Your photograph attached so that everyone can view it (please do this yourself in the future):



www.sparweb.ca

There is extensive corrosion on this part.
It conceals all other traces of damage.
If this shaft failed recently, then the obvious cause is corrosion. Period.
If it failed a very long time ago, and allowed to corrode since, then traces of the real cause may be lost now.

www.sparweb.ca

Have you got a picture of an unfailed part for comparison?

It'd be odd not to see the characteristic 45 degree shear if its a torsion failure.

Cheers

Greg Locock


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This is the other end. Here you can see teeths of coupling are damaged/wear in a specific regular pattern. Only area of approx 5mm is damaged in this coupling and on the other end splines teeths are of 5mm approx length. This drive shaft has allowed play of 5mm in axial direction

In addition to what Sparweb said
Got a side view of the failed male coupling half, its hard to work out what exactly we are seeing with a photo with no discernible depth.

What is the time in service(hours & calendar) plus is it the same for both halves.

The female coupling half, clockwise or counter clockwise rotation and is the power transmitted on to or out of it (trying to work out which side is the wear face of the teeth).

Plus piston or turbine engine?

This coupling condition is freshly removed. Iam trying to workout that is it due to loss of lubrication or due to impact. This side is connected to the main drive shaft through this coupling and on its right is tail drive shaft of helicopter. Periodically it is greased once a year and then tightened with a o ring packing as well. There is a impact on ground and drive shaft was decoupled and placed some meters away. So this condition of teeth wearing is due to impact which i dont think so or due loss of lubrication. condition of coupling removed from another helicopter which was greased 8 months ago is attached.however before posted pictures are those which were lubricated 6 months before and then reopened at the time of failure or decoupled.

Thank you for the additional pictures.
I know I sound like I jumped to conclusions in my first reply, but I would like to know more. Can you describe the sequence of events in the helicopter accident?
Not just starting from the failure, but instead starting from the phase of flight, kind of operations the helicopter was used for, and step by step series of events, as far as they are known to you?
For example, it would be an important difference if the helicopter were engaged in a cruising flight at the time of the accident, versus being engaged in a vertical cargo lifting operation. If you cannot tell us these details, then please just let us know that it is not known or cannot be told.

www.sparweb.ca

Sir, helo was in hover taxi and took off 3 minutes ago. When suddenly, tail rotor stopped and it went into spin and crashed on ground. Main rotor blades also came into comtact with tail drive shaft. So exactly it is unknown after looking drive shaft that what was the initial damage which initiated this process. Drive shaft is broken from many points. As per my experience, after looking at coupling, i think the teeths conditions of coupling indicates fretting wear and tear and then it slipped on the day of incident. Which resultant into loss of power transmission to tail rotor. Because shaft is showing multiple damages from main rotor blades and ground impact damages. Difficult to ascertain whthr there is fatigue cracking type of phenomenon on them

One technique of accident investigations that I have read about (it is not my specialty) is a microscopic evaluation of the corroded areas. When one fracture is suspected as the first point of failure, then it will accumulate evidence in its cracks over time, which the abrupt breakage during the accident cannot cause.

For example, let us say that a crack may be in a driveshaft for a year before causing an accident. In this example, the crack will grow when the helicopter is operating, and it will corrode when it is not. The area of that crack would have deep corrosion pitting that had developed over that year, which would not be possible for a part recently broken. An investigator could look at that corroded part, and compare it to a recent fracture, and know that the more corroded part was the start. Once that "old" corrosion can be found, then the depth and cause of the corrosion can be examined more closely which will help identify the initial cause.

www.sparweb.ca

What is the goal of this investigation?

Exactly, the same way we are doing investigation to identify, the first point of failure which lead to sequence of events. This coupling appears to us faulty due presence of fretting wear of teeths in a specific pattern and corrosion . Obviously this doesnot happened at the time of impact with ground. What is your general observation regarding condition of coupling

@3ddave. To identify the root cause which lead to failure of tail rotor drive shaft faipure. What is your views about this coupling condition

I don't see the point of you knowing the root cause. You are not the maker of the helicopter so understanding how this failed does not give you information that you can use.

Asking the maker to determine if the helicopter was badly maintained might save the failure of another helicopter, but if maintaining it is a problem, then not having a helicopter at all is the safer option.

Let the maker of the helicopter determine the cause of the failure and involve the country civil aviation regulatory agency.

@RotaryHelos
FYI it appears there is debris in the coupling, teeth show abnormal wear and damage. this coupling could have seized. thus causing damage to the shaft.
metal in between the teeth is very bad. if this coupling is inspected and repacked with grease once a years. it will need more frequent inspection and greasing.
corrosion was a good possibility initial cause of damage. all conjecture with the limited info.
a formal failure analysis will be required. all parts will require sent to metlab. an electron microscope analysis will be required.
far as 3DDave advice it sounds very sound. take very careful pictures of each details of the assembly. and cataloged. as not to accidently
remove evidence. good luck.

Do You have any photos of the damaged and undamaged... corresponding male and female... TRDS couplings? Please post... looking through dirty/contaminated grease is like trying to read an off-angle X-ray... without knowing what we are looking at.

I presume that... in addition to Your NTSB-equivalent accident investigator... the helo manufacturer [???] sent a representative to the accident investigation!? IF NOT... WHY??? The NTSB/OEM will-be [should-be] remarkably well equipped to look at/adjudicate EVERY FACET of the damage to the helo in excruciating detail... and help/guide you likewise.

For my one-and-only Class A MIL helo mishap investigation, we had an outstanding Sikorsky investigator on-site within 1-week of the mishap... astonishing level of expertise/experience [lots of this type have crashed over the years].

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Rotary Helos… are You there?????????????

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Yes iam here

To amplify the questions from WKTaylor: We remain very curious about the circumstances of your investigation.
Most of us who use this forum are aware of the safety standards involved, including the ICAO obligations to conduct investigations of such accidents. Therefore your questions do pique our interest.

Within the bounds of professional discretion, what can you tell us about the tools and methods available to you as you investigate?

www.sparweb.ca

Thanks for the guidance. Questions which i asked in the start was very basic one as we were at the start of finding root cause. Visual inspection tool was adopted to find cause of failure. Aparently, we reached to a conclusion that gasket between coupling torn out which lead ro dryness of grease and subsequent failure. We have forwarded all damaged parta for SEM and fracture analysis as well. Details i will share once received

Thank you for the update. I look forward to it.
Are you able to say anything about the gasket failure, for its cause? Can you describe the location of the gasket in the coupling (internal or external)? Are there other ways for the grease to escape? Or for the opposite: ingress of moisture which may have been trapped?
Or were some of these possibilities ruled out somehow?

www.sparweb.ca

Since YOU are investigating this failure... and details are sparse... it seems reasonable to presume You are the on-site investigator for both the Authorities and Manufacturer...? My curiosity is elevated...

Rotary helos… For clarity.. please advise...

NYCO G353 Grease has an MoS2 filler... typically used in extreme pressure/wear applications... and most greases like this are silvery-gray/black... the photos show distinctly hardened tan-to Brown... more similar to NYCO G355... a conventional lithium-based grease.

Environmental seal failures often introduce atmospheric moisture to grease... which tends to 'thin conventional greases'. The photos show an oddly stiffened-dry appearance... which seems inconsistent with environmental seal failures... except in extreme hot/dry desert climates... which can also expose critical unsealed joints to micro-fine dust...

Is this a one-of-a-kind [prototype] in flight testing?
Ultralight, light or heavy weight category?
Manned? Unmanned?
Amateur-built experimental?
Corporate experimental?
Military experimental?
What kind of 'iron-bird' TRDS drive-train testing was accomplished prior to certification for flight?

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

BTW... FYI, ONLY...

Aeroshell publishes a great reference book on [their] lubricants [greases, oils, SFLs, etc] and fuels... but it is worth downloading as a general-knowledge reference...

Aeroshell E-Book...
https://www.shell.com/business-customers/aviation/...

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Rotary Helos… are You there?????????????

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Hmmm... Rotary helos has not logged-on after reading my last couple of posts...

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Hello, iam back. Actually i was away from internet and trying to spend internet free month. We have got the analysis report of coupling. It failed due to loss of grease from packing which gradually resulted into overheating and material deterioration. So, the point is we have introduced periodicity of greasing after 6months to prevent such failure

@Wk taylor, your analysis were spot on, loved to stay in contact with you on technical analysis. Hope you share your email ID with me. Then i can share further details of aerial vehicle

Hello, iam back. Actually i was away from internet and trying to spend internet free month. We have got the analysis report of coupling. It failed due to loss of grease from packing which gradually resulted into overheating and material deterioration. So, the point is we have introduced periodicity of greasing after 6months to prevent such failure

thanks for the feed back and clouser