Ignition and Electrical System Notes

Ignition System

  • Two purposes:
    • Start the engine
    • Keep the engine running
    • Achieved by igniting a fuel-air mixture via an electric spark.
  • Dual ignition system:
    • Efficient combustion.
    • Redundancy (back-up).

Components

  • Magneto.
  • Ignition harness.
  • Ignition switch.
  • Spark plugs.
  • Impulse coupling.

Magnetos

  • Generates high voltage current to the spark plugs.
  • Driven by the engine; separate from the electrical system.
  • Most light airplanes have two magnetos (dual ignition system).

Spark Plugs

  • Located within the cylinders.
  • Provide a timed spark to ignite the fuel-air mixture.
  • Each cylinder has two spark plugs.
  • High voltage from the magneto causes a spark to jump across the gap.
Fouling
  • Aviation Gasoline (AVGAS) contains lead.
  • Lead build-up can cause spark plugs to misfire.
  • Most likely with low engine RPM and rich mixture.
  • Check during magneto checks.

Ignition Lead Shielding

  • High voltage current generates radio waves.
  • Shielding prevents interference with radio communications.
  • Faulty shielding causes clicking sound in the headset, varying with engine RPM.

Ignition Switch

  • OFF: Both magnetos are deactivated/earthed.
  • R: Right magneto active; L is earthed.
  • L: Left magneto active; R is earthed.
  • BOTH: Both magnetos are active.
  • START: Engages starter motor.

Starter Motor

  • Battery powered.
  • Rotates the propeller to start the engine.

Impulse Coupling

  • Provides a hot, retarded spark for easier starting.
  • Usually only the left magneto has an impulse coupling.

Pre-Take Off Magneto Check

  • Check normal operation of each Magneto and switches.
  • Verify all spark plugs firing normally.
  • When switching from BOTH to either L or R, there should be an RPM drop.
  • Checking the 'grounding wire', also known as the P-lead
Possible Faults
  • RPM drop more than recommended, engine runs smoothly: Incorrect magneto timing.
  • RPM drop more than recommended, engine runs roughly: Potential spark plug fouling.
  • No RPM drop: Fault in wiring or switches; both magnetos still 'live'.
Ignition System Operation
  • BOTH to L or R: Should cause a drop in RPM.
  • OFF: Earths the primary windings; no spark. If engine continues, magneto is live.
Dead-Cut Check
  • Momentarily turn magneto switch OFF before engine shutdown.
  • Sudden power loss should be apparent.
  • Engine then shut down with mixture control to ICO.
  • If engine continues with switch OFF, system is not grounded.

Electrical System

General

  • Powers lights, radios, nav aids, instruments, starter motor, etc.
  • Ignition system is independent.
  • Light aircraft typically use DC (14 or 28 volts).
  • DC provided by battery, generator, or alternator.

Supplies to:

  • Avionics
  • Flap systems
  • Fuel boost pumps
  • Lights - internal and external
  • Stall warning
  • Retractable landing gear
  • Starter motor
  • Fuel gauges
  • Trim motors

Bus Bar

  • Common source of electrical energy.
  • Alternator connected to the bus bar.

Battery

  • Stores electricity (voltage).
  • Powers starter motor.
  • Provides backup power if the alternator fails.
  • Rated in ampere-hours (amps).
  • Emits flammable hydrogen gas; compartment must be vented.

Alternators

  • Driven by the engine.
  • Generates electricity while the engine is running.
  • Recharges the battery.

Master Switch

  • ALT: Connects alternator to bus bar.
  • BAT: Connects battery to the electrical system.
  • Normally switched ON together.
  • Never leave the aircraft with the battery switch ON

Monitoring Systems

Centre Zero Ammeter
  • Measures battery-charge current.
  • Right of centre indicates charge; left indicates discharge; centre is zero.
Left Hand Zero Ammeter or Load Type Ammeter
  • Measures total output of the alternator
  • Ammeter shows zero when battery is on and engine not running or engine running and alternator off
  • After engine starts, ammeter indicates alternator output
Voltmeters
  • Monitors system voltage.
  • 12V system uses a 12V battery with 14V alternator output.
  • 24V system uses a 24V battery with 28V alternator output.
  • Prior to start-up, voltmeter reads battery voltage; after start-up, it reads alternator output.

Fuses & Circuit Breakers

  • Protect equipment from electrical overload.
  • Fuses melt; circuit breakers trip to break the circuit.
  • Circuit breakers are rated according to the number of amps they will tolerate before breaking
  • Circuit breaker should only be reset once after waiting at least 90 seconds
  • Current in circuit measured in amps