SAC design engineers have considered all these scenarios during the latest MGB upgrade and re-engineering.

For me it would be beneficial if a statement was released by SAC stating the root cause issue identified and the engineered solution arrived at. This would be a positive step forward in re-instilling confidence in the revised MGB design.

Safe flying

Max

[industry insider]

Max, Sikorsky has briefed all operators at length on this subject in line with continuing to supply new castings at no cost. Your operator should be able to brief you fully.

[maxwelg2]

Max, Sikorsky has briefed all operators at length on this subject in line with continuing to supply new castings at no cost. Your operator should be able to brief you fully.

Thanks industry insider, I've requested an update directly from Cougar today, I'll wait for their response.

Safe flying

Max

[One_S92_Inst]

For your viewing pleasure:

New Addition to Sikorsky360 - S-92 Helicopter Main Gearbox Improvement Information - CCS-92-AOL-10-0011

MGB Foot Crack Analysis Tool for HUMS!!! - CCS-92-AOL-10-0008

[500e]

I I
Nice to hear that SK are supplying Free casings I expect they are reimbursing down time & engineering costs as well ??

[immaengineer]

Has anyone seen a vibratory analysis on the Gearbox feet as they relate to the airframe?

[ifsknt]

500e. I have not heard of those costs being reimbursed to our company. However, all recent product improvements have been free, not just the MGB related ones. We never got that from EC with the 332L or the EC225.

[500e]

IFSKNT
Nice to hear about casing and product improvements?? so your Co are standing ENG costs and loss of revenue on a multi million $ product that appears to be faulty, makes you wonder.
I am not saying that it is only SK EC or only the helicopter industry that charge indirectly the end user for design shortcomings, and expect them to pick up the bill.

[Grunt92]

immaengineer.
On the HUMS down load, which in the company I work for, we down load after every flight, which is normally about 4 to 5 hrs duration the gearbox foot vibration analysis can be viewed, although I can't make head nor tail of it, ( awaiting the company course). It is then sent away, electronically, direct from the laptops, to our main base and SAC for proper analysis, along with the TGB shaft info, T/Rotor pivot bearing retainer, info, and also the other vibration reports for all the rest of the aircraft.
During Gearbox changes the airframe mount holes are measured for rotation of the bushings and also a maximum/minimum dimensions between holes.

[immaengineer]

Understood... Thanks... I look forward to seeing the data on the specific foot that seems to be the focal point of the cracking...

[squib66]

If SAC knew I doubt they would share. All indications are they have no real understanding of the failure mode.

[Oldlae]

Grunt92
Surely, you have hit the spot, bushings shouldn't rotate, holes shouldn't change dimensions, must be something wrong with the designed interference fits/clearances.

[immaengineer]

Or it could be a supplier problem, and SAC didn't inspect properly... The supplier could be fudging things up. From my research I found out this box is a derivative from the Black Hawk. The Black Hawk has been in service for decades so I think any little problems should have been worked out over the years...

[212man]

From my research I found out this box is a derivative from the Black Hawk

No sh*t Sherlock!

[immaengineer]

Thanks for the sarcasm, I can tell you are very well liked... and SAC doesn't build their own MGB housings... They are supplied

[I Build 92's]

And while the foot print is the same and the overall appearance is similar, totally different design gear box.....

[VeeAny]

http://www.caa.co.uk/docs/33/EASA_EAD_2010-0170-E_1%5B1%5D.pdf


Posted just for info, probably woth merging with the S92 Gearbox Crack Thread in a few days.

[Lonewolf_50]

Found out recently that the Navy is looking to go from Mag to Al main gear boxes (and I think Intermediate/Tail rotor as well) in the next few years, even though it created a small weight penalty to do so. (Something less than 50 lb, I think, but I may be wrong about that).

Given that S-92 operators seem to operate quite a bit in salt water environment, is a switch to Aluminum gear boxes a design option under consideration for later production?

If that's close hold/company info, apologies if it seems out of order to ask.

[Jack Carson]

I believe that Sikorsky uses either Rockhard or Sermatech coatings on all main and tail gear box housings. This was incorporated during S-92 development to improve the durability of the housings. The initial concept behind the development of the S-92 was for all of the major dynamic components to be capable of being retro-fitted on future Blackhawk/Seahawk models. As such the basic layout (i.e. foot print) would match that of the Hawk line. This design constraint may in part be responsible for some of the on going problems. The Blackhawk’s design gross weight was 18600 lbs. and its maximum all up weight of was approximately 24000 lbs. 28000 lbs may be a stretch for the present configuration.

[[email protected]]

But Sikorsky sold the S-92 boasting that it was based on Blackhawk dynamics which therefore gave it strong lineage and assured quality - clearly not true:ugh:

[immaengineer]

Switching from MAg to ALm will be a great weight penalty... 150lbs IMO... With addition of all the other upgrades 200lbs wouldn't be farfetched to me...

[Horror box]

Switching from MAg to ALm will be a great weight penalty... 150lbs IMO... With addition of all the other upgrades 200lbs wouldn't be farfetched to me

Not exactly a great weight penalty over 26500lbs. Get rid of three of the six FG's if you really want to save weight, as they don't make much difference anyway. Alternatively take approximately seven minutes less fuel, either way it is not going to be a huge show stopper.

[212man]

Introducing the increased MGW (somewhere North of 27,200) will obviously help offset any additional weight, too. Though whether that's on hold pending the phase 3 MGB I've no idea.

[riff_raff]

"Switching from MAg to ALm will be a great weight penalty... 150lbs IMO"

immaengineer,

The weight difference between a sand cast aluminum or magnesium gearbox housing is usually not much. There are several reasons for this.

First, while magnesium is less dense than aluminum (.064 vs .098 lb/cu.in.), the minimum wall thickness most sand foundries will pour is .16 inch for mag and .12 inch for aluminum. Lightweight aircraft gearbox housings tend to have lots of surface area (especially with cast lube system pipes) with minimum thickness. So that extra .04 inch of mag can make a difference with a large housing.

Second, maintaining accurate gear geometry under load is very important. Housing deflections can cause problems with gear and bearing life, so having a stiff housing structure is critical. Aluminum has a higher modulus than mag (10.3 vs. 6.5 x 10^6 lb/sq.in.), so it gives better stiffness for a similar structural configuration.

Third, if the gearbox is in an area that must meet a fire rating, large thin wall areas in a mag housing must sometimes be thickened to meet the burn through requirement.

As for using cast mag in a marine environment, this is something that the Navy (NAVAIR) usually frowns on, mostly due to corrosion/maintenance issues. There are some newer sand cast mag alloys (WE43) that have decent corrosion resistance, but they are much harder to cast than the more common mag alloys such as ZE41.

riff_raff

[industry insider]

212
MGW increase will be to 27700 according to recent briefs by SIkorsky. No impact on or from the MGB issue but a modified IGB will be required to go to the IGW.

[pasptoo]

27700 according to recent briefs by SIkorsky. No impact on or from the MGB issue

What about Frame cracks then? Will they just disappear?

[riff_raff]

industry insider,

You seem to have a good knowledge of the S-92 TR drivetrain. Another poster mentioned that the S-92 TR drivetrain components could be retrofit to the H-60 model. Is this true?

You mentioned that the S-92 TR drive intermediate gearbox would need to be upgraded to get the 1200 lb (?) increase in GW. What needs upgrading? Is it bearings, gears, lube system, housings, couplings, or some combination of these?

Also, do you have any idea how much margin (if any) there is in the rest of the S-92 TR drivetrain components with the CT7-8A's?

I'd appreciate any insight you could provide. TIA.

riff_raff

[industry insider]

Riff

Its gear coating only for the IGB gears themselves I believe. I think that the H60 and S-92 have tended to diverge rather than converge in component terms. The CT7-8A engine itself will probably be the limiting factor for any more IGW programs but there is more engine power on the way with another variant coming.

[riff_raff]

industry insider,

Thanks for the info.

I believe CT7-8C was designed to go into the H-60 model at some future point, but I was told that the H-60 MRGB is already torque limited under some conditions with the CT7-8A.

riff_raff