CASA 06 Structures Combined units

Insulated Wire

A metal conductor covered with a dielectric or insulating material

Why do aircraft wires contain stranded conductors?

For flexibility

What layers/materials may aircraft wire insulation provide?

Dielectric insulation

How is a conductor's resistance related to its length?

Directly proportional

the longer the wire

the higher its resistance

How is a conductor's resistance related to its cross-sectional area?

Inversely proportional

the larger the cross-sectional area

the lower its resistance

Why is copper ideal for aviation wiring?

It is non-magnetic (doesn't interfere with sensitive flight equipment)

Why is copper wire easier to terminate than aluminium of the same current rating?

Copper conductors have a smaller cross-sectional area for the same current rating

When is aluminium wire often used instead of copper?

Where large amounts of current must be carried for long distances

What is a property of aluminium wire compared to copper regarding resistance?

It is non-magnetic but has a slightly higher specific resistance

What is the specific electrical resistance of aluminium compared to copper?

Aluminium (2.8 µΩ/cm) is 1.6 times higher than copper (1.73 µΩ/cm)

For equal linear resistance

how much bigger must an aluminium conductor's diameter be than copper's?

What is the specific mass of copper compared to aluminium?

Copper (8.9 g/cm3) is 3.3 times higher than aluminium (2.7 g/cm3)

For identical linear resistance

how does an aluminium conductor's weight compare to copper's?

What weight saving can aluminium provide for large conductors carrying high current over long distances?

Approximately 50% weight saving

Why is aluminium wire rarely used for general aircraft wiring?

When exposed to vibration

List situations where aluminium wire should not be used.

Attached to engine-mounted accessories

Conductor Plating

Coating exposed copper and aluminium wire with tin

Important AWG

American Wire Gauge

What is the typical smallest gauge wire used in aircraft?

22 or 24 gauge wire

What is the largest AWG wire size and its alternate name?

0000

What is the diameter of a 24 AWG wire?

0.51 mm

What is the diameter of a 0000 AWG wire?

11.7 mm

List materials used for aircraft wire insulation.

Polyvinyl chloride (PVC)

What voltage rating are most aircraft wire insulators rated to?

600 V

Does wire insulation affect its resistance?

No

What determines how much heat is generated in a wire

potentially burning its insulation?

What does the current limit a conductor can withstand depend on?

How hot the conductor can get before it burns the insulation

Insulation Resistance

The resistance to current leakage through the insulation materials

What instrument measures insulation resistance without damaging the insulation?

An insulation tester (megger)

Why might clean

dry insulation with cracks or faults show a high insulation resistance value yet still need replacement?

List the four things the term 'cable' can include in aircraft electrical installations.

Two or more insulated conductors in a common insulating material

one or more insulated conductors with an overall shield (shielded cable)

two insulated conductors twisted together (twisted pair)

a single insulated centre conductor with a metallic braided outer conductor (coaxial cable)

What is the only wire type permitted for installation and use on aircraft?

Approved aircraft wiring

Important QPL

Qualified Products List

What is the majority of aircraft wiring made from?

Stranded copper

Conductor Stranding

Constructing wire by combining groups of smaller wires gently twisted or wrapped together for flexibility

Why is stranded wire used instead of solid conductors in aircraft?

It provides more flexibility and reduces metal fatigue/conductor breakage due to flight vibration and flexing

Bonding

A process that grounds components in an aircraft that are not otherwise electrically connected

List what a correctly bonded structure serves to do.

Minimise electrical damage

Shielding

A method of intercepting electrical energy from a wire's magnetic field and shunting it to electrical ground

What type of current has an especially bad effect on electronic equipment via magnetic field interference?

AC or pulsating DC

What is shielding typically made from?

Tin-plate or cadmium plated copper wire

Airframe Wire

Wire specifically designed for component interconnection wire in the aircraft's airframe

How many insulation coverings does airframe wire have?

Two

What is an example use of airframe wire?

Aircraft undercarriage leg wiring

What is a benefit of airframe wire's high tensile strength?

Reduces vulnerability to damage during installation

What is the minimum conductor strand count and AWG size for airframe wire?

Must not be less than 19 conductor strands or conductor size less than 18 AWG

Why is airframe wire stiff and springy

making installation more difficult?

Why is soldering difficult on airframe wire conductors?

They are plated with corrosion resistant alloys

Interconnect/Hook-up Cable

Cable designed for component interconnection inside the protection of the fuselage

When can hook-up cable be used as airframe wire?

Only where there is a secondary insulation covering for abrasion protection

Why are interconnect cables more prone to damage when pulled through pressure bungs?

They are normally of smaller overall diameter than equivalent gauge airframe wire

What is the minimum conductor strand count for interconnect cables?

Must not be less than 19 conductor strands

What additional support should wires smaller than 20 AWG have at terminations?

Connector grommets

Where should small gauge interconnect wires (below 20 AWG) not be used?

Applications subjected to excessive vibration

Equipment Wire

Wire (also called standard 'wire') intended for use within aircraft equipment

What is equipment wire also known as in the USA?

Module Wire

Fire Resistant Cable

Insulated single wire for use in flight critical circuits

Where is fire resistant cable predominantly used?

In the engine compartment of aircraft

Since what year has asbestos not been permitted in the manufacture of fire resistant wire?

1993

How should wire of the fire-resistant type with no manufacture date be treated?

As containing asbestos

Where does the definition of fire resistant cable insulation retention come from?

AC 43.13-1B

Why should 'fire resistance' not be confused with 'high temperature'?

Fire resistant types have other characteristics (like fluid resistance) that are usually poorer than a non-fire resistant high temperature cable

How long must fire resistant wire maintain insulative quality in the presence of fire?

30 minutes

What do fire resistant conductors have a thick coating of?

An alloy with a higher melting point than copper

List the layers of fire resistant cable construction (from inside out).

Nickel Clad High Strength Copper Alloy Conductor

High Temperature Cable

Wire with a temperature rating of 125°C or higher (though some manufacturers use ratings as low as 90°C)

What are the two key components ensuring high-temperature wires are suitable for an application?

The wire's temperature rating and current capacity

What is a wire's temperature rating defined as?

The maximum continuous temperature the wire can withstand during its lifetime

What is current capacity defined as?

The maximum current an insulated conductor can safely carry without exceeding insulation/jacket temperature limitations

What happens if a cable is undersized for its current load?

Heat produced may exceed the cable's temperature rating and the cable may be compromised

Thermocouple

A pair of dissimilar metallic conductors that detect and measure temperature changes

What material is used for thermocouple extension leads?

The same material as the thermocouple itself (e.g. Iron-constantan

How are thermocouple extension wires typically arranged?

Paired in a braided jacket and colour coded

For gas turbine engine temperature measurement

how are thermocouples typically arranged?

What is the insulating material of thermocouple harness cables?

Silicone rubber or PTFE-impregnated fibreglass

Where does a thermocouple harness typically terminate?

At an engine or firewall junction box

What is the insulating material of thermocouple extension cables normally made of?

Polyvinyl type

Bonding Cable/Straps

Used to bond shock mounted components to the main structure

What material must bonding cable conductors be made from?

Copper or aluminium

Why should bonding straps not produce galvanic corrosion?

To avoid damage from dissimilar metal corrosion at the bonding joint

What must be true of a bonding cable's length?

Long enough not to interfere with movable component operation but not so long as to increase resistance

What is the purpose of sacrificial aluminium washers between a copper bonding strap and aluminium stringer?

To concentrate potential galvanic corrosion in the washer rather than the structure

When are copper bonding straps used instead of aluminium alloy?

To bond parts made of stainless steel

Where can detailed information on bonding strap construction be found?

Advisory Circular 43.13-1B

What should always be the primary maintenance reference document for bonding cables/straps?

The approved Aircraft Maintenance Manual (AMM)

Important AMM

Aircraft Maintenance Manual

Important IPC

Illustrated Parts Catalogue

Power Cable

Cables running from power sources to bus bars

What must protect power cables from overload?

Circuit protection devices

How does aluminium conductor size compare to copper for power cables of the same weight?

Aluminium conductor is usually 2 AWG sizes larger than copper despite being half the weight

How is aluminium conductor vibration susceptibility reduced in power cables?

Making the conductor from more strands

Why are copper cables used between engine-mounted generators and the fuselage rather than aluminium?

To avoid areas of high vibration

Give an example of power cables used in aircraft.

Generator feeder cables

Data Cables

Cables covered by the classification of interconnect cables/wires

Why can data cable conductors be lighter than power cables?

They conduct very little current and do not require overload protection