This chapter covers the fundamental principles, maintenance practices, and troubleshooting procedures for aircraft electrical systems. It encompasses the knowledge required for airframe maintenance technicians to safely inspect, repair, and maintain electrical components including wiring, circuit protection devices, motors, generators, batteries, and associated control systems. The material aligns with FAA regulations and accepted industry standards as outlined in AC 43.13-1B and 14 CFR Part 43.
Key Concepts
1. Electrical Circuit Fundamentals
Voltage, Current, and Resistance Relationships
The relationship between voltage, current, and resistance is governed by Ohm's Law: V = I × R. In aircraft systems, this relationship is critical for diagnosing faults:
Voltage drop occurs when current flows through resistance. Excessive resistance in a circuit reduces the voltage available at the load.
A dim light with correct voltage at the source indicates excessive resistance somewhere in the circuit, most commonly at connections, grounds, or switches.
Current draw is directly proportional to the load and inversely proportional to resistance. An increase in current draw beyond design specifications indicates a fault condition.
Circuit Configurations
Aircraft electrical systems use both series and parallel configurations:
Series circuits have a single path for current flow. An open in any component interrupts the entire circuit.
Parallel circuits provide multiple paths. Each branch operates independently, allowing selective operation of loads.
2. Wiring and Cable Construction
Wire Types and Ratings
Aircraft wiring must meet stringent specifications for temperature, voltage, and mechanical strength:
Conductor material: Copper is standard, with tin or nickel plating for corrosion resistance.
Insulation types: Materials are selected based on temperature rating, resistance to fluids, and abrasion resistance. Common types include:
PVC (polyvinyl chloride) for general-purpose use
Tefzel for high-temperature and abrasion-resistant applications
Kapton for high-temperature areas
Gauge selection: Wire gauge must match the current-carrying capacity required by the load and the circuit length. Using undersized wire creates excessive voltage drop and heat generation.
Wire Identification and Marking
Wires are identified by:
Wire number printed on the insulation
Color coding for circuit identification
Size designation (AWG - American Wire Gauge)
3. Circuit Protection Devices
Fuses
Fuses provide one-time protection against excessive current. They must be replaced with the correct amperage rating as specified in the wiring diagram.
Circuit Breakers
Circuit breakers provide resettable protection and serve as switches in some installations:
Thermal breakers respond to heat generated by current flow
Magnetic breakers respond to magnetic fields produced by current
Trip characteristics: Breakers trip when current exceeds their rated value for a specified time
Critical principle: Circuit breakers must be sized to protect the wiring, not the load. Installing a higher-rated breaker than specified compromises wire protection and creates a fire hazard.
Common Breaker Failure Modes
Immediate trip upon reset indicates a short circuit in the protected circuit
Intermittent tripping suggests high current draw from motor deterioration or mechanical binding
Breaker not tripping when it should indicates a faulty breaker or an open circuit
4. Electrical Components and Systems
Solenoids and Relays
These electromagnetic switching devices control high-current circuits with low-current control signals:
Solenoid construction: Contains a coil that creates a magnetic field when energized, pulling contacts together
Relay contacts: Can become pitted or worn, causing intermittent operation
Troubleshooting: If voltage is present at the coil and coil resistance is within limits, check the ground path and contact condition
Electric Motors
Aircraft use various electric motors for flaps, landing gear, trim tabs, and fuel pumps:
Series-wound motors provide high starting torque
Shunt-wound motors provide constant speed
Permanent magnet motors are simple and reliable
Motor failure modes:
Open field winding: Motor will not run; no current flow
Short circuit: Excessive current draw, breaker trips
Mechanical binding: Motor runs but output shaft does not move, or excessive current draw
Generators and Alternators
Charging systems convert mechanical energy to electrical energy:
Alternators produce AC voltage rectified to DC, with output controlled by a voltage regulator
Generators produce DC voltage directly, with output controlled by a voltage regulator
Field circuit: Controls output by varying field current; a tripped field breaker indicates a short in the field circuit
Charging system characteristics:
Output varies with RPM; many alternators produce minimal output at idle
A fully charged battery with low electrical load may show discharge at idle - this is normal
Continuous discharge during operation indicates charging system failure
5. Batteries
Battery Types
Lead-acid batteries: Common in general aviation; require electrolyte maintenance
Nickel-cadmium (Ni-Cad) batteries: Used in turbine aircraft; require specific charging procedures
Battery Maintenance
Electrolyte level: Must be maintained; overfilling causes leakage
Corrosion control: Electrolyte spills must be neutralized with baking soda solution
Battery box integrity: Must be inspected for corrosion damage
Ventilation: Battery compartments must be properly vented to prevent hydrogen accumulation
Battery Safety
Always disconnect the battery before electrical maintenance
Secure the disconnected lead to prevent accidental contact
Neutralize any spilled electrolyte before cleaning
6. Bonding and Grounding
Purpose of Bonding
Electrical bonding provides a low-resistance path for electrical continuity between metallic components:
Static discharge prevention: Critical for fuel systems to prevent spark ignition
Lightning protection: Provides a path for lightning current
Electrical system return: Ensures proper circuit operation
Radio interference reduction: Provides shielding effectiveness
Bonding Requirements
Resistance: Critical bonding paths must have resistance less than 0.003 ohm (3 milliohms)
Fuel system bonding: Fuel filler caps, fuel lines, and related components must be bonded
Corrosion protection: Bonding jumpers must be inspected for corrosion and replaced when deteriorated
Grounding
Proper grounding is essential for circuit operation:
A poor ground adds resistance, reducing voltage available to the load
Ground paths must be clean, tight, and corrosion-free
Ground connections should be checked when troubleshooting dim lights or inoperative components
7. Wiring Installation and Protection
Routing Requirements
Wires must be routed to avoid:
Heat sources: Engine bleed air ducts, exhaust systems, and other high-temperature areas
Sharp edges: Metal edges that can chafe insulation
Moving parts: Control cables, pulleys, and mechanical linkages
Fluid contamination: Fuel, oil, and hydraulic fluid
Protection Methods
Grommets: Required where wires pass through holes in bulkheads or structure
Clamps and stand-offs: Secure wires and maintain separation from hazards
Conduit: Provides additional protection in vulnerable areas
Sleeving: Protects against abrasion and high temperatures
Wire Bundle Practices
Bundles must be properly supported at specified intervals
Wires in bundles must be identified for maintenance
Bundles must not be routed where they can be stepped on or used as handholds
8. Wiring Repair and Replacement
Repair Standards
Per AC 43.13-1B Chapter 11:
Damaged insulation: Wires with cracked, brittle, or chafed insulation must be replaced
Broken conductors: Replace the entire wire from origin to termination when possible
Splicing: Allowed only with approved methods (crimp splices with heat shrink); not recommended in bundles unless necessary
Prohibited methods: Twist-on connectors, electrical tape, and soldering alone are not approved
Wire Replacement
When replacing wires:
Use same gauge or larger
Match insulation type and temperature rating
Follow original routing or improve routing to eliminate hazards
Install proper protection (grommets, clamps)
9. Electrical System Troubleshooting
Systematic Approach
134.Verify the problem: Confirm the reported symptom
135.Check power source: Battery voltage, alternator output
136.Check circuit protection: Breakers, fuses
137.Check switches and controls: Verify proper operation
138.Check wiring: Visual inspection for damage, loose connections
139.Check components: Test individual components
140.Check grounds: Verify ground path integrity
Common Fault Symptoms and Causes
Symptom
Likely Cause
Dim light
Poor ground, high resistance connection
Light inoperative
Open circuit, blown bulb, faulty switch
Motor runs but no movement
Mechanical failure (broken linkage)
Motor inoperative
Open winding, faulty switch, no power
Breaker trips immediately
Short circuit
Breaker trips intermittently
High current draw, motor deterioration
Ammeter shows discharge
Charging system failure, high load
Gauge reads empty
Open circuit in sender
Intermittent operation
Loose connection, worn contacts
10. Maintenance Documentation
Logbook Entries
Per 14 CFR 43.9, after performing maintenance, an entry must be made in the maintenance records containing:
Description of work performed
Date of completion
Mechanic's signature and certificate number
Major vs. Minor Maintenance
Minor repairs: Routine component replacement (alternator, battery, navigation light) - logbook entry only
Major repairs/alterations: Require FAA Form 337
100-hour inspections: Must be documented per regulations
Return to Service
Aircraft may be returned to service after maintenance by an authorized person
Documentation must be complete before return to service
14 CFR 43.13(a) requires using methods and practices acceptable to the Administrator
Important Formulas and Specifications
Ohm's Law
V = I × R (Voltage = Current × Resistance)
Power Formula
P = V × I (Power = Voltage × Current)
Voltage Drop Limits
Acceptable voltage drop: Typically less than 0.5 volt for 14-volt systems
Critical bonding resistance: Less than 0.003 ohm
Wire Gauge Selection
Based on current-carrying capacity and circuit length
Must account for voltage drop and temperature rise
Circuit Breaker Sizing
Must protect the wire, not just the load
Must match wiring diagram specifications
Common Relationships Between Concepts
Voltage Drop and Resistance
Excessive resistance anywhere in a circuit reduces voltage at the load
Poor connections, corroded terminals, and damaged wires all add resistance
Testing voltage at the load with the circuit loaded reveals resistance problems
Circuit Protection and Wire Size
Breaker rating must be matched to wire gauge
Undersized wire with oversized breaker creates fire hazard
Correct breaker protects the entire circuit
Grounding and Circuit Operation
All circuits require a complete path to ground
Poor grounds cause voltage drops and intermittent operation
Ground integrity is as important as power supply integrity
Motor Current and Mechanical Load
Mechanical binding increases current draw
Increased current may trip breakers
Motor deterioration (worn bearings, brushes) affects current draw
Charging System and Battery State
Alternator output varies with RPM
Battery state affects charging system operation
A fully charged battery with low load may show discharge at idle
Bonding and Safety
Bonding prevents static discharge in fuel systems
Corroded bonding jumpers compromise safety
Bonding resistance must be verified after maintenance
Documentation and Airworthiness
Proper documentation proves compliance with regulations
Maintenance records must be accurate and complete
Return to service requires proper documentation
Regulatory References
14 CFR Part 43
43.9: Maintenance record entries
43.13: Performance standards for maintenance
43.15: Inspection requirements
AC 43.13-1B
Chapter 11: Electrical systems maintenance and repair standards
Provides acceptable methods for wiring repair, component testing, and system troubleshooting
FAA AMT Handbooks
Provide detailed information on electrical system theory and maintenance practices
Summary
Aircraft electrical systems require systematic troubleshooting, proper maintenance practices, and strict adherence to regulatory standards. Understanding the fundamental relationships between voltage, current, and resistance, combined with knowledge of component construction and failure modes, enables the technician to diagnose and repair faults efficiently. Safety considerations, particularly regarding battery handling, circuit protection, and fuel system bonding, are paramount. Proper documentation ensures regulatory compliance and maintains the aircraft's airworthiness record.
