Sikorsky S-92: From Design to Operations
Lots of potential problems with retaining that filter bowl. Key elements appear in the report but not all of the potential problems are highlighted. Using a stud certainly has the advantage of bringing the regularly-dismantled threaded joint out to where the maintainer can observe it properly and makes nut replacement simple. However, at the same time, it provides for two threaded joints that can cause problems instead of just one. Then there is the length of the stud which is not great enough to allow significant stretch under preload and therefore is highly prone to loosening. (Locking wires and such like don't prevent loosening if aspects of the threaded fastener design are poor.)
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Jim,
Reading that report, I am not so sure that the studbolts were as easy as you say for the maintainer to properly observe. Task 63-24-02-210-001 of the Sikorsky AMM Rev 13 issued 5/11/08 required use of an inspection mirror and a 10x magnifying glass. Used individually at a workbench that seems reasonable but not so easy to use together in situ, I would think.
It may have been due to that difficulty, I would suggest, that between 5/11/08 and 23/3/09 none of the S-92A operators reported finding any damaged studs while ‘performing’ the enhanced inspection. However, after the FAA issued EAD 2009-07-53 on 23/3/09 and all titanium studs were replaced with steel ones, operators returned 59 titanium studs with varying degrees of galling. That was out of a fleet at the time numbering around 80 aircraft, so roughly a 25% return/reject rate. Those studbolts were replaced in the field whereas when the problem first came to light in 2008 it was held to be an overhaul facility task which doubtless delayed preventative action and tragically cost 17 lives.
I agree with your other observations on the filter bowl attachment.
Reading that report, I am not so sure that the studbolts were as easy as you say for the maintainer to properly observe. Task 63-24-02-210-001 of the Sikorsky AMM Rev 13 issued 5/11/08 required use of an inspection mirror and a 10x magnifying glass. Used individually at a workbench that seems reasonable but not so easy to use together in situ, I would think.
It may have been due to that difficulty, I would suggest, that between 5/11/08 and 23/3/09 none of the S-92A operators reported finding any damaged studs while ‘performing’ the enhanced inspection. However, after the FAA issued EAD 2009-07-53 on 23/3/09 and all titanium studs were replaced with steel ones, operators returned 59 titanium studs with varying degrees of galling. That was out of a fleet at the time numbering around 80 aircraft, so roughly a 25% return/reject rate. Those studbolts were replaced in the field whereas when the problem first came to light in 2008 it was held to be an overhaul facility task which doubtless delayed preventative action and tragically cost 17 lives.
I agree with your other observations on the filter bowl attachment.
I find it hard to believe that both the CHC maintenance techs and Sikorsky field service reps considered this to be an acceptable repair procedure.
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Didn't it? The TSBC report (1.18.3.2) seems to imply that it did.
"The initial visual examination by CHC maintenance personnel and a Sikorsky field representative revealed that two of the three MGB oil filter bowl studs had fractured and the filter bowl had partially separated from the MGB causing a total loss of oil. One of the failed studs had been repaired on 09 June 2008 (see 1.18.3.4)."
"The initial visual examination by CHC maintenance personnel and a Sikorsky field representative revealed that two of the three MGB oil filter bowl studs had fractured and the filter bowl had partially separated from the MGB causing a total loss of oil. One of the failed studs had been repaired on 09 June 2008 (see 1.18.3.4)."
I was not aware of that inspection procedure. My intention was to compare accessibility with a threaded hole (or helicoil or insert) in the parent assembly.
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From the same report, however:
1.12.5 “…Testing of the occurrence and exemplar studs and nuts showed that after 13 to 17 assembly cycles, the nut self-locking feature was significantly damaged… “
Curiously, although Cougar CH191 and worldwide fleet maintenance records were averaging around 200 – 220 hours between filter replacements, the CHC VH-LOH had averaged only 72.6 hours over 17 replacements, so the self locking effectiveness may have been lost anyway.
Sikorsky did not recommend renewing the nuts at every filter removal/replacement until September or November 2008, several months after the CHC failure.
So, at least as far as nut (and preload) retention is concerned, it may have been little worse than refitting a worn self locking nut. Neither is desirable but only the improvised repair looks obviously suspect. The studs of both failed, at much the same time, the fatigue damage possibly having been largely done under the original attachment conditions, as later with CH191[/I][/FONT].
1.12.5 “…Testing of the occurrence and exemplar studs and nuts showed that after 13 to 17 assembly cycles, the nut self-locking feature was significantly damaged… “
Curiously, although Cougar CH191 and worldwide fleet maintenance records were averaging around 200 – 220 hours between filter replacements, the CHC VH-LOH had averaged only 72.6 hours over 17 replacements, so the self locking effectiveness may have been lost anyway.
Sikorsky did not recommend renewing the nuts at every filter removal/replacement until September or November 2008, several months after the CHC failure.
So, at least as far as nut (and preload) retention is concerned, it may have been little worse than refitting a worn self locking nut. Neither is desirable but only the improvised repair looks obviously suspect. The studs of both failed, at much the same time, the fatigue damage possibly having been largely done under the original attachment conditions, as later with CH191[/I][/FONT].
Looking at at photo of the failed filter housing attachment it appears the type of locknut used is an MS21043. This is an A286 corrosion resistant steel all-metal locknut with silver plating finish that is widely used in the aerospace industry, and it uses a deformed thread locking feature located at the upper end of the nut. These nuts are qualified to MIL-DTL-25027, which includes testing to verify the locking feature's prevailing (locking) torque remains within specified limits for a minimum number of installation/removal cycles.
I'd estimate the stud in the filter housing photo is maybe 1/4" diameter. One common issue with all-metal deformed thread locknuts at smaller sizes like this is difficulty in accurate control of prevailing torque during manufacture, and maintaining consistent prevailing torque over repeated installation/removal cycles. For this reason, every aerospace manufacturer I have worked with has fastener installation procedures that require checking the prevailing torque of every locknut at each installation to verify it is within specified limits. These procedures are normally carried over to the maintenance instructions since it is a safety/reliability compliance issue with the threaded fastener dual locking function requirement contained in FAR 29 & FAR 27.
The prevailing torque problem becomes worse as the thread size gets smaller. These #4-40 size locknuts are often used to secure electrical connectors, and I have seen situations where maybe 10% or more of brand new ones had out-of-tolerance prevailing torque and had to be scrapped. I think one reason for this is the sampling rate used for acceptance, which is only 5 samples per 10,000 parts.
One last thing I would point out about the locknuts shown in the photo of the filter housing linked above is that they appear to be silver plated, which would be standard for an MS21043 locknut. It was reported that the mating studs were titanium alloy. It is well known that silver coatings should not be used in contact with titanium, since it can cause embrittlement of the titanium. Here's what MMPDS has to say about it: "Under certain conditions, titanium, when in contact with cadmium, silver, mercury, or certain of their compounds, may become embrittled. Refer to MIL-HDBK-1568 for restrictions concerning applications with titanium in contact with these metals or their compounds."
S92 at the White House today, along with the VH-3 and other Sikorsky products.
White House Highlights Sikorsky During 'Made In America' Event - Hartford Courant
White House Highlights Sikorsky During 'Made In America' Event - Hartford Courant
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Cougar S92 Payload increase of 1200 lb
Anyone know the details on what is involved with the modification to the S92 fleet to increase payload by 1200 lbs? Passengers are getting curious.
The upgrade was bought in back in 2015. Transport Canada approved it back in April this year.
https://www.verticalmag.com/news/sik...lopmentfors92/
Google is your friend.
“[We’re] making it very simple to retrofit the gross weight expansion, so for an increase of 1,200 pounds, it’s simply a 10-pound kit for the aircraft in the field,” said Audrey Brady, director of S-92 programs at Sikorsky. “[It involves] a series of doublers and stringers, clad to the aircraft, to get it structurally available for the higher gross weight.”
Google is your friend.
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Increasing to the higher gross weight is a lot more then just some structural upgrades. There are many components that the life reduces and part numbers change. So components on the lower gross weight aircraft cannot be used on the higher gross weight and vice versus. That is a big increase in costs to operate with the higher gross weight regardless if you actually operate the aircraft at the higher gross weight. If you have both types in your fleet you have a lot of expensive parts to keep in stock that can only be used on the one type. This is the main rotor parts , main rotor servo´s etc. If you plan to upgrade your aircraft do your homework!
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I don't suppose you care to share with us why you think as you do?
Or....you just happy to be moaning about something hoping you will find someone to console you over your misery.
It is time for upgrades to the 92 no doubt.....as all aircraft can be improved with new technology that has come into being since it was first designed and built.
Hopefully, Sikorsky is in the process of sorting out what needs improving and will come forth with an innovative update.
As the FAA has to be involved in some parts of any significant upgrade.....that might prove to be an obstacle re certification issues.
For sure....Pilots and Customers should be making their desires, concerns, and comments known to Sikorsky for their consideration during the process.
Or....you just happy to be moaning about something hoping you will find someone to console you over your misery.
It is time for upgrades to the 92 no doubt.....as all aircraft can be improved with new technology that has come into being since it was first designed and built.
Hopefully, Sikorsky is in the process of sorting out what needs improving and will come forth with an innovative update.
As the FAA has to be involved in some parts of any significant upgrade.....that might prove to be an obstacle re certification issues.
For sure....Pilots and Customers should be making their desires, concerns, and comments known to Sikorsky for their consideration during the process.
Last edited by SASless; 19th Dec 2018 at 11:16.
SAS and company here have a point about it being time for an S-92 upgrade. Not due to just the time/technology consideration, but the inevitable GW creep and mission expansion. Note that the original S-92 design was laid down in the 1990’s for a 23000 lb machine. During an extended design and development period this weight morphed to 26500. Now it is 27700 for the civil and VH version, and 28650 for the Canadian MHP, all the while with the same power train and rotors ( except for some engine upgrades ). We did a flight loads test program preparatory to a Portuguese Coast Guard proposal at 31,800lbs ( my recollection-could be off by a bit ) and up to 9000 ft, and that data clearly indicated that rotor growth was indicated. I should add that increasing the number of main blades to 5 or better yet 6, would not only restore the maneuver and performance envelope as the GW increases, but also result in a better vibration situation, accompanied by a reduction of the parasitic weight of the vibration attenuation systems. Some of my former colleagues at SA have heard this before-going back to the initial S-92 design stage, but it was decided, and quite frankly I concurred with the strategy to put a four bladed system on the 92, providing a pre designed, pre-flight tested, pre qualified, upgraded rotor system for the UH-60 at an appropriate future time. At this point, however, and addressing only the 92, its time to increase the number of blades for the next growth version. A smart design would be back-retrofittable.
On the cockpit/controls subject, a couple of comments. First, the FBW system on the Canadian MHP has received excellent marks from both the SA and Canadian pilots, and should, one would think be a natural upgrade to the civil version. My opinion is based on the fact that the FAA has yet to address FBW in Part 29 and its attendant Advisory Circular ( which instructs on how the applicant is to meet the Part 29 standards ), and thus, folks like the AW 609 team are obtaining certification based upon “ special conditions “. I haven’t seen those special conditions published anywhere, but I’m guessing ( and I do mean guessing ) that AW briefs the FAA on how their system works and the FAA flies it, assesses whether it does what AW says, and then approves it. ( That Part 29 is two decades plus behind current flight control technology is a subject for separate discussion ). Anyway, by this line of thinking, putting the FBW system into the FAA certified S-92 should be relatively easy.
The cockpit equipment discussion is also valid. During initial design, Honeywell proposed their Epic cockpit, along with an updated version of their S-76 autopilot. That was up against a Collins proposal for cockpit equipment and a Sikorsky/Hamilton Standard SAS/autopilot. That the UTC based system ( Ham STD is a UTC company and had built all the AFCS hardware for all models since the S-58 ) was selected was understandable, but disappointed a segment of SA who had perhaps a better feel for the direction and capability the operators were looking for. Understatement. Back to the point raised: certainly the technology has advanced demonstrably since the basic design of the 92 and those capabilities are valid candidates for inclusion in what one might call a “ S-92B- Model “.
Last edited by JohnDixson; 19th Dec 2018 at 16:22. Reason: added words
The early 76's had the in house HamStandard SAS/Autopilot system but that quickly fell into disfavor with Sperry Helipilot Systems becoming the preferred choice as I recall.
Sikorsky had been working conceptual design layouts for a 5 bladed S92 hub going back at least 10 years ago. With the original VXX, CSAR-X, and MHP kerfuffle, I think a lot of these upgrades (presumably including the ID-MGB III capable of actual 30 minute loss of lube) fell by the wayside as the 92 had not found a large lucrative customer at that point.