nuckolls.bob(at)cox.net
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 Posted: Fri Nov 14, 2008 7:16 am Post subject: Lessons Learned, Lessons Forgotten, Tools for Sharing |
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At 09:53 PM 11/13/2008 -0800, you wrote:
| Quote: |
We found a way to not have any electrons for keeping the engine alive.
Easy enough not to have our problem, but very critical that you do it
right. The fat wire from the battery to the contactor shorted out on a
brake line and filled the cabin with smoke. Shutting off the master didn't
help since it was downstream of the short.
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When the industry accepted the notion that fat wire
protection wasn't useful or necessary in TC aircraft,
they were also cognizant of a need for due-diligence in
the installation of said wires . . . separation and
support being chief amongst concerns.
The scenario cited above could also have caused a
landing accident. If the short had been a soft fault,
it might have slowly eroded a hole in a brake line
causing loss of brakes.
Here's an excerpt from a thread that ran though the
List 4 years ago:
-----
IMHO, the only advantage of zip-ties is reduced time for installation. I
don't recommend using them in the engine compartment, they get brittle with
time and can let loose without warning.
This is mostly a materials issue. Like bolts, you can purchase
cable ties in a wide variety of materials and qualities . . . and
like bolts, you cannot deduce the any secrets of fabrication by
simply looking at it.
Not all plastics are equal but most all plastics are relatively
cheap . . . so it's not difficult to purchase cable ties fabricated
from known materials selected for their suitability to the task.
If one wishes to acquire the Cadillac of cable ties made from the
same stuff as the wire insulation of choice, you can purchase
Tefzel ties from lots of places not the least of which is our
friends a Steinair. See:
http://www.steinair.com/cableties.htm
Any supplier worth his salt will KNOW where his inventory comes
from, what it's made of and MAY be able to advise for or against
the use of any particular product in certain applications. When
you buy that big jar of 1000 cable ties at Harbor Freight for $9.95,
what you see is what you get and you cannot deduce much from simply
seeing.
For ties that you do not wish to look and or touch for a very
long time, it's worth your time and effort to purchase ties
by name brand manufacturers where the material and it's features
are cited. Looking for UV, ozone, and/or hydrocarbon resistant
products. These are NOT the generic nylon cable wraps offered by
the vast majority of consumer oriented suppliers including
Walmart and Harbor Freight.
Tensioning zip-ties is also problematic. If you overtension, they WILL eat
the insulation with enough vibration/time.
There are at least two underlying issues here. Insulation is hard
to "eat" . . . but certain insulations like Teflon are soft enough
to flow under continuous pressure exceeding its compression strength
combined with heating cycles.
I have also seen wire-ties eat into engine mounts and aluminum when
installed improperly
There have been suggestions in this thread concerning abrasion of
metal tubes like engine mounts due to the improper use of cable ties.
I'll suggest that ANY form of wire attachment can become a problem
for metals IF the attachment is loose enough to allow motion -AND-
you add dust and grit to the space between the metal surface and
whatever is riding against it.
I had a power steering hose simply lay against a brake line in
my '57 Chevy for a bunch of years. I lost brakes when a hole
blew out in a section of the brake line thinned by continuous
motion of a rubber hose lubricated with gritty grease. This didn't
even involve a wire tie, "Adel" clamp or similar technology. These
kinds of things CAN and DO happen and it has nothing to do with
the type of retention technology and a LOT to do with craftsmanship.
If you undertension, wire-ties they don't hold well. They have a tendency
to slightly loosen after initial installation. To cure the above problems,
you might consider a wire tie install tool.
There are cable tie installation tools that feature adjustable
and repeatable tensioning and cutoff adjustments. Many factories
use them (including Raytheon Aircraft) and they've proven useful.
They're not cheap. I've never bothered to own one. The range of
acceptable tensions for wire ties is large and it's not hard to
apply them by hand in a way that offers long service life.
Wire-ties also have an affinity for human skin. You'll find this out down
the road when you reach up behind your panel to do something and shortly
thereafter donate a small amount of blood to the nylon god.
This is not so much an issue with the tie but how the tail is cut off.
Avoid using the classic diagonal wire cutter (dikes) . . . they
part the material by driving two symmetrical wedges together and
the finished cut protrudes from the tie buckle and is sharp. Use
flush cutters like:
http://www.action-electronics.com/cutters.htm
http://www.home-jewelry-business-success-tips.com/wire-cutter.html
http://www.aeroelectric.com/Pictures/Tools/Wire_Cutters/Flush-Cutters.jpg
. . . and trim the tail off squarely and flush with the buckle
and you'll not have to bleed on the project at any time in the
future.
For shear beauty (and no cuts on your hands) nothing can beat tying your
wires together the old fashioned way. See the aeroelectric site for the
technique. Yes, it's time consuming! and requires a certain amount of
learned skill.
Probably no more effort than required to learn the use
of a cable-tie installation tool. However, there's
nothing 'magic' about the techniques suggested in:
http://aeroelectric.com/articles/cable_lace/cable_lace.html
the choice of MATERIALS is still just as critical. You
wouldn't want to tie up your wire bundles with kite
string.
Adels work great. They also weigh more and are time consuming to install.
I'd use them to use larger wiring harnesses and for all wiring attachment
in the engine compartment unless no other option exists.
We're talking about two related but different tasks. The cable tie
is used in many places to simply hold the bundle of wires together.
The MS21919 (Adel) series clamps . . .
http://aeroelectric.com/articles/adel.html
. . . are certainly adequate to many applications. This doesn't
mean that other materials and technologies are not recommended.
Proper use of any technology is dependent upon understanding
it's limitations.
On occasion, a bundle of wires needs to be support on the airframe
either for the purpose of simply holding the bundle in place -or-
to prevent the bundle from coming into adverse contact with
parts of the airplane.
Like all things on an aircraft, each situation will require a balance of
needs and limitations...
Exactly. The situation is further improved with a thoughtful
analysis of cause and effect for both materials selection and
techniques applied to their use.
---------------
We who make a living swinging hammers and CAD keyboards
on airplanes experience the "red flag event" when any two
objects come into contact with each other in an uncontrolled
manner be it fluid lines, wires, control cables, etc.
The drill is to (1) fasten them together with appropriate
retention and buffering as illustrated in the cable lacing
article cited above or (2) separate them.
| Quote: | Dragged the battery down to 5 volts.
|
I presume this was a voltage measured on the ground after
the smoke cleared. It's clearly indicative of a fault that
drained the battery well past the "used up" voltage level
of 10 volts. It would be interesting to have the carcasses
of shorted wire and brake line to inspect . . .
IMPORTANT . . . some readers of this thread may be thinking
that a fuse would have prevented this. NO. ANL's and their
cousins would probably have carried the current necessary to feed
this fault and discharge this battery. It's a much more practical
mitigation of risk to concentrate on installation technique than
to add protection for lack of craftsmanship.
| Quote: | We do have one magneto and that's what kept it running long enough to get
on the ground. Very, very important that this wire not be allowed to short,
|
Yup!
| Quote: | especially if both of your ignitions are on it.
|
Please don't run both electrically dependent ignitions from
the same power source. Better yet, make at least one of
the ignition systems a self-powered Emag.
These are USEFUL events to know about, contemplate,
and analyze for discovery of lessons-learned (or
reminders of lessons forgotten).
These are the things that grey-beards in the
factories do as sort of second-nature and it's become
one of the most important services I can offer my
customers. The problem is that fewer and fewer of my
contemporaries get their hands dirty working on real
hardware . . . and fewer and fewer of our production
line folks are trained for or expected to think for
themselves nor are they given such information to
contemplate. It's just not the ISO9000 way.
Bottom line is that while our TC airframes grow in
parts count and operational complexity, the OBAM
aircraft community is becoming the true expert in
practical, low-risk aircraft design, fabrication
and operation.
Recall that several years ago we here on the
AeroElectric-List mounted an effort to help defend
Matronics from a silly, knee-jerk lawsuit.
If we can put on the white hats and come rushing
over the hill to rescue a good thing under attack,
then we must certainly nurture it too. It may be
heartwarming to offer somebody a Thanksgiving dinner
with all the trimmings . . . but there are 364
OTHER days in the year too.
If I were to send 1800 List members a bill for
their share of my services for the year at my
current rates, it would be a hell of a lot more
than $10 apiece. If Matt finds it necessary to
give up this endeavor for lack of support of the
hardware, then we suffer loss of a critical
communication tool. Be assured that I cannot
spend the $time$ necessary to support the same
mission one-on-one through my telephone or email!
I've donated over $1000 in product to this cause,
you folks can do a lot more for a lot less if
you all participate.
Do it now. Sent your $10 or more right now. The money
we're talking about won't buy you a lunch at Bennigan's.
This is MORE IMPORTANT than care and feeding
of the occasional predatory lawyer. Go to . . .
http://matronics.com/contribution
Bob . . .
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