FAA Powerplant Written TestChapter 8 · 40 practice questions

Chapter 8: Ignition and Starting Systems

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Ignition and Starting Systems

Chapter Overview

This chapter covers the fundamental principles, components, maintenance practices, and troubleshooting procedures for aircraft engine ignition systems. It addresses both reciprocating engine magneto systems and turbine engine electronic ignition systems, with emphasis on airworthiness standards, regulatory requirements, and approved maintenance practices. The material prepares maintenance technicians to inspect, service, troubleshoot, and repair ignition systems in compliance with FAA regulations and manufacturer specifications.

Reciprocating Engine Ignition Systems

Magnetos: Principles of Operation

Magneto Electromagnetic Induction - FAA A&P Prep Magneto Electromagnetic Induction MAGNETO ASSEMBLY N S PRIMARY (coarse wire) SECONDARY (fine wire, thousands of turns) GROUND BREAKER POINTS CAPACITOR DISTRIBUTOR OUTPUT VOLTAGE 15,000–20,000V ✓ INDEPENDENT No aircraft electrical system required ROTATING PERMANENT MAGNET Primary: ~250V Secondary: 15–20kV ELECTROMAGNETIC INDUCTION Faraday's Law: EMF = −N dΦ/dt Voltage step-up via turns ratio FLUX BUILD/COLLAPSE 14 CFR Part 43, AC 43.13-1B — Magneto Ignition System

A magneto is a self-contained electrical generator that produces the high-voltage spark required to ignite the air-fuel mixture in reciprocating engine cylinders. Unlike automotive systems that rely on battery power, magnetos generate their own electrical energy through electromagnetic induction, making them independent of the aircraft's electrical system.

The magneto operates on the principle of magnetic flux change. A rotating permanent magnet induces current in primary and secondary coil windings. The primary circuit consists of low-voltage windings (typically 200-300 turns of heavy wire), while the secondary circuit contains thousands of turns of fine wire that step up the voltage to 15,000-20,000 volts required for spark generation.

Key components of a magneto include:

  • Permanent magnet rotor
  • Primary and secondary coil windings
  • Breaker points (contact points)
  • Condenser (capacitor)
  • Distributor block and rotor
  • Impulse coupling or induction vibrator (for starting)
  • P-lead (shielded grounding lead)

Breaker Points and E-Gap

Breaker Points and the E-Gap - Ignition System Timing Breaker Points & the E-Gap — Ignition Timing AC 43.13-1B CORRECT E-GAP (4-Cylinder Magneto) N S Magnetic Flux (Φ) E-GAP Points Close Points Open Points close → current builds magnetic field in primary coil. At max flux (E-gap), points open → field collapses → secondary winding induces high voltage. Strong spark ✓ INCORRECT E-GAP (Retarded Timing) N S 15° off Flux (Φ) — Early Break BREAK True E-Gap Points open too early — before magnetic field reaches maximum. Result: weak spark, misfiring, reduced engine performance, hard starting in damp conditions. Weak spark ✗ ⚠ Misfire E-GAP ADJUSTMENT: Set breaker points to open at the exact moment of maximum magnetic flux. Use a timing light or degree wheel. Typical gap: 0.003–0.005 in. (AC 43.13-1B §1-120).

Breaker points are mechanical switches that control the timing of spark generation by interrupting the primary circuit current. When the points close, current flows through the primary winding, building a magnetic field. When the points open at the precise moment of maximum magnetic flux (the E-gap), the collapsing field induces a high voltage in the secondary winding.

The E-gap is the critical angular position of the magneto rotor where the magnetic flux is at its maximum rate of change. Proper E-gap adjustment ensures that the breaker points open at exactly the right moment to produce maximum spark energy. Incorrect E-gap results in weak sparks, misfiring, and reduced engine performance.

Maintenance requirements for breaker points:

  • Inspection for pitting, burning, or wear at every 100-hour inspection
  • Replacement when worn beyond manufacturer's limits
  • Cleaning alone is not acceptable for pitted points
  • E-gap must be reset whenever points are replaced
  • Internal timing must be verified after any breaker point service

Magneto Timing

Internal vs. External Magneto Timing - FAA A&P Prep Internal vs. External Magneto Timing INTERNAL TIMING Breaker points vs. rotor position (E-gap) E-gap Pts Set during assembly/overhaul Fixed relationship — no adjustment in service unless overhauled EXTERNAL TIMING Magneto firing vs. piston position (°BTDC) TDC BDC 25° Set when installing magneto on engine Expressed in degrees BTDC Typical: 20°–25° BTDC @ idle Cylinder TDC BDC 25° BTDC Compression Power TOO ADVANCED Detonation Overheating CORRECT Peak pressure at ~15° ATDC TOO RETARDED Power loss Engine damage ⚠ Risk of catastrophic failure Piston motion animated — spark fires at 25° BTDC AC 43.13-1B — Ignition timing verification required E-gap: rotor position where breaker points just open — maximum magnetic flux change

Magneto timing refers to the relationship between the magneto's spark delivery and the engine's piston position. Two distinct timing operations exist:

Internal timing: The relationship between the breaker points opening and the magneto rotor position (E-gap). This is set during magneto assembly or overhaul.

External timing: The relationship between the magneto's firing point and the engine's piston position, expressed in degrees before top dead center (BTDC). This is set when installing the magneto on the engine.

The manufacturer's maintenance manual or Type Certificate Data Sheet specifies the exact timing range. Operating outside this range can cause:

  • Detonation (pre-ignition)
  • Engine overheating
  • Loss of power
  • Increased fuel consumption
  • Engine damage

Timing procedure essentials:

  1. Set the engine to the specified degrees BTDC on the compression stroke
  2. Install the magneto with the rotor aligned to the timing mark
  3. Verify timing using the timing light method or timing meter
  4. Check both magnetos independently
  5. Record the timing values in the maintenance records

P-Leads and Ignition Shutdown

P-Lead Grounding and Ignition Shutdown - FAA A&P P-Lead Grounding & Ignition Shutdown NORMAL: Switch OFF → Engine Shutdown IGNITION OFF P-lead (shielded) GND MAGNETO Primary CKT SPARK PLUG NO SPARK ENGINE STOPPED ✓ FAULT: Damaged P-lead Shield IGNITION OFF BREAK MAGNETO HOT! SPARK FIRING! ENGINE ROUGH RUNNING MAINTENANCE RULE — AC 43.13-1B "P-lead damage requires replacement, not temporary repair." A damaged P-lead shield can leave a magneto hot — a serious safety hazard. P-LEAD CONDITION COMPARISON Condition Switch OFF Result Engine State Required Action Healthy P-lead Grounds primary ✓ Engine stops ✓ No action Damaged P-lead Intermittent ground Runs rough / hot REPLACE P-LEAD 14 CFR §23.1367 / AC 43.13-1B — Ignition system maintenance and P-lead integrity requirements

The P-lead is a shielded wire that connects the magneto's primary circuit to the ignition switch. When the ignition switch is in the "OFF" position, the P-lead grounds the primary circuit, preventing spark generation and stopping the engine.

Critical safety considerations:

  • A broken or damaged P-lead shield can cause intermittent grounding
  • Intermittent grounding may cause engine roughness or failure to shut down
  • A faulty P-lead can leave a magneto "hot" even with the ignition switch off
  • Always verify magneto grounding before rotating the propeller
  • P-lead damage requires replacement, not temporary repair

The shielding on P-leads is essential for electromagnetic interference suppression and proper grounding. Tape repairs do not restore shielding integrity and are not acceptable maintenance practices.

Ignition Harness and Spark Plugs

The ignition harness distributes high-voltage current from the magneto distributor to each spark plug. Harness components include:

  • Shielded leads with conductive cores
  • Terminal connectors at both magneto and spark plug ends
  • Insulation and shielding layers
  • Support clamps and routing hardware

Harness inspection criteria:

  • Chafing or wear on insulation
  • Loose or corroded terminals
  • Broken shielding
  • Improper routing near heat sources or moving parts
  • Missing or damaged support clamps
  • Signs of arcing or corona discharge

Spark plug installation requirements:

  • Use manufacturer-specified anti-seize compound on threads
  • Torque to the exact value in the engine maintenance manual
  • Proper gap setting per manufacturer specifications
  • Correct plug type and heat range for the engine
  • Never use automotive installation practices

Incorrect torque can damage the plug or cylinder head threads, while improper gap affects spark energy and engine performance.

Turbine Engine Ignition Systems

Exciter Units

Exciter Unit High-Energy Ignition System Exciter Unit High-Energy Ignition System Aircraft DC Bus 28 VDC Input (Nominal) 28 VDC EXCITER UNIT High-Energy Ignition Capacitor Discharge Type CAP SCR XFMR 15-25 kV IGNITER PLUG Shunted Gap Type COMBUSTION CHAMBER Turbine Engine Ignition Zone TROUBLESHOOTING PROCEDURE — FAA AC 43.13-1B / A&P ACS Standards 1 Check Input Voltage Verify 28 VDC at exciter input terminals with multimeter (AC 43.13-1B) V 2 Inspect Igniter Plug Check gap per manufacturer specs (typically 0.040- 0.060 in). Inspect for fouling, erosion, cracks GAP 3 Inspect Ignition Leads Check for chafing, corrosion, loose connections, and proper shielding All checks OK? → Replace exciter unit per manufacturer specifications (AC 43.13-1B, FAA A&P ACS) ⚠ WARNING: High-voltage ignition systems can deliver lethal shocks. Always de-energize and discharge system before servicing. Follow 14 CFR Part 43 and manufacturer maintenance manual procedures. CHARGING... 2-20 J/spark High-Energy Ignition System — Exciter Unit converts 28 VDC to 15,000-25,000 V pulses (2-20 J) for igniter plugs

Turbine engines use high-energy ignition systems to ignite the fuel-air mixture in the combustion chamber. The exciter unit converts low-voltage DC aircraft power (typically 28 VDC) into high-energy electrical pulses delivered to the igniter plugs.

Exciter characteristics:

  • Output voltages typically range from 15,000 to 25,000 volts
  • Energy output measured in joules (typically 2-20 joules per spark)
  • Solid-state circuitry in modern systems
  • Hermetically sealed for environmental protection

Troubleshooting considerations:

  • Low exciter output may indicate internal failure
  • Check input voltage before condemning the exciter
  • Verify igniter plug gap and condition
  • Inspect leads for damage or poor connections
  • Follow manufacturer's troubleshooting procedures

Igniter Plugs

Igniter plugs are similar in function to spark plugs but designed for the extreme conditions of turbine engine combustion chambers. They must withstand:

  • Extremely high temperatures
  • High combustion pressures
  • Continuous vibration
  • Thermal shock

Maintenance requirements:

  • Regular inspection for electrode erosion
  • Gap checking and adjustment per manufacturer specifications
  • Cleaning to remove carbon deposits
  • Replacement at specified intervals
  • Proper torque during installation

Safety Procedures and Regulations

Regulatory Requirements

14 CFR 65.81 and 65.83 govern the privileges and responsibilities of certificated mechanics. A certificated mechanic may supervise and be responsible for maintenance performed by others, but must personally observe the work and determine it is satisfactory before signing a maintenance release.

14 CFR 43.15 requires that all maintenance be performed using methods, techniques, and practices prescribed in the manufacturer's maintenance manuals or Instructions for Continued Airworthiness. This regulation also mandates that safety precautions be observed during all maintenance operations.

14 CFR 91.7 requires that no person may operate an aircraft that is not in an airworthy condition. Any discrepancy found during inspection must be corrected before the aircraft is returned to service.

14 CFR 65.85 authorizes certificated mechanics to perform maintenance within the limitations of their rating. Replacing magneto points is a standard maintenance practice within the privileges of an A&P mechanic.

Safety Wiring Requirements

Safety wiring is a critical locking method used to prevent fasteners from loosening due to vibration. This is especially important on engine components such as magnetos, where fastener failure could have catastrophic consequences.

Approved safety wiring practices (per AC 43.13-1B Chapter 7):

  • Use only approved materials (typically stainless steel wire)
  • Install with proper tension and direction
  • Ensure the wire pulls the fastener in the tightening direction
  • Route to prevent interference with adjacent components
  • Replace any missing or damaged safety wire before return to service

Torque alone is not sufficient to prevent fastener loosening under vibration. Thread-locking compounds are not approved substitutes for safety wire on critical engine components.

Fire Safety Considerations

Ignition system wiring routed near fuel lines presents a serious fire hazard. Chafed wiring can create sparks that could ignite fuel vapors. Maintenance actions must include:

  • Repairing damaged wiring using approved methods
  • Rerouting wiring to maintain adequate separation from fuel lines
  • Correcting the cause of chafing (missing clamps, improper routing)
  • Thorough inspection of the affected area for hidden damage

Troubleshooting Procedures

No Spark at Spark Plug

When a magneto produces spark at the distributor block but not at the spark plug, the problem is isolated to the components between the distributor and the plug:

  • Faulty spark plug
  • Broken or shorted ignition lead
  • Poor connection at either end
  • Damaged terminal or connector

The magneto itself is functioning correctly in this scenario. Troubleshooting should focus on the harness and plug.

Excessive RPM Drop During Magneto Check

The magneto check is a standard run-up procedure that verifies ignition system health. FAA standards specify:

  • Maximum allowable RPM drop: 10% of rated RPM
  • Maximum difference between magnetos: 5% of rated RPM

An excessive drop on one magneto indicates:

  • Faulty spark plugs
  • Damaged ignition leads
  • Internal magneto problems
  • Incorrect timing

Rough running on one magneto confirms a malfunction that must be corrected before return to service.

Intermittent Grounding Issues

Intermittent grounding can cause:

  • Engine roughness
  • Uncommanded shutdown
  • Failure to shut down when the ignition switch is off

Troubleshooting steps:

  1. Inspect P-leads for damage or broken shielding
  2. Check all ground connections
  3. Verify ignition switch operation
  4. Test magneto grounding circuits
  5. Replace damaged components per manufacturer instructions

Inspection Procedures

100-Hour Inspection Requirements

During a 100-hour inspection, the ignition system requires thorough examination:

Magneto inspection:

  • Breaker point condition and wear
  • E-gap adjustment
  • Internal timing verification
  • Safety wire condition on mounting bolts
  • P-lead condition and security
  • Distributor block condition
  • Rotor and contact points

Harness inspection:

  • Insulation condition
  • Chafing or wear
  • Terminal security
  • Routing and support
  • Shielding integrity

Spark plug inspection:

  • Electrode condition
  • Gap setting
  • Insulator condition
  • Thread condition
  • Proper torque

Documentation Requirements

All maintenance must be properly documented in the aircraft maintenance records. Logbook entries must include:

  • Description of work performed
  • Parts replaced (with part numbers)
  • Inspection findings
  • Compliance with manufacturer's instructions
  • Signature and certificate number of the responsible mechanic

Summary of Key Relationships

  • Breaker points and E-gap: Worn points require replacement and E-gap reset to maintain proper ignition timing
  • Magneto timing and engine performance: Incorrect timing causes detonation, overheating, and power loss
  • P-lead integrity and safety: Damaged P-leads can leave magnetos hot, creating serious safety hazards
  • Safety wiring and vibration: Critical fasteners require safety wire to prevent loosening under vibration
  • Exciter input and output: Low output may indicate low input voltage or exciter failure
  • Harness condition and spark delivery: Damage between distributor and plug prevents spark delivery regardless of magneto condition

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