FAA Airframe Written TestChapter 11 · 40 practice questions

Chapter 11: Aircraft Electrical Systems

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Chapter: Aircraft Electrical Systems

Overview

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

Ohm's Law and Voltage Drop - Aircraft Electrical System Fault Diagnosis Ohm's Law and Voltage Drop FAA A&P General - Aircraft Electrical Systems (14 CFR §65.75, AC 43.13-1B) BATTERY 28 VDC SW HIGH R CORRODED CONNECTION LANDING LIGHT GND VOLTMETER AT SOURCE 28.0 V VOLTMETER AT LOAD 14.2 V VOLTAGE DROP 28.0 - 14.2 = 13.8 V OHM'S LAW: V = I × R Voltage Drop = Current × Resistance A dim light with correct source voltage indicates excessive resistance in the circuit Check connections, grounds, and switches per AC 43.13-1B ! I = 0.5A R = 27.6Ω R = 28.4Ω E = 28V AC 43.13-1B Ch. 11

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 - Series vs Parallel Circuit Behavior Series vs Parallel Circuit Behavior — Aircraft Electrical Systems FAA A&P Prep · AC 43.13-1B SERIES CIRCUIT Single current path 28V DC L1 L2 L3 OPEN OPEN = ALL LAMPS GO DARK Key behavior: I_total = I_1 = I_2 = I_3 One open path → zero current everywhere V_total = V_1 + V_2 + V_3 (Kirchhoff's law) PARALLEL CIRCUIT Multiple independent branches 28V DC L1 L2 L3 OPEN = ONLY L2 GOES DARK L1 & L3 REMAIN LIT Key behavior: V_total = V_1 = V_2 = V_3 I_total = I_1 + I_2 + I_3 (branch currents add) Each branch operates independently SERIES • Single path for current flow • R_total = R1 + R2 + R3 • Used in: current-limiting circuits PARALLEL • Multiple independent paths • 1/R_total = 1/R1 + 1/R2 + 1/R3 • Used in: aircraft lighting, avionics buses 14 CFR § 43.13-1B Acceptable methods for electrical system maintenance & inspection AC 43.13-1B Chapter 11

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 Breaker Operation and Sizing - FAA A&P Aircraft Electrical Systems CIRCUIT BREAKER OPERATION & SIZING THERMAL CIRCUIT BREAKER Bimetallic strip bends when heated by overcurrent OVERCURRENT TRIP! RESET MAGNETIC BREAKERS Respond to current-induced magnetic fields Faster trip than thermal — used for sensitive equipment BREAKER SIZING — PROTECT THE WIRE 14 CFR 23.1357 / AC 43.13-1B Chapter 11 14 AWG WIRE Max continuous: 15A CORRECT: 15A Protects wire at rated ampacity Trips before wire overheats ✓ Compliant with AC 43.13-1B Load: 12A normal / 20A fault OVERSIZED: 30A Too high for 14 AWG wire Wire melts before breaker trips ✗ Fire hazard — NOT compliant WIRE OVERHEATING! Normal Danger SIZING RULE Breaker rating ≤ wire ampacity (14 CFR 23.1357) Protect the wiring — not just the load Ref: AC 43.13-1B Ch. 11 §11-69, FAA A&P ACS ! FAA A&P Exam Prep — Aircraft Electrical Systems | SkyLicense

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

Common Breaker Failure Modes - Aircraft Electrical Systems Common Breaker Failure Modes — Aircraft Electrical Systems MODE 1: IMMEDIATE TRIP Short circuit in protected circuit CIRCUIT BREAKER TRIPS POWER FEED SHORT Reset → Instant Trip Indicates short circuit in protected circuit AC 43.13-1B §11-84 MODE 2: INTERMITTENT TRIP High current draw / motor binding CIRCUIT BREAKER CYCLING MOTOR BINDING I = HIGH Intermittent Tripping Motor deterioration or mechanical binding AC 43.13-1B §11-85 MODE 3: FAILED TO TRIP Faulty breaker / open circuit CIRCUIT BREAKER STUCK CLOSED DEAD SHORT Breaker Fails to Trip Faulty breaker mechanism or open circuit condition AC 43.13-1B §11-86 FAA A&P Exam Prep — Aircraft Electrical Systems | 14 CFR §23.1357, AC 43.13-1B
  • 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

Solenoid and Relay Operation - Aircraft Electrical Systems Solenoid and Relay Operation — Low-Current Control of High-Current Circuits SOLENOID — MASTER CONTACTOR Typical: Master Relay / Starter Contactor 28V HIGH-CURRENT PATH (up to 200A) COIL (CONTROL WINDING) CONTROL SWITCH M STARTER Energizing coil → magnetic field pulls armature → contacts close → high current flows to load RELAY — WORN CONTACTS Pitted / burned contacts → intermittent operation CONTROL CIRCUIT (LOW CURRENT) COIL ARC LOAD CIRCUIT (HIGH CURRENT) LOAD PITTED CONTACTS • Arcing erodes contact surface • High resistance → heat → more arcing • Causes intermittent circuit operation AC 43.13-1B: inspect & replace SYMPTOMS • Load drops out intermittently • Relay chatters / buzzing • Voltage drop across contacts Check with voltmeter (mV drop) Worn contacts cause resistance and arcing — replace relay per manufacturer's maintenance manual FAA A&P General — Aircraft Electrical Systems — Solenoid & Relay Operation

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
  • Worn brushes: Sparking, reduced output, intermittent operation
  • 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

Bonding Requirements and Limits - Aircraft Electrical Systems Bonding Requirements and Limits AC 43.13-1B Chapter 11 • FAA A&P Exam Prep Component A (e.g., Fuel Tank) Component B (Airframe Ground) Bonding Resistance 0.0012 0.0058 Ω 0.0018 Ω 0.0025 Ω milliohms (mΩ) 3 mΩ LIMIT ✓ CRITICAL BONDING PATH Resistance must be less than 0.003 Ω (3 mΩ) ⚠ FUEL SYSTEM COMPONENTS All fuel system components must be bonded to airframe ✗ INSPECTION REQUIRED Inspect jumpers for corrosion Replace when deteriorated ⚡ CORROSION DETECTED Reference: AC 43.13-1B Ch. 11 § 11-65 • 14 CFR § 23.867 • FAA A&P Airframe Knowledge Bonding jumpers: minimum 20 AWG copper strap or equivalent • Clean tight connections required Bonding Requirements and Limits - Aircraft Electrical Systems
  • 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

  1. Verify the problem: Confirm the reported symptom
  2. Check power source: Battery voltage, alternator output
  3. Check circuit protection: Breakers, fuses
  4. Check switches and controls: Verify proper operation
  5. Check wiring: Visual inspection for damage, loose connections
  6. Check components: Test individual components
  7. Check grounds: Verify ground path integrity

Common Fault Symptoms and Causes

SymptomLikely Cause
Dim lightPoor ground, high resistance connection
Light inoperativeOpen circuit, blown bulb, faulty switch
Motor runs but no movementMechanical failure (broken linkage)
Motor inoperativeOpen winding, faulty switch, no power
Breaker trips immediatelyShort circuit
Breaker trips intermittentlyHigh current draw, motor deterioration
Ammeter shows dischargeCharging system failure, high load
Gauge reads emptyOpen circuit in sender
Intermittent operationLoose 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.

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