SkyLicence
Chapter II

Standard Practices - Wiring, Connectors, Crimping, Bonding and EWIS

SkyLicence study guide with diagrams.

Overview

This chapter covers the fundamental principles and practices for aircraft electrical wiring interconnection systems (EWIS), including proper techniques for wire selection, installation, termination, bonding, and troubleshooting. The material focuses on the standards outlined in AC 43.13-1B and industry best practices for maintaining reliable electrical systems in aircraft.

Key Concepts

Wire Selection and Sizing

Wire Sizing and Ampacity — Matching Breaker Rating Wire Gauge and Ampacity — Sizing vs Breaker Mandatory correspondence between AWG, ampacity, and breaker size (AC 43.13-1B) 18 AWG Wire Copper conductor Maximum ampacity 20 A ✓ Correct breaker 20 A Adequate protection ✗ Oversized 30 A ⚠ Wire overheating ✗ Undersized 10 A ⚠ Nuisance tripping 20 AWG Wire Copper conductor Maximum ampacity 11 A ✓ Correct breaker 10 A Adequate protection ✗ Oversized 20 A ⚠ Wire overheating ✗ Undersized 5 A ⚠ Nuisance tripping 22 AWG Wire Copper conductor Maximum ampacity 5 A ✓ Correct breaker 5 A Adequate protection ✗ Oversized 10 A ⚠ Wire overheating ✗ Undersized 2 A ⚠ Nuisance tripping Rule: The breaker must be sized according to the wire's ampacity (AC 43.13-1B) 18 AWG wire = 20 A max | 20 AWG = 11 A max | 22 AWG = 5 A max Correct Incorrect

Wire gauge selection is critical for safe aircraft operation. The American Wire Gauge (AWG) system defines wire sizes, with smaller numbers indicating larger diameter conductors. Current-carrying capacity (ampacity) varies by wire size:

18 AWG: Rated for 16-20 amps in aircraft applications
20 AWG: Rated for 7.5-11 amps
22 AWG: Rated for approximately 5 amps

When selecting wire for a circuit, the wire must be rated for at least the circuit breaker rating. Circuit breakers must be replaced with identical type and current rating components. Using a higher-rated breaker could allow wire overheating without tripping, while a lower rating causes nuisance tripping.

Diagram — Standard Practices - Wiring, Connectors, Crimping, Bonding and EWIS EWIS Standard Practices — Wiring, Connectors, Crimping & Bonding 1. Wire Selection & Sizing AWG 18 — 16-20A CB: 15A AWG 20 — 7.5-11A CB: 7.5A AWG 22 — ~5A CB: 5A ⚠ CB rating ≤ wire ampacity 2. Insulation Types by Zone Standard: PVC / Nylon High-Temp: PTFE / Kapton / Tefzel >200°C Wheel Wells: Sealed Conduit Protection from fluids, mud, debris 3. Crimping Process Wire Strip Term Crimp Tool ✓ Proper Crimp Section 4. Bonding & EWIS Integration Bonding Strap Low impedance path to airframe Prevents static discharge Connector Backshell, Grommet Pin/Insert retention Pins Troubleshooting Visual inspection Continuity / IR tests Connectors Wiring/CB Bonding Insulation Ref: AC 43.13-1B

Wire Insulation Types

Different aircraft zones require specific insulation materials:

Standard areas: PVC and nylon insulation
High-temperature areas (near engines, exhaust): Teflon (PTFE), Kapton, or Tefzel insulation rated for 200°C or higher
Wheel wells: Wiring requires protection from hydraulic fluids, water, mud, and debris using sealed conduit or protective sleeving

Bend Radius Requirements

Proper bend radius prevents conductor and insulation damage:

Single wires: Minimum bend radius of 10 times the wire diameter
Coaxial cables: Minimum bend radius of 6 times the cable diameter
Wire bundles: Larger radius may be required depending on bundle size

Stripping and Conductor Preparation

Proper stripping technique is essential:

Use the correct gauge wire stripper to avoid nicking or cutting conductor strands
Nicked conductors create stress risers that can break under vibration
Any nicked or cut strands require wire replacement per AC 43.13-1B
Strip length should allow the conductor to fill the terminal cup without excess exposed conductor

Soldering Techniques

Solder Joint Defects — Cold, Dry, Wicking, Ball Soldering defects — dry joint, wicking, cold joint CORRECT JOINT (REFERENCE) Concave fillet Bright and smooth surface Complete wetting of the pin Concave profile (meniscus) Contact angle < 90° Good metallurgical bond High mechanical strength HIGH RELIABILITY Minimal contact resistance THE FOUR SOLDERING DEFECTS DRY JOINT no wetting Appearance: • Dull, granular, porous • Solder "beads up" • Contact angle > 90° CONSEQUENCE High resistance, heating, possible breakage under vibration COLD JOINT movement during cooling Appearance: • Rough, dull surface • Visible cracks • Deformed joint CONSEQUENCE Intermittent connection, electrical arcs, random failure WICKING stiff zone migration Appearance: • Solder under insulation • Wire stiff when bent • Deformed or melted insulation CONSEQUENCE Wire breakage under vibration, loss of flexibility SOLDER BALL no wetting >90° Appearance: • Spherical shape • No adhesion to pin • Solder in "beads" CONSEQUENCE Nonexistent or very resistive connection, risk of short circuit Golden rule: a correct joint has a bright concave fillet — any dull, granular, or spherical appearance indicates a defect.

Solder Types:

Rosin core solder (60/40 or 63/37 tin/lead): Standard for aircraft electrical connections
Acid core solder: For plumbing only, causes corrosion in electrical applications
Silver solder: For high-temperature applications only

Proper Soldering Procedure:

35.Clean and tin the soldering iron tip for good heat transfer
36.Tin the stripped wire before soldering to ensure proper solder flow
37.Heat the joint (both wire and terminal), not the solder
38.Apply solder to the joint, allowing it to flow and create a concave fillet
39.Use a heat sink between the joint and insulation to prevent wicking

Common Solder Joint Defects:

Cold solder joint: Dull, rough, or cracked appearance; poor mechanical and electrical connection
Dry joint: Solder has not flowed onto the pin due to insufficient heating
Solder wicking: Solder flowing under insulation, creating stress risers
Solder ball: Too much solder or poor wetting

Crimping Procedures

Crimping — Procedure and Quality Control Crimping — Procedure and Quality Control 1. Stripping Clean cut Precise length. Strands intact. 2. Insertion Stop Push until stop. Insulation flush. 3. Crimping Height Calibrated/approved tool. Respect "Crimp Height". 4. Pull Test Force Validate retention. No movement allowed. Quality Control: Crimp Cross-Section CORRECT Complete metal-to-metal contact Optimal mechanical strength INCORRECT Voids / Trapped air Risk of overheating & failure

Critical Parameters:

Crimp height: Must be within manufacturer-specified range; tools must be calibrated
Crimp position: Centered over the terminal barrel
Tool selection: Ratcheting crimp tools specific to terminal type; standard pliers are unacceptable

Crimp Quality Verification:

Perform pull test after crimping to verify proper conductor deformation
Wire should be inserted until insulation is flush with the back of the contact
Double crimping is not recommended

Connector Installation

Circular Connectors:

Pins must be properly locked into the insert; pushed-back pins cause intermittent contact
Bent pins must be replaced, not straightened
Coupling rings should turn smoothly; difficulty indicates cross-threading, damage, or contamination

Coaxial Connectors:

Precise preparation is critical: center conductor, dielectric, and braid must be trimmed to exact lengths
Improper preparation causes impedance mismatch and signal loss
Shield must be properly captured in the connector

Connector Backshells:

Provide strain relief by clamping the cable
Seal connector from moisture and contaminants
Enable 360-degree termination for cable shield

Bonding and Grounding

Bonding and Grounding — Bond Check Resistance Measurement Bonding and Ground — Resistance Measurement (Bond Check) PHYSICAL INSTALLATION Stripped Surface (Bare Metal) Stripped Surface (Bare Metal) Bonding Strap MEASUREMENT & CRITERIA 0.0021 Ω MILLIOHMMETER (Micro-ohmmeter) ACCEPTANCE CRITERION R < 2.5 mΩ (0.0025 Ω) Maximum allowable bonding resistance Note: Use the 4-wire method (Kelvin) to eliminate lead resistance. OBJECTIVE: Prevent static & ground loops

Bonding Requirements:

Maximum allowable resistance: Less than 2.5 milliohms (0.0025 ohms) per AC 43.13-1B
Measurement requires milliohmmeter or micro-ohmmeter using four-wire (Kelvin) technique
Standard multimeters cannot accurately measure such low resistances

Bonding Strap Installation:

Mating surfaces must be clean, bare metal (no paint, corrosion, or anodizing)
Use conductive hardware (stainless steel)
Corrosion on bonding straps requires replacement and surface cleaning

Ground Studs:

Designated single-point connection for multiple bonding straps
Provides low-impedance reference point
Prevents ground loops

Ground Loops:

Ground Loops — Cause and Prevention Ground Loop — Cause and Prevention POOR BONDING — GROUND LOOP AVIONICS A (receiver) AVIONICS B (transmitter) signal ground A ground B COMMON GROUND POINT fuselage R1 (0.5 Ω) R2 (0.5 Ω) parasitic current (noise) noisy signal (interference) Multiple ground paths create a potential difference → parasitic current in sensitive avionics. GOOD BONDING — SINGLE POINT GROUND AVIONICS A (receiver) AVIONICS B (transmitter) signal ground A ground B SINGLE GROUND POINT fuselage R ≈ 0 Ω clean signal — no interference Single ground path → no parasitic current. All avionics are at the same potential. Correct bonding: resistance < 2.5 mΩ. Principle: a single bond to the single ground point avoids ground loops and noise in avionics.
Unintended current paths in ground system
Cause interference and noise in sensitive avionics
Prevented by proper bonding and single-point grounding

Wire Bundle Installation

Wire Bundle Routing — Separation and Spacing Wire Bundle Routing — Separation and Spacing Side View — Bundle Below Conduits Hydraulic / Fuel Line Leak WIRE BUNDLE 2 in min. Separation min 2 in Oxygen zone — strict separation 90° Crossing — Power / Signal Power Cable Signal Cable 90° ✓ Reduced EMI Clamp Spacing Clamp Clamp Clamp Clamp Clamp 8 in max. 8 in max. 8 in max. 8 in max. High-vibration areas: • Lacing tape preferred • Reduced spacing recommended • Enhanced abrasion protection Separation rules: • Wiring BELOW fluid lines • 2 in min. from hydraulic/fuel/oxygen lines • Power/signal crossing at 90° AC 43.13-1B Ch. 11-13 | AME Training — Transport Canada

Routing Requirements:

Wire bundles must be routed below fluid lines to prevent fire risk
Minimum 2-inch separation from hydraulic, fuel, and oxygen lines
Power and signal wiring must be separated to prevent EMI
Signal cables should cross power feeders at 90 degrees to minimize inductive coupling

Support and Securing:

Clamp spacing: Maximum 8 inches for typical bundles
Clamps must be snug but not compress wires
Clamps should be non-conductive (nylon or rubber)
Place clamps away from splices and connectors (minimum 1 inch)

Bulkhead Penetration:

Use rubber or plastic grommets to prevent chafing
Grommets provide smooth, non-abrasive surface

Cable Ties:

Must not be so tight as to indent wire insulation
Cut ties must be replaced immediately
In high-vibration areas, lacing tape is preferred over plastic ties

Splice Requirements

Use approved mechanical crimp splices with insulated sleeves or heat shrink
Stagger splices in bundles to prevent large lumps that cause chafing
Soldered splices are generally not recommended
Twisting and taping is not an approved method

Wire Identification

Use heat-shrinkable or slip-on wire markers with permanent markings
Place markers within 2 inches of each termination point
Markers must be resistant to fluids and abrasion
Tape or felt-tip markers are not acceptable

Terminal Block Installation

Bare conductor must be fully captured by terminal clamping mechanism
Insulation should be flush with terminal to prevent exposed conductor
Follow manufacturer's torque specifications for terminal screws
Correct hardware order: flat washer, terminal, lock washer, nut

Shielded Cable Termination

Shield Termination Methods:

Connect to connector backshell for 360-degree coverage
Twist braid into pigtail for connection to ground point (keep pigtail short)
Drain wire provides easier termination point for shield connection

Shielded Twisted Pair:

Twisting cancels induced noise from magnetic fields
Shield provides protection against electric fields
Both wires are equally exposed to external fields

Important Regulations and Procedures

AC 43.13-1B Requirements

Acceptable Practices:

Wire ampacity charts for proper sizing
Bonding resistance less than 2.5 milliohms
Minimum bend radius of 10 times wire diameter
Maximum support spacing of 8 inches
Use of approved crimp tools and terminals
Rosin core solder for electrical connections

Prohibited Practices:

Nicked or cut conductor strands
Acid core solder for electrical connections
Straightening bent connector pins
Using tape as substitute for grommets
Over-tightening cable ties

Troubleshooting Approach

Systematic Method:

147.Verify circuit breaker status (tripped or not)
148.Check power at the unit
149.Inspect antenna and cable (for RF systems)
150.Check connections and wiring continuity
151.Test component operation

Common Failure Points:

Antenna systems: Coaxial cable most common failure point
Fuel quantity systems: Poor grounding/bonding causes erratic readings
Strobe lights: High-voltage power supply common failure
Landing gear indication: Position switches most common failure
Communication systems: Poor antenna connections, faulty bonding
Navigation receivers: Antenna system problems cause radial errors

Bond Check Procedure

160.Use milliohmmeter with four-wire (Kelvin) technique
161.Clean mating surfaces to bare metal
162.Measure between component and aircraft primary ground
163.Acceptable reading: Less than 2.5 milliohms
164.Reading of 0.5 ohms indicates poor connection requiring repair
165.Reading of 0.001 ohms indicates excellent bond

Relationships Between Concepts

Wire Protection and Environmental Factors

The level of wire protection required correlates directly with environmental exposure:

High vibration areas: Require lacing tape, proper clamp spacing, service loops
High temperature areas: Require specialized insulation materials
Wheel wells: Require sealed conduit and contaminant protection
Bulkhead penetrations: Require grommets to prevent chafing

Bonding and System Performance

Proper bonding affects multiple systems:

Fuel quantity systems: Poor bonding causes erratic capacitance readings
Communication systems: Poor bonding causes interference and noise
Antenna systems: Poor bonding degrades ground plane and performance
Lightning protection: Proper bonding provides low-impedance path
EMI control: Proper bonding prevents ground loops and interference

Crimp Quality and Reliability

Crimp quality depends on multiple factors:

Tool calibration: Ensures correct crimp height
Proper insertion: Wire fully seated in contact
Pull test verification: Confirms mechanical connection
Centered crimp: Ensures even compression

Solder Joint Quality Factors

Heat control: Prevents insulation damage and wicking
Proper tinning: Ensures good metallurgical bond
Heat sink placement: Protects components and insulation
Joint appearance: Concave fillet indicates proper wetting
Chapter 1Reciprocating Engine — Theory & ConstructionChapter 3Human Factors

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