Nick. Good post!

A question well asked about "the system" (with all of those folks in it) remains: how was the lesson learned in the incident in Australia (previous to this tragic crash) spread among the interested parties? A variety of "if we know this, what do we do with that information" arises every time things are discovered in the field. This question is informed by the case of a swashplate bearing and the eventual addition of the bearing monitor panel, in another model of aircraft.

Design and Engineering
 

Nick thanks for the insightful and (at least for me) illuminating information on the engineering considerations - well put.

LW50 and NF ask excellent questions.

On the question of weight: Would these 17 fatalities been avoided if the crew has SS6 floats appropriate for the Atlantic and been more comfortable ditching? SS4 is only good for ditching along side those in d'Nile after all.

I wonder how the trade off was made on the failure to achieve 30 minutes running after a loss of lubrication. There was over 2 years from the failed test to entry into service and over 6 until Cougar. So time to act. Too expensive (like the Ford Pinto) or too heavy? Another Sikorsky helicopter had a total oil loss in Canada in 2002 so the alarm bells should have been ringing with Sikorsky and the 'extremely remote' argument was already 'optimistic'. Or should we just going to hear the FAA were to blame for letting Sikorsky deliver the aircraft?

NF. Look again, the reasoning is recorded in the report.

1.6.3 S-92A Main Gearbox

The oil filter bowl is attached to the MGB housing by three equally spaced titanium alloy stud and self-locking nut assemblies. The manufacturer decides what type of fastener to use, as there is no specific rule that defines the type of fastener to be used for what application. In selecting a fastener, aircraft manufacturers typically use similar product history, published material specifications (i.e. American Society for Testing and Materials), and/or develop their own process specifications.

Titanium is desirable in certain applications due to its corrosion resistance and lighter weight as compared to steel. Sikorsky selected the titanium alloy stud because these had been used successfully on other Sikorsky products, such as the CH-53E Sea Stallion which utilizes six titanium studs to attach the oil filter bowl to the MGB. Also, Sikorsky declared that there was no reported history of in-service titanium stud failures.

The fastening system for the oil filter bowl of the S-92A used anodized titanium alloy studs, silver-plated steel nuts and cadmium-plated steel washers. Anodizing and plating increases resistance to corrosion, wear and galling.

http://http://www.bst-tsb.gc.ca/eng/rapports-reports/aviation/2009/a09a0016/a09a0016.asp#sec1_6_3

As always I stand to be corrected, but I think you will find the UH-60 series MGB has titanium studs and still the case today for the reasons noted above.

Read the following paragraph as to why the S-92 was unique in having problems with the titanium studs.

1.6.3.3 S-92A Main Gearbox Oil and Filter Replacement

The impact of this crash was sufficient for the sponsons (and their integral dinghies) to detach as I recall. I very much doubt sea state six floats would have made any difference to the poor souls with multiple broken bones who drowned in the freezing cold water.

You can talk about the design flaw and the inadequate run-dry time for decades if you like. The simple truth is that if the Captain had listened to his co-pilot, who twice told him that they had reached the words 'Land Immediately' in the checklist, there would have been no accident to survive, merely a controlled ditching.

I think you are missing the point, which is that the sea state at the time was above 4 and perhaps that was a factor in the captain's denial about the need to ditch - ie he knew it would not be a safe ditching and possibly felt that continued flight was the safer option. But of course we don't know that.

SAR RC sounds like your answer to my question is either "possibly" or "probably". Unless you think someone believed the oil loss marketing hype and pressed on for the Cape...

Another conclusion from what you write is that survivability features mitigate design flaws (among other things) and if a manufacturer runs out of options and has to say "Land Immediately" in the RFM they shouldn't begrudge investing in them! If they do perhaps it's a sign they aren't going to make much of an effort either

I can understand if Sikorsky are sensitive on survivability. Flack have a C$200k research contract to look at the S-92 exit windows and see if they can actually be opened in a real scenario. The CAA / UK oil company cabin size project has made the S-76 an 8 seater (and any one here who is Xtra Broad shouldn't argue with that considering there have been a trio of S-76 ditchings in recent years), its floatation performance is poor and the 76D isn't likely to survive until next HeliExpo under new owners. Plus I believe the European survivability rule making proposal to change Part 29 hits the streets in a few weeks.

And the SH-60 (now MH-60) family as well.

Hilife Thanks for taking the time to do that research.

Hilife / Lonewolf_50 Contacts at Fort Rucker insist the H-60 started with stainless steel bolts not titanium studs.

I see "the CH-53E Sea Stallion which utilizes six titanium studs". Why only three studs on the S-92A? Weight again?

With even just 4 studs, geometrically the filter housing would have been less susceptible to the loss of one stud.

Really?! Wasn't galling the failure mode here?! This just begs more questions than it answers.

This question by Lonewolf_50 remains valid:

I understand this comment to mean the H-53 disaster after a quality control deception by a Sikorsky supplier discussed here: http://www.pprune.org/rotorheads/202...g-failure.html
and
At Sikorsky Aircraft, A Flight Gone Wrong - NYTimes.com

Are you referring only to what secures the oil filter to the main module (actually, the sump) or all of the studs holding the transmission together?

As soon as the FAA introduce a means for employees to raise concerns about poor certification decisions, some in industry challenge the idea!
FAA Safety Review Prog delaying Aircraft Cert? | JDA Journal

Lonewolf_50
The filter.

Ah. Was thinking all studs ...

There was quite a bit of discussion on this thread about the design issues with the S-92 MRGB. As I pointed out, in my opinion, the use of just 3 fasteners to attach the oil filter housing did not provide a sufficient level of fault tolerance in the event one of the fasteners were to fail.

Also, in my opinion, titanium fasteners should never be used for an application that involves regular removal and re-installation of the fastener, such as attachment of an oil filter housing . Titanium fasteners work well for situations where the fastener is installed under carefully controlled conditions and never is removed. But due to titanium's inclination to gall and inherent notch sensitivity, it does not make a good fastener material for service applications. The miniscule weight savings provided by using titanium studs rather than more reliable A286 cres studs for this application cannot be justified, in my opinion.

I won't go into the obvious design issues in the EC225 MRGB.

Your not reading the report are you NF.

I say again, read the following paragraph as to why the S-92 was unique in having problems with the titanium studs (historically not an issue on the 53 and 60 Series with millions of flight hours between them) and note that the design scheduled number of times that the oil filter bowl should have been removed during the life of the MGB was just five, but Cougar was averaging around 220 hours instead of the anticipated 500 to 1000 hours.

1.6.3.3 S-92A Main Gearbox Oil and Filter Replacement

When digested, then take a butchers at points 1. to 5. of the conclusions.

3.0 Conclusions

3.1 Findings as to Causes and Contributing Factors

1. Galling on a titanium attachment stud holding the filter bowl assembly to the main gearbox (MGB) prevented the correct preload from being applied during installation. This condition was exacerbated by the number of oil filter replacements and the re-use of the original nuts.

2. Titanium alloy oil filter bowl mounting studs had been used successfully in previous Sikorsky helicopter designs; in the S-92A, however, the number of unexpected oil filter changes resulted in excessive galling.

3. Reduced preload led to an increase of the cyclic load experienced by one of the titanium MGB oil filter bowl assembly attachment studs during operation of CHI91, and to fatigue cracking of the stud, which then developed in a second stud due to increased loading resulting from the initial stud failure. The two studs broke in cruise flight resulting in a sudden loss of oil in the MGB.

4. Following the Australian occurrence, Sikorsky and the Federal Aviation Administration (FAA) relied on new maintenance procedures to mitigate the risk of failure of damaged mounting studs on the MGB filter bowl assembly and did not require their immediate replacement.

5. Cougar Helicopters did not effectively implement the mandatory maintenance procedures in Aircraft Maintenance Manual (AMM) Revision 13 and, therefore, damaged studs on the filter bowl assembly were not detected or replaced.

Indeed.

As confirmed by Hilife...

* I assume you mean the TBO

...the expected MTBUR of the filters was massively optimistic meaning the comments on the success of titanium on other fleets are irrelevant.

Any news on the new MGB being designed?

So why were the filters being removed so frequently? Where there other issues that led to this practice?

The filter mesh was smaller than the naturally occurring particulates in canned oil, so they clogged prematurely. Some operators bought filtering equipment, but most didn't (and why should they?)

We have some S-92s contracted at the moment. They fly 8 hours per day each on busy days. In 18 months, we have had 2 or 3 MGB Oil Filters pop into bypass mode which have been discovered on turnaround. Each time we have consulted the OEM but it has never caused much additional maintenance other than some checks.

So far, we haven't had a technical RTB. As a customer, I find the S-92 is pretty reliable, it always starts, the oil stays in the boxes and passengers like it and trust it.