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S-92 gearbox crack

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S-92 gearbox crack

Old 29th Jul 2010, 15:07
  #21 (permalink)  
 
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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.
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Old 29th Jul 2010, 16:55
  #22 (permalink)  
 
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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.
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Old 29th Jul 2010, 17:41
  #23 (permalink)  
 
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Hopefully I won't step a foot on this a/c! Some scary stuff. Let's all stick to S-76's.
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Old 29th Jul 2010, 19:06
  #24 (permalink)  
 
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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.
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Old 29th Jul 2010, 19:21
  #25 (permalink)  
 
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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.
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Old 29th Jul 2010, 21:53
  #26 (permalink)  
 
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Still waiting for answers

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

Last edited by maxwelg2; 30th Jul 2010 at 13:21.
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Old 2nd Aug 2010, 01:58
  #27 (permalink)  
 
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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

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
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Old 2nd Aug 2010, 02:02
  #28 (permalink)  
 
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I believe that the MGB housing is magnesium not aluminium.
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Old 2nd Aug 2010, 11:53
  #29 (permalink)  
 
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Yea, It's Mag, and it goes through all of those inspections you mentioned as well...
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Old 2nd Aug 2010, 23:51
  #30 (permalink)  
 
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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.
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Old 3rd Aug 2010, 13:04
  #31 (permalink)  
 
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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.
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Old 3rd Aug 2010, 19:44
  #32 (permalink)  
 
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S92 MGB Foot Inspection

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
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Old 4th Aug 2010, 10:40
  #33 (permalink)  
 
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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.
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Old 4th Aug 2010, 12:20
  #34 (permalink)  
 
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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.
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Old 4th Aug 2010, 13:36
  #35 (permalink)  
 
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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.
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Old 4th Aug 2010, 14:01
  #36 (permalink)  
 
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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 ?.
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Old 4th Aug 2010, 14:38
  #37 (permalink)  
 
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...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
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Old 4th Aug 2010, 15:05
  #38 (permalink)  
 
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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.
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Old 4th Aug 2010, 16:01
  #39 (permalink)  
 
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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
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Old 6th Aug 2010, 02:57
  #40 (permalink)  
 
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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
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