Thread: credibiltiy
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Old February 20th 10, 01:10 AM posted to rec.aviation.rotorcraft
Stu Fields
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Default credibiltiy


"The OTHER Kevin in San Diego" skiddz "AT" adelphia "DOT" net wrote in
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On Mon, 8 Feb 2010 16:16:59 -0800, "Stu Fields" wrote:

It seems that if there are two possible causes for a helicopter accident,
the preferred one for the manufacturer is the one for which they have no
fault. That certainly is the best for them. However, lets say that a low
hour helicopter suffers a failure directly attibutable to fatigue.
Further
lets assume that the helicopter had had a prior series of hard landings
or
other beyond normal stress loadings. Now lets further assume that the
fatigue failure occurred at a point in the helicopter drive system where a
diameter change was machined into the shaft without any radius or attempt
at
a proper fillet which yielded a strong stress riser.
Lets say that the kit manufacturer is very aware that a number of kits
have
been sold with the same machining flaw.


Should the kit manufacturer issue a service advisory statement advising
all
owners of those ships of a potential safety issue caused by those parts?


You'd hope so.

What should their action be? Recall and supply exchange parts for no
charge?


If it's a machining/design defect, absolutely. Since you mention in a
later post the machine shop was given drawings that don't indicate any
type of fillet where the diameter of the shaft changes, I'd lean
towards a design defect and expect the manufacturer to replace the
shaft on their dime.

Recall and supply exchange parts for their cost?


At a minimum.

Change the
machining process and ignore the other parts out there?


If I'm following what you said, it's not a machining process issue.
It's that the designer (or CAD guy) didn't call out that fillet.

How about sell the business to someone else and just duck and hope that
nothing bad ever comes from the above?


Would that "absolve" the previous owners of any liability, especially
if they were aware of the issue prior to peddling the biz? Common
sense (and the litigous nature of the States these days) says no but
we all know how short in supply THAT is.

This wouldn't have anything to do with your upcoming project, would
it?

Hi Kevin.

Things have moved on. I have found a jagged groove in the main rotor shaft
that is approx 0.020 deep with jagged bottom. I've got a photo from an
optical comparator for more detail. This groove occurs just at the top of a
roller bearing that is the last point of support of the shaft. There is a
rubber seal at this point. Manufacturers instructions were to remove the
spring from the inside of this seal to avoid possible gouging of the shaft.
Well evidently the rubber seal is capable without the spring of generating
this jagged groove. The manufacturer has said that they have seen these
before and they do not constitute a hazardous condition!!
This is a soft (non-heat treated) TI shaft and all my books say that TI
doesn't handle fatigue well if the surface of the shaft is rough with
imperfections. I could be wrong, but I don't believe that the manufacturer
has done a detailed fatigue analysis of the shaft with this kind of groove.
Hell with what I've read about fatigue, it would take a world famous
structural engineer with a bunch of fatigue design experience to convince me
to fly that shaft. Even then I would want to try to borrow a bunch of money
from him before my flight to test his surety.
Apparently, based on written statements, the manufacturer doesn't believe
that any of the accidents that occured were the fault of the poor fillets.
Even in spite of the accident investigating agencies statement that the
fatigue failure had occurred where the fillet was a sharp corner. There
were reportedly other events of exceptional stresses that possibly could
have started a crack in these highly stressed points.
The mfr. has issued drawings that specify radiuses at the flillet locations
and have offered a "Speedy-Sleeve" to protect the shaft from seal wear. All
new ships are supposed to have these mods.
The manufacturer has offered to provide these parts to owners of older
ships. However, there has been no indication that the owners will receive
any cost breaks.
Aah the world of experimental aviation!!! I have sure got a new
appreciation for the depth of the education available to the people who
build, or buy kit aircraft.

Upcoming project has had a closer look at the serious points and so far it
looks good. That isn't to say that I'm not finding some warts on it, but so
far, with the exception of adequate room for a helmet, the warts look easily
handleable.