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Recently, the FAA and the various military services have been doing studies
on wiring failures and come up with some startling results.  Much of this
work has been prompted by unexplained failures like TWA800 and generic aging
aircraft issues.

Throughout aviation history, manufacturers have been trying to use ever
cheaper and lighter forms of wiring insulation.  While less important for the
size of our Lancairs, this trend does have an effect on what we can buy and
how much it costs.  If Boeing shifts 50% of the US aircraft wire usage to a
particular kind of insulation, it becomes a lot cheaper for us to use the
same stuff.

Older mil spec wire used to be PVC/glass/nylon where a soft and highly
insulating layer was covered by a layer of fiberglass and this was covered by
an abrasion resistant layer.  The abrasion resistant layer was prone to
cracking and failure if the bend radius was violated, and the stuff was
expensive;  it had very good service history though.

Along comes single layer PTFE.  Cheaper, lighter, thinner, better bend
radius.  The problems now emerging are that if the bend radius is violated,
there are no outward signs.  However, the PTFE will "cold flow" and the
conductor will move closer to the surface of one side of the bend.  This is
also a problem wherever the wire is clamped.  Abrasion of PTFE is also a
major issue in that it only takes abrasion of about half the insulation
thickness before rest cracks through to the conductor and allows arcing.  
PTFE next to PTFE tend to eat each other in turbine applications unless the
wire bundle is properly combed (all wires parallel).

Last, we have Kapton.  VERY cheap, VERY light, VERY, VERY thin, mostly used
in power feed cables due to the large diameter and high weight of the other
insulations.  Unfortunately, very unreliable.  

A Navy study of P-3 airframe wiring came up with a wide matrix of number of
failures per foot of wire based on type and location on the airframe.  
Amazingly enough the data can be summarized in two points:

1.  Thinner insulation fails more often.

2.  Dirty or wet areas of the airframe or areas that have motion (like wheel
wells and control surface wells) have more wiring failures than clean areas
of the airframe that have no wire motion (like behind the panel).

Your tax dollars paid for this. <LOL>

Add to this the seemingly obvious data that tight bends and debris on the
wires create problems.  Lessons here are to respect the bend radii of the
wire type and gauge and clean up all those metal and composite shavings that
got dropped on the wire bundle under the hole you just drilled.

The thin insulation thing really floored me.  Kapton has been in service for
over 30 years and they're just figuring this out!  The studies and
recommendations were published in Aviation Week, 4/2/01, page 92.  
Recommended reading for anyone doing their own wiring.

Eric
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Please send your photos and drawings to marvkaye@olsusa.com.
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