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

system.

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?

and relays.
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

conductors.

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 -

wiring's condition.
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

conductors.

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

Continued on following page...

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Table of Contents for the Digital Edition of Pilot's Guide to Avionics 2017-2018

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Pilot's Guide to Avionics 2017-2018 - Cover3
Pilot's Guide to Avionics 2017-2018 - Cover4
http://www.brightcopy.net/allen/pilotsguide/18-19
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