Overview This chapter covers the fundamental principles and practices of aircraft electrical, avionics, and instrumentation systems maintenance. It encompasses troubleshooting methodologies, regulatory compliance requirements, inspection procedures, and corrective actions for common electrical and avionics system failures. The material integrates practical troubleshooting techniques with the regulatory framework established by Canadian Aviation Regulations (CARs) and industry standard practices outlined in AC 43.13.
Key Concepts Explained in Detail Troubleshooting Methodology Systematic Approach to Electrical Fault Diagnosis
Electrical Troubleshooting — Systematic Progression
Electrical Troubleshooting — Systematic Progression
1
Check
circuit breakers
and fuses
AC 43.13
2
Measure voltage
at system
terminals
Multimeter
3
Inspect for
looseness,
corrosion,
damage
CAR 571
4
Examine for
chafing,
frayed
insulation
AC 43.13
5
Test individual
components
(transponder,
PFD, etc.)
6
Record findings
in the
aircraft
logbook
CAR 571, 605
• CAR 571 — Maintenance: approved data, release, investigation of tripped breaker before reset
• CAR 573 — Modifications/repairs: independent inspection after modification, contact manufacturer if discrepancy
• CAR 605 — Continuing airworthiness: safety equipment operational, document corrective actions
• AC 43.13 — Acceptable methods: inspect connections, check breakers, clean terminals, replace wires
Common faults:
corrosion (connectors, terminals) • frayed wiring (replace, never temporary repair) • loose connections (lock, tighten) • intermittent issues (flickering, disconnections)
Electrical system troubleshooting follows a logical progression that begins with the simplest and most probable causes before moving to more complex diagnostics. The fundamental principle is to verify power supply and connections before suspecting component failure.
Initial Troubleshooting Steps:
Tripped Breaker — Investigate Before Reset
Circuit Breaker Tripped — Investigation Before Reset
STEP 1 — DETECTION
Circuit breaker tripped
or fuse blown
(Common cause of failure)
STEP 2 — DIAGNOSIS
Investigation before reset
Never reset without
identifying the cause
STEP 3 — CAUSES
Temporary overload
Short circuit
Faulty component
STEP 4 — VISUAL AND PHYSICAL INSPECTION
Damaged wiring
Frayed insulation
→ Replace the wire
Loose connections
Visible corrosion
→ Clean / tighten
STEP 5 — DOCUMENTATION
Record the findings
Logbook
CAR 571 / 605 requirement
Replacement authorized
Identical rating and type
Only
STEP 6 — RESET
Cause identified and fixed
Cause not identified
RESET AUTHORIZED
Functional test required
Verify CAR 571 compliance
DO NOT RESET
Ground the aircraft
Defer release (CAR 605)
REGULATORY REMINDER
• CAR 571: Mandatory investigation
before reset
• CAR 573: Independent inspection
after modification
• AC 43.13: Good troubleshooting
practices
Check circuit breakers and fuses – These are the first line of defense in electrical systems. A tripped breaker or blown fuse indicates either a temporary overload or a persistent fault. Never reset a breaker without investigating the cause.
Verify power supply – Measure voltage at the component's input terminals. Low or absent voltage points to supply issues rather than component failure.
Inspect connections – Loose, corroded, or damaged connections account for a significant percentage of electrical system failures. Connectors should be visually inspected and physically verified for secure mating.
Examine wiring – Look for chafing, frayed insulation, broken conductors, or signs of overheating.
Intermittent Fault Diagnosis Intermittent problems present unique challenges because they may not be reproducible during ground testing. The key approach involves:
Thorough visual inspection of all wiring and connectors in the affected circuit
Checking for loose connections that may vibrate or shift during flight
Verifying connector integrity – Ensure locking mechanisms function properly
Reviewing maintenance logs for previous related issues or modifications
Diagram — Electrical, Avionics & Instrumentation
Dépannage électrique et avionique — Approche systématique
1. Symptôme observé
2. Collecte d'informations
3. Causes probables (simples → complexes)
4. Tests et mesures
5. Correction / Remplacement
Échec du test
Cadre réglementaire
• CARs (RAC)
• AC 43.13
• Manuel du constructeur
• Fiche technique
Outils de diagnostic
• Multimètre numérique
• Testeur de continuité
• Oscilloscope
• Analyseur de circuits
Légende :
Symptôme / Échec
Test / Correction
Analyse / Causes
Flux principal
Approche logique : du plus simple au plus complexe — Conforme à la norme AC 43.13
Electrical System Components and Maintenance
Electrical and Avionics — Architecture
Electrical and Avionics — Architecture
ALT / GEN
28V DC
BATTERY
24V DC
+
-
MASTER
SWITCH
MAIN BUS
(DISTRIBUTION)
GROUND (GND)
BRK.
AVIONICS
(COM, NAV, GPS)
BRK.
INSTRUMENTS
(PFD, MFD, EICAS)
BRK.
LIGHTING
(NAV, STROBE, CABIN)
BRK.
OTHER SYSTEMS
(ELT, GEAR, FLAPS)
Current Flow (Educational Animation)
Protection (Breaker/Fuse)
Component / Equipment
Circuit Breakers and Fuses Circuit breakers serve as both protective devices and indicators of system health. A tripped breaker demands investigation before resetting. The technician must:
Identify the cause of the trip (overload, short circuit, component failure)
Document findings in technical records
Replace only with components of identical rating and type
Batteries and Power Systems Battery maintenance requires attention to:
Terminal condition – Corrosion indicates potential acid leakage or environmental exposure
Voltage levels – Low voltage during engine start can cause system flickering
Proper installation – Correct terminal orientation and torque specifications prevent shorts
Wiring and Connectors Wiring integrity is critical for system reliability. Key maintenance actions include:
Replacing damaged wiring – Frayed insulation, chafing, or broken conductors require replacement, not temporary repairs with tape
Re-routing harnesses – When chafing against structure is discovered, the harness must be rerouted or protected
Replacing corroded connectors – Corrosion degrades electrical performance and reliability
Securing connections – Connectors must be properly seated and locked
Avionics Systems Primary Flight Displays (PFD) PFD troubleshooting focuses on:
Power supply verification – Check circuit breakers and voltage at the display
Connection integrity – Loose or corroded connections cause intermittent flickering
System calibration – Inaccurate readings may require calibration verification
Autopilot Systems Autopilot malfunctions often stem from:
Wiring and connector issues – Intermittent disconnections frequently result from poor connections
Disconnect switch functionality – A stuck switch requires system deferral until repaired
Power supply stability – Voltage fluctuations can cause system disengagement
Communication and Navigation Systems Troubleshooting these systems involves:
Audio panel power – Check fuses and circuit breakers first
Transponder testing – Requires external power source for ground checks
Navigation system calibration – Verify before further diagnosis
Emergency Locator Transmitter (ELT) ELT maintenance priorities:
Battery voltage check – Insufficient power is a common cause of failure
Connection verification – Secure all electrical connections
Operational testing – Confirm activation and signal transmission
Instrumentation Systems Pitot-Static Systems Accurate altitude and airspeed readings depend on:
Static port condition – Blockages cause erroneous readings
Pitot tube integrity – Ensure unobstructed airflow
System leaks – Check for leaks in lines and connections
Altimeter Accuracy When altimeter readings are incorrect:
First check static system for blockages or leaks
Verify electrical connections if static system is clear
Consider recalibration only after eliminating other causes
Important Regulations and Procedures Canadian Aviation Regulations (CARs) CAR 571 – Maintenance and Manufacturing Standards This regulation governs the standards for maintenance work. Key requirements include:
All maintenance must be performed using approved data
Functional checks must verify system accuracy and operation
Damaged components must be replaced, not temporarily repaired
Maintenance releases require documentation of all work performed
CAR 573 – Maintenance Organizations This regulation addresses organizational requirements:
Independent inspections are required after modifications
Discrepancies in technical data must be clarified with manufacturers
Technical records must document all maintenance actions
Non-compliance with airworthiness directives must be reported
CAR 605 – Aircraft Requirements This regulation covers operational requirements:
Safety-critical components must be operational before flight
Inoperative equipment may require aircraft deferral
Maintenance releases cannot be issued for non-compliant aircraft
AC 43.13 – Acceptable Methods, Techniques, and Practices This advisory circular provides industry-standard guidance for:
Electrical system troubleshooting – Systematic approach to fault diagnosis
Connector maintenance – Proper cleaning, inspection, and securing procedures
Corrosion prevention – Cleaning and protective coating application
Wiring repairs – Replacement criteria and installation standards
Battery maintenance – Terminal cleaning and installation procedures
Standard 566 – Maintenance Standards This standard addresses:
Functional check requirements after maintenance
System testing under various operating conditions
Documentation of test results
Common Relationships Between Concepts Power Supply and System Performance The relationship between power quality and system operation is fundamental:
Low voltage causes flickering displays, intermittent operation, and system failures
Voltage drops during engine start indicate battery condition issues
Circuit breaker trips protect against overloads but indicate underlying faults
Connection Integrity and Reliability Poor connections manifest in predictable ways:
Loose connections cause intermittent failures that may appear only during vibration
Corroded terminals increase resistance, causing voltage drops and heat generation
Damaged connectors lead to complete circuit failure
Regulatory Compliance and Safety The regulatory framework ensures safety through:
Documentation requirements – All maintenance must be recorded for traceability
Independent inspections – Verify modification integrity before return to service
Airworthiness directive compliance – Mandatory actions for known safety issues
Maintenance release procedures – Formal certification of airworthiness
Troubleshooting Progression Effective troubleshooting follows a logical sequence:
121. Verify power supply – Check breakers, fuses, and voltage
122. Inspect connections – Look for loose, corroded, or damaged connectors
123. Examine wiring – Check for chafing, fraying, or damage
124. Test components – Only after eliminating supply and connection issues
125. Document findings – Record all actions and results
Corrosion Management
Corrosion — Effect Chain on Electrical Connections
Corrosion — chain of effects on electrical connections
TP14038E-EA Ch.5 — Electricity, avionics, and instrumentation
CAUSE-EFFECT CHAIN
STANDARD RESPONSE — MAINTENANCE
ROOT CAUSE
Corroded connection
Connectors, battery terminals,
wire harnesses exposed to moisture
PHYSICAL EFFECT
Increased resistance at connection point
Corrosion creates an insulating barrier
that impedes current flow
ELECTRICAL CONSEQUENCES
Voltage drop + localized heating
High resistance causes voltage drop
and generates heat
OBSERVABLE SYMPTOMS IN FLIGHT / ON GROUND
Flickering lights (cabin, navigation)
Intermittent autopilot disconnection
Complete failure of the affected electrical system
Detection
STEP 1 — INSPECTION
Check for battery acid leaks
Visually inspect connectors
and terminals for corrosion
STEP 2 — CORRECTIVE ACTION
Replace corroded connectors
Do not clean if replacement is possible
— this is the recommended best practice
STEP 3 — PROTECTION
Apply protective coating
To new terminals and connectors
in accordance with the maintenance manual
STEP 4 — VERIFICATION & DOCUMENTATION
Test the circuit — confirm compliance
Document the action in the logbook
Requirements: RAC 571, 573, 605 — AC 43.13
Corrosion affects electrical systems through:
Increased resistance at connection points
Potential for complete circuit failure
Risk of acid leakage from batteries
Progressive damage if not properly treated
The standard response involves:
Inspecting for source of corrosion (e.g., battery leaks)
Cleaning affected areas
Applying protective coatings
Replacing severely corroded components
System Interference After Modifications Modifications can introduce unexpected interactions:
Electromagnetic interference between new and existing systems
Power supply loading issues from additional equipment
Wiring routing conflicts causing chafing or signal degradation
Configuration management problems from incomplete documentation
Addressing these requires:
Comprehensive functional testing
Independent inspection per CAR 573
Manufacturer consultation for discrepancies
Proper documentation of all changes
