Pilot's Guide to Avionics 2017-2018 - 67
plastic occurred only where needed to connect new
Even more enjoy baby-boomer status.
equipment to the electrical system.
Chances are about even that the wiring harness of
But what about elsewhere? The wires feeding
the "average" or older aircraft is due for replacement -
or maybe for its second, maybe even third time. A few
wingtip recognition and anti-collision lights, connecting
signs point toward replacement.
fuel-level sensors to panel readouts, connecting the
battery to the generating source to the overall electrical
But weighing when to recommend replacing old
wires and wiring harnesses should take into account
So what else influences the health and viability of
the connective structures - the synapses-like
components such as breakers or fuse blocks, switches
aircraft wiring harnesses?
All it takes is a good look at every annual inspection
The calendar's consequences
to discover the condition of wiring harness and runs of
How long does wiring last? Years.
How long is its insulation effective? With modern
Teflon or Tefzel wiring, decades. Old plastic? Probably
The starting point: Inside the airframe
already past its prime.
and behind the panel
Many factors affect the longevity of wiring, starting
with the type of conductor - solid, twisted strands,
A peek at the harnesses and connectors provides
coaxial, or ribbon - and the insulation materials used.
some insight into the aircraft's past. Finding a nest
Other issues known to influence wires as they age
of wires insulated with woven cotton or rayon tubing
means old stuff. Such wiring typically consisted of a
include the wiring system's design, use, maintenance,
solid-copper or solid-aluminum core wrapped in paper
repair and the environment in which it exists. Harness
or thin rubber, then sheathed in the fabric.
components subject to extremes of heat also affect
Like flight instrument technologies of the same era -
And then there's the unrelenting impact of time -
gyroscopic instruments and air-data gauges - carried
forward after World War II, those were the technologies
age. We know what happens when insulation breaks
of the day.
down or wires work-harden and start to fracture.
Cracks develop from years of flexing under varying
Generators gave way to alternators, tube-type radios
to transistors, and transistors to digital circuitry. Wiring
G loads. Bends to pass through airframe parts
advances often didn't keep up with other upgrades.
can become fracture points for both insulation and
Fabric-insulated wiring remained through much of
the 1950s but started losing out to plastic-insulated
The repetitive cycles of heat and cold exacerbate
wire in the early 1960s. That early plastic product is
the aging of wires under the cowling. Connectors can
now of an age to be brittle and prone to cracking or
corrode and the plastics in some connections can also
breaking in stressed areas - such as where wires
suffer with age. Then, with wire damage hidden out of
curve or in long unsupported runs.
sight, things can go, well, haywire.
Wires can short to other wires, burning out fuses
Chances are, however, such old aircraft wired with
fabric-insulated conductors enjoyed some upgrades
or tripping breakers. Cracked, exposed wiring can arc
to the wiring harnesses - often behind the panel to
to the airframe, weakening the aluminum airframe at
accommodate avionics and instrument upgrades in
the ensuing years. But it's possible the upgrade to
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