Cessna 170, Service Manual

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Cessna 170 Service Manual
Electrical System
14
3. Test the circuits final ground at the end of the circuit.
4. Test the primary item/accessory both for power and with a temporary ground.
5. Test/eliminate each connection/item between the final ground and the current limiter.
6. Murphy’s 6
th
rule of electricity: Never assume a part (not even a new one) is faultless purely by
visual inspection. TEST IT!
ELECTRIC WIRE
GENERAL . Aircraft service imposes severe environmental conditions on electric wire. To assure
satisfactory service, inspect the wire at regular intervals for abrasions, defective insulation, condition of
terminal posts, and buildup of corrosion under or around swaged terminals.
AIRCRAFT ELECTRICAL WIRE.
Use aircraft quality wire. Correct wire selection is dependent upon
knowledge of current requirements, operating temperatures, and environmental conditions involved in
the particular installation. (Authors Note: The Cessna 170 was originally designed and built utilizing
MIL-W-5086A wire or previous standards. Some of these wires were insulated with PVC (poly-vinyl-
chloride) that emits poisonous gases when ignited. While adequate for continued use in circuits of good
condition, these standards are no longer acceptable for replacement, repair, and/or new installations.
The most common aircraft electrical wire in modern applications is MIL-W-22759/16 that is constructed
of a copper conductor that has been tinned and insulated with ETFE (ethylene/tetraflourethelene.) This
is commonly referred to by its commercial moniker “Tefzel”. )
a. Conductors. Copper conductors are coated to prevent oxidation and to facilitate soldering. Tinned
copper or aluminum wire is generally used in installations where operating temperatures do not
exceed 221°F (105°C). Aluminum wire shall be restricted to size 6 and larger. Aluminum wire shall
neither be directly attached to engine mounted accessories nor installed in other areas of severe
vibration. It shall not be installed where frequent connections and disconnections are required. All
installations of aluminum wire shall be relatively permanent. Aluminum wire shall not be used
where the length of run is less than 3 feet, in areas where corrosive fumes exist. It is not
recommended for use in communication or navigation systems.
Note : Aluminum and Silver
coated wire was never used in Cessna 170s originally, but this information is being supplied in the
event any is found subsequently installed.
SILVER COATED WIRE is sometimes found and used where temperatures do not exceed
392°F (200°C). CAUTION: An inflammability hazard exists when silver or silver plated conductors
impressed with direct current potential are saturated with water/glycol solutions (anti -freeze/de-icing fluids.)
The positive (cathodic) may be of any conductive material. If the anode and cathode are in sufficient
proximity to permit current (in the milliampere range) to flow through a glycol solution that has contaminated
the space between the two conductors, oxidation is rapid and an intensely hot flame appears. This
phenomenon is not known to occur when the anode is other than silver or when the impressed voltage is
alternating current.
Nickel coated copper wire is sometimes used for temperatures up to 500°F (260°C). Nickel coated wire is
more difficult to solder than tinned or silver coated wire, but with proper techniques, satisfactory connections
can be made.
b. Insulation. Silicone rubber is rated at 392°F (200°C), is highly flexible, and self -extinguishing
except in vertical runs. Polytetrafluoroethylene (TFE Fluorocarbon) is widely used as high
temperature insulation. It will not burn, but will vaporize when exposed to flame. It is resistant to
most fluids. Fluorinated ethylene propylene (FEP Fluorocarbon) is rated at 392° F (200°C), but will
melt at higher temperatures. Other properties of FEP are similar to TFE.
c. Thermal and Abrasion Resistant Materials. Glass braid has good thermal and abrasion qualities
but moisture absorption is high. Asbestos and other minerals provide high temperature and flame
resistance, but are highly absorbent. Moisture absorption is reduced by use of silicone rubber, TFE,
or other saturants. Nylon is widely used in low temperature wires for abrasion and fluid resistance.
Polyimide, a new material, has excellent thermal and abrasion resistant characteristics.
d. Wire Selection. When selecting wire, refer to structural and environmental characteristics. Wire
normally used for chassis wiring, in enclosed areas, or in compact wire harnesses protected by
molded or braided coverings, usually has low abrasion resistance. Wire used to interconnect units,