Mod 6c Electrical Hardware

Insulated Wire

• provides: dielectric insulation, thermal protection, abrasion resistance, moisture resistance, fire resistance, and fluid resistance.

Conductor Materials

• Resistance is directly proportional to length → Longer wire = higher resistance.

• Resistance is inversely proportional to cross-sectional area → Larger cross-sectional area = lower resistance.

Copper or Copper Alloy Wire

• Non-magnetic – won’t interfere with sensitive flight equipment.

• Malleable, conductive, and corrosion resistant.

• Smaller cross-sectional area than aluminium (same current rating).

• Easier to bend, shape, and terminate.

Aluminium or Aluminium Alloy Wire

• Used for high current over long distances.

• Non-magnetic.

• Higher specific resistance than copper → requires a larger cross-sectional area.

• Approximately 50% weight saving compared to copper.

• Can crystallise (work-harden) and break under vibration.

• Rarely used for general aircraft wiring.

• Do not use on engine-mounted accessories or in areas with:

  • Corrosive fumes

  • Severe vibration

  • Mechanical stresses

  • Frequent disconnection

Conductor Plating

• Copper and aluminium oxidise when exposed to oxygen.

• Oxidation is accelerated by aircraft temperature and pressure variations.

• Plated with tin, silver, or nickel to minimise oxidation.

Wire Size

• Measured using the American Wire Gauge (AWG) system.

• Larger AWG number = smaller wire.

• Typical aircraft wire sizes: 24 AWG (smallest) to 0000 (4/0) AWG (largest).

• 24 AWG = 0.51 mm, 0000 (4/0) AWG = 11.7 mm.

14 AWG

20 AMPs

12 AWG Stranded & Solid

25 AMPs

10 AWG

30 AMPs

8 AWG

40 AMPs

6 AWG

55 AMPs

2 AWG

95 AMPs

1/0 AWG

125 AMPs

Aircraft Wire Insulation

• Materials: PVC, nylon, Teflon®, various polymers, and glass cloth braid.

• Most insulation is rated to 600 V.

Wire Insulation Temperature Rating

• Insulation does not affect wire resistance.

• Resistance generates heat, which can burn the insulation.

• Current limit depends on the insulation temperature rating.

• Different types of insulation have different temperature ratings.

Insulation Resistance

• Resistance to current leakage through the insulation.

• Measured with an insulation tester (megger).

• Used to determine the condition of insulation.

• Cracked or faulty insulation is not serviceable, even if insulation resistance is high.

Electrical Cable

includes:

• 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).

• Single insulated centre conductor with a metallic braided outer conductor (coaxial cable).

Aircraft Wire

• Only approved aircraft wiring is permitted.

• Must pass rigorous testing before being added to the Qualified Products List (QPL).

• OEM/military wire must come from approved wire mills.

• Majority of aircraft wiring is made from stranded copper.

Conductor Stranding

• Stranded wire = smaller wires twisted or wrapped together.

• Provides more flexibility than solid conductors.

• Reduces metal fatigue.

• Reduces conductor breakage caused by flight vibration and flexing.

Bonding

• Grounds components not otherwise electrically connected.

• Protects against static/electrostatic charge build-up and lightning strike discharge.

• Correct bonding:

  • Minimises electrical damage.

  • Prevents high potential differences (sparks).

  • Reduces electric shock risk.

  • Reduces interference with radio communications and navigational aids.

  • Provides a low resistance electrical return path in earth-return systems.

Shielding

• Current-carrying wires produce a magnetic field.

• Magnetic fields can interfere with aircraft instrumentation and control systems.

• AC and pulsating DC have the greatest effect on electronic equipment.

Airframe Wire

• Designed for component interconnection in the aircraft airframe.

• Normal or medium weight wire.

• Two insulation coverings for improved abrasion protection.

• Can be used without conduit in harsh conditions.

• High tensile strength reduces installation damage and breakage.

Airframe Wire Construction

• Must have at least 19 conductor strands and be 18 AWG or larger.

Airframe Wire Characteristics

• Multiple thick insulation layers for harsh conditions.

• Stiff and springy, making installation more difficult.

• Corrosion-resistant plated conductors make soldering difficult.

Interconnect/Hook-up Cable

• Designed for component interconnection inside the fuselage.

• Lightweight with one insulation layer.

• Requires secondary insulation/conduit if used as airframe wire.

• Smaller diameter than equivalent airframe wire.

• More flexible, saving weight and space.

• More prone to damage when pulled through pressure bungs.

Interconnect Cable Construction

• Must have at least 19 conductor strands.

• Small gauge wires: use high-strength alloy conductors.

• Wires smaller than 20 AWG:

  • Additional clamps.

  • Group with at least 3 other wires.

  • Additional support at terminations.

• Do not use where there is:

  • Excessive vibration.

  • Repeated bending.

  • Frequent disconnection from screw terminations.

Equipment Wire

• Used within aircraft equipment (LRUs).

• Smaller conductor sizes than hook-up wire.

• More strands for flexibility and good solder joints.

• Can be single-strand if secured to a solid structure.

• Thinner, less springy insulation.

• Not designed for interconnect wiring.

Fire Resistant Cable

• Used in flight critical circuits.

• Resists flames up to 1093 °C (2000 °F) under short-time emergency conditions.

• Predominantly used in the engine compartment.

Warning: This Wire May Contain Asbest

• Asbestos has not been permitted since 1993.

• Undated fire resistant wire should be treated as containing asbestos.

• Contact Environmental Health if asbestos is identified or suspected.

• Replace with non-asbestos alternatives where feasible.

• Fire resistance ≠ high temperature.

• Use fire resistant cable only where required.

• More susceptible to contamination and abrasion.

Fire Resistant Cable Construction

• Multiple insulation layers for fire protection.

• Maintains insulation for 30 minutes to allow the aircraft to land.

• Conductors have a thick alloy coating with a higher melting point than copper.

Fire Resistant Cable Construction Diagram

inside → outside

• Nickel clad high strength copper alloy conductor – conductor with a high melting point coating.

• Inorganic barrier – thermal insulation against fire.

• Polyimide tape – maintains electrical insulation at high temperatures.

• PTFE tape – outer protective layer (abrasion/chemical protection).

High Temperature Cable

• Temperature rating of 125 °C or higher (some manufacturers use 90 °C).

• Different from fire-resistant cable.

• Can be single- or multi-conductor.

• May have a fibreglass braid or Kevlar-fibre jacket.

• Two key factors:

  • Temperature rating – maximum continuous operating temperature.

  • Current capacity – maximum current without exceeding insulation/jacket temperature limits.

• Undersized cable overheats, reducing cable life and compromising the cable.

Thermocouple Cable

• Used to detect and measure temperature changes.

• Made of a pair of dissimilar metallic conductors.

• Extension leads must be the same material as the thermocouple.

• Extension wires are paired in a braided jacket and colour coded.

• Correct installation is required to maintain accuracy.

• Gas turbine engines use multiple thermocouples arranged in a harness.

Thermocouple Cable Construction

• Harness insulation: silicone rubber or PTFE-impregnated fibreglass.

• Harness terminates at an engine or firewall junction box.

• Extension cable insulation: polyvinyl, as it is not subject to high temperatures.

Bonding Lead

Shock Mount with Bonding Strap

Bonding Cable Construction

• Made from copper or aluminium.

• Must have sufficient surge capacity to carry the whole discharge.

• Material must not produce galvanic corrosion.

• Length: long enough for movement, not so long that resistance increases.

• Sacrificial aluminium washers prevent galvanic corrosion between copper straps and aluminium structure.

• Aluminium alloy straps are used in most cases.

• Copper straps are used to bond stainless steel, cadmium plated steel, copper, brass, or bronze.

• Always use the bonding cables/straps specified in the AMM and IPC.

Bonding Strap on a Flight Control

AC 43.13-1B Section 15

Grounding and Bonding

Power Cables

• Runs from power sources to bus bars, between bus bars, and from bus bars to equipment.

• Protected by circuit protection devices.

• Aluminium conductors are ~50% lighter than copper but are usually 2 AWG sizes larger.

• Aluminium conductors have more strands to reduce vibration cracking (work hardening).

• Copper cables are used between engine-mounted generators and the fuselage.

• Generator feeder cables are an example of power cables.

Generator Feeder Cables

Example of larger size cables

Data Cables

• Connect different aircraft systems.

• Carry voltage signals, not power → conduct very little current.

• Lighter conductors.

• Do not require overload protection.

• Types: single conductor & earth, single screened, twisted pair, screened twisted pair, coaxial, triaxial, multiple screened, and fibre optic cables.

Multicore Screened and Jacketed Cables

• Up to four cores, twisted together.

• May be jacketed or screened and jacketed.

• Screening is usually a braid with 85% surface coverage.

• Replacement cable must not reduce the screening standard.

• Cores are colour coded for easy identification.

Data Bus

• Must be replaced only with the cable specified by the Design Authority.

Unshielded Twisted Pair (UTP)

Shielded Twisted Pair (STP)

Twisted Pair Cable

• Twisted wires cancel magnetic fields, reducing interference.

• Twisting reduces unwanted induced signals.

• UTP (Unshielded Twisted Pair) – no ground shield.

• STP (Shielded Twisted Pair) – ground shield reduces external signal interference.

High Tension Cables

• Operating voltage over 600 V.

• Mainly used in engine ignition systems (up to 40,000 V).

• Single-core stranded, insulated, and metal braided screened to prevent interference.

• Also used in: strobe systems, unregulated frequency wild generators, and hybrid/electric aircraft battery packs.

• Spec 55: 600 V (single wall), 1000 V (dual wall).

Piston Engine Ignition Cable Construction

• Silicone jacket – heat, cold, chemicals, moisture.

• Reinforcing member – strength & terminal retention.

• Silicone insulation – dielectric & heat performance.

• Conductive silicone layer – EMI/RFI suppression.

• Fibreglass braid – strength.

• Carbon impregnated fibreglass core – conductivity.

• Kevlar® reinforcement – strength.

• Ferrite layer – EMI/RFI suppression.

• Wire-wound layer – EMI/RFI suppression.

High Tension Cable - Picture

Spec 55 Insulated System - Single Wall

Spec 55 Insulated System - Dual Wall

Ignition Cable (Piston Engine)

High-Tension Ignition Cable Terminations

• Connected using special end fittings with springs or contact caps, secured to the conductor, insulation, and threaded coupling assembly.

High-Tension Ignition Cable End Exploded View

High-tension ignition Cable Termination Picture

High Tension Cable for Piston Engine

Warning - Igniter Plug

• Igniter plug discharge can be up to 2,000 amps.

• Low battery voltage does not reduce discharge power, only the rate of spark discharge.

• HEIU output voltage can cause significant injury.

• Isolate input power and wait at least 1 minute before working on the system.

• Short the capacitor before working inside the HEIU.

• Coil and magneto systems have higher voltage but lower current than HEIUs.

1: High temperature "Class E" silicone jacket to withstand heat, cold, chemicals and moisture.

2: Reinforcing member for added strength and terminal retention.

3. Silicone insulation for superior dielectric and heat performance.

4. Conductive silicon layer for high temperature performance and EMI/RFI noise suppression.

5. Fiberglass braid for maximum strength

6. Carbon impregnated berglass core for superior support conductivity.

7. Core reinforced with KEVLAR®.

8. Layer impregnated with Ferrite.

9. Wire-wound layer to suppress EMI/RFI noise.

Spark Plug Wires

• Carry over 20,000 volts from the distributor cap to the spark plug.

• Fibre core carries the high voltage.

• Older metallic core wires caused electrical interference.

• Some have a locking connection at the distributor cap.

• To remove: remove the distributor cap first, squeeze the terminals together, then remove the spark plug wire.

Coaxial Cabel

• Used to connect electronic equipment to antennas.

• Prevents radio frequency interference (RFI).

• Two conductors:

  • Centre conductor covered by a dielectric.

  • Outer conductor covered by a protective sheath.

• Do not kink, squash, stand on, or drop tools on the cable – it will become unserviceable.

Coaxial Cable Construction

• Centre conductor: solid or stranded copper (plain, tinned, silver plated, or gold plated).

• Dielectric: polyethylene or Teflon®.

• Outer conductor: fine wire braid.

• Outer jacket: protects against weather, fluids, mechanical and electrical damage.

• Used in: communication, navigation, and capacitance fuel quantity indicating systems.

Antenna connected using co-axial connector

Coaxial Cable Construction

Coaxial Cable Advantages

• Shielded against electrostatic and magnetic fields.

• Does not radiate or pick up external interference.

• Common impedance: 75 Ω.

• Damage that changes the cable’s coaxiality changes its impedance, degrading or stopping equipment operation.

• After insulation testing, always short the cable before reconnecting to prevent equipment damage.

Co-axial Cable Diagram

Typical Co-axial Lead with BNC Connector

Impendence Calculations of Coaxial Cable

Triaxial Cable

Three Parts (Remember: Signal → Return → Noise)

• Centre wire → Carries the signal.

• First shield → Return path for the signal.

• Second shield → Carries external noise to ground.

Used For

• Antenna cable for very sensitive signal detection systems.

• Example: Automatic Direction Finder (ADF).

Multiple Layers of Shielding

Triax Cable

MIL-W-22759 Aircraft Wire

• is commonly used in new aircraft wiring installations.

• Always consult the manufacturer's Service Manual (AMM) and Service Bulletins (SBs) before replacing aircraft wiring.

• Check for the correct:

  • Wire type

  • Wire size

• All aircraft wire must have its type identification imprinted along its length.

Memory Trick

"22759 = Check Manual, Check Size, Check Stamp."

MIL22759

Crimping

• Attaches a terminal/contact to a conductor without solder.

• Uses compression (deformation) of the terminal barrel around the conductor.

• Creates a homogenous mass between the conductor and terminal.

• Produces a strong mechanical and electrical connection

A Good Crimp should ?

• Be as strong as the conductor.

• Have good electrical conductivity.

• Not introduce impedance or signal discontinuity.

• Use the correct combination of:

  • Conductor

  • Crimp barrel/terminal

  • Crimp tool

Advantages of Crimping

• Faster and easier fabrication.

• Uniform operation.

• Good electrical conductivity.

• Lower voltage drop.

• Strong connections (as strong as the conductor).

• Eliminates solder-related problems (solder slop and flux).

• No dry solder joints

Crip Types

• Ring terminal

• Slotted terminal

• Hook-type terminal

• Ring terminal (lug) is the most commonly used.

• Preferred because it virtually eliminates circuit failure due to terminal disconnection.

Memory Tip

Ring = Reliable (most secure, least likely to disconnect).

Splices

• A method of connecting conductors using a crimped connector.

• Usually a permanent connection using a self-insulated splice connector.

• Permitted only if it does not affect the reliability or electromechanical characteristics of the wiring.

• Maximum of one splice is allowed in any one wire segment between two connectors or disconnect points.

• Crimper calibration must be checked before use by verifying the calibration label/sticker is current.

• Refer to MIL-C-22520 for crimping tool requirements.

• TIR (Total Indicator Reading) measures the total deviation from the true centre line after a contact is crimped.

Crimping Tool Calibration Label

Contact Size and Deformation by crimper

MIL-C-22520 Crimping Tool

Types of Electrical Connectors

• M (Motion Picture) Series (1925):

• Mechanical locking ring to prevent accidental disconnection.

• P (Paramount Studios) Series:

  • Introduced die-cast shells, moulded pin inserts and latch locking devices.

• F (Fox Studios) Series:

  • Introduced the threaded coupling ring.

  • Adapted for the Douglas DC-1 and later aircraft.

  • Led to the AF firewall connector, the first hostile environment connector for aircraft.

• Most common aircraft connectors are AN (Army-Navy) and MS (Military Specification).

• AN connectors have been superseded by MS connectors (same part number, AN replaced with MS).

• Installed on wiring that is frequently disconnected.

• Power supply side is typically connected to the socket to reduce the chance of an accidental short when connectors are separated.

• Commonly used on avionic components with multi-pin connectors for easy maintenance.

Military Connector Specifications

• Standardise aircraft and military connectors.

• Ensure reliability, performance and environmental protection.

• Define:

  • Housing dimensions

  • Insert layout

  • Operating characteristics

• Approved manufacturers are listed on the Qualified Products List (QPL).

• Crimped contacts are generally used.

• Use environmental-resistant connectors where there are fluids, vibration, shock or corrosion.

QPL

Qualified Products List

Millarary Specs

ensure reliability, performance and environmental protection.

Plug

Receptacle

Connector Housings

Connector Identification

• Designed to meet military specifications.

• Adapted for use in most aircraft.

• Reference identification must remain legible throughout the life of the aircraft.

Military Specication Connector Identication

• Military specifications (MIL specs) vary greatly.

• Part number descriptions can vary between MIL specs.

• Refer to the applicable MIL spec and part number to determine a connector's exact characteristics.

MIL Spec Part Number Breakdown