Ignition Systems


  • The ignition system provides the spark to ignite the mixture in the cylinders
  • Most small aircraft use a direct-cranking electric starter system
  • The systems consists of:

  • Together, the system enables starting an aircraft's engine
  • Most standard certificated aircraft incorporate a dual ignition system with two individual magnetos, separate sets of wires, and two spark plugs per cylinder to increase reliability of the ignition system through redundancy
    • This is designed so if one magneto or spark plug fails, the other is unaffected and will continue to run normally, with a slight decrease in power
  • In a spark ignition engine the ignition system provides a spark that ignites the fuel/air mixture in the cylinders and is made up of magnetos, spark plugs, high-tension leads, and the ignition switch [Figure 1]


  • Pilot Handbook of Aeronautical Knowledge, Ignition System
    Pilot Handbook of Aeronautical Knowledge, Ignition System
  • Magnetos are self-contained engine-driven units to supply electrical current to the spark plugs
    • A permanent magnet is used to generate this current completely independent of the aircraft's electrical system
  • The magneto generates sufficiently high voltage to jump a spark across the spark plug gap in each cylinder
  • The system begins to fire when the starter is engaged and the crankshaft begins to turn
    • It continues to operate whenever (or however) the crankshaft is rotating
    • See more in the hand propping section
  • Each magneto operates independently to fire one of the two spark plugs in each cylinder
  • Pilot Handbook of Aeronautical Knowledge, Ignition System
    Pilot Handbook of Aeronautical Knowledge, Ignition System
  • Magneto Check:

    • Magneto checks test the ignition system's operation and are a critical element to ground checklists
    • When checking magnetos there a few conditions must be satisfied to ensure proper functionality:
      1. The magneto grounding wires are connected
        • If not connected, there will be no drop in RPM when that magneto is selected
      2. The drop in RPM falls within the recommended limits as defined in the Pilot Operating Handbook
        • A drop, but continued operation of the engine, ensures the aircraft can fly on a single magneto, albiet at reduced performance
        • If equipped, pilots should also monitor EGT for a rise on all cylinders
        • Note that when checking EGT, a drop in a single cylinder's EGT is one way to identify the malfunctioning magneto
      3. The differential drop between magnetos is within limits as defined in the Pilot Operating Handbook
        • Excessive differentials demonstrate a malfunction in one of the magnetos
      4. If the drop is greater than the recommended limits as defined in the Pilot Operating Handbook
        • Consider running the engine to a cruise/near-cruise RPM and leaning to peak EGT
        • Run lean of peak at high RPM for about 30 seconds
        • If magneto checks after a second or even third time, have the engine looked at by a mechanic
    • Although magneto checks are normally performed on the ground, they may also be performed in flight
    • Note that aging magnetos can manifest themselves and provide a warning through hot starts, before ever performing the magneto check
    • Magneto checks are usually performed before takeoff to:
      • Allow the aircraft to warm up
      • Check condition prior to takeoff (taxi may foul)
      • Avoid propwash concerns in the ramp area
  • Failed Magneto Checks:

    • If an anomoly in the magneto check occurs, the ignition system is not operating correctly
    • Anomolies are not meeting one of the previous mentioned conditions during the check, or something unexpected excessive rough engine idling during the check
    • A common culprit are the spark plugs
    • Another cause for rough idle could be poor ignition timing
      • Low EGTs and high CHTs are a likely corresponding indication

Electronic Ignition:

  • Electronic ignition systems are becoming more common, supplementing or all-out replacing magnetos
  • These systems maintain the same functionality, but change the interface from an ignition switch with magneto selection usually via a key, to push-start systems
  • These systems generally maintain aircraft performance as compared to those with magnetos, but reduce maintenance costs through the use of a more simple system

Spark Plugs:

  • Spark plugs provide the source of ignition
  • Each cylinder has two spark plugs which improves combustion of the fuel/air mixture, results in a slightly higher power output, and provides redundancy

Ignition Switch:

  • Pilot Handbook of Aeronautical Knowledge, Start Switch
    Pilot Handbook of Aeronautical
    Knowledge, Start Switch
  • The operation of the magneto is controlled in the flight deck by the ignition switch [Figure 3]
  • The switch has five positions:

    1. OFF
    2. R (right)
    3. L (left)
    4. BOTH
    5. START
  • With RIGHT or LEFT selected, only the associated magneto is activated while BOTH uses the two simultaneously
  • A malfunctioning ignition system can be identified during the pre-takeoff check by observing the decrease in rpm that occurs when the ignition switch is first moved from BOTH to RIGHT, and then from BOTH to LEFT
    • The permissible decrease is listed in the AFM or POH
    • If the engine stops running when switched to one magneto, the rpm drop exceeds the allowable limit, or no drop occurs, do not fly the aircraft until the problem is corrected
  • The cause could be fouled plugs, broken or shorted wires between the magneto and the plugs, or improperly timed firing of the plugs
  • Following engine shutdown, turn the ignition switch to the OFF position
  • Even with the ignition, battery, and master switches in the OFF position, if the ground wire between the magneto and the ignition switch becomes disconnected or broken, the engine could accidentally start if the propeller is moved with residual fuel in the cylinder because it requires no external power
    • If this occurs, the only way to stop the engine is to move the mixture lever to the idle cutoff position
  • The video below is an example of a magneto check
    Always follow the procedures and limits listed in the AFM/POH for your aircraft when performing this check
    (i.e., 125 RPM is this aircraft's maximum drop, yours may be different)
  • Pilot Handbook of Aeronautical Knowledge, Start Switch
    Pilot Handbook of Aeronautical
    Knowledge, Start Switch

Starter/Starter Motor:

Starting An Aircraft:

  • Most aircraft have starters that automatically engage and disengage when operated, but some older aircraft have starters that are mechanically engaged by a lever actuated by the pilot
  • The starter engages the aircraft flywheel, rotating the engine at a speed that allows the engine to start and maintain operation
  • Pilot Handbook of Aeronautical Knowledge, Typical Starting Circuit
    Pilot Handbook of Aeronautical Knowledge,
    Typical Starting Circuit
  • Pilot Handbook of Aeronautical Knowledge, Typical Starting Circuit
    Pilot Handbook of Aeronautical Knowledge,
    Typical Starting Circuit
  • Electrical power for starting is usually supplied by an on-board battery, but can also be supplied by external power through an external power receptacle
  • When the battery switch is turned on, electricity is supplied to the main power bus bar through the battery solenoid
  • Both the starter and the starter switch draw current from the main bus bar, but the starter will not operate until the starting solenoid is energized by the starter switch being turned to the "start" position
  • When the starter switch is released from the "start" position, the solenoid removes power from the starter motor
  • The starter motor is protected from being driven by the engine through a clutch in the starter drive that allows the engine to run faster than the starter motor [Figure 4]
  • When starting an engine, the rules of safety and courtesy should be strictly observed
    • Clear the area visually and make a call, "Clear prop!"
  • In addition, the wheels should be chocked and the brakes set, to avoid hazards caused by unintentional movement
  • When using a starter, don't hold the started for more than a few seconds or it can burn out due to excessive heat
  • To avoid damage to the propeller and property, the aircraft should be in an area where the propeller will not stir up gravel or dust


  • During normal combustion, the fuel/air mixture burns in a very controlled and predictable manner
  • In a spark ignition engine the process occurs in a fraction of a second
  • The mixture begins to burn at the point where it is ignited by the spark plugs, then burns away from the plugs until it is all consumed
  • This type of combustion causes a smooth build-up of temperature and pressure and ensures that the expanding gases deliver the maximum force to the piston at exactly the right time in the power stroke [Figure 5]

Abnormal Combustion:

  • Pilot Handbook of Aeronautical Knowledge, Spark Plug Anomalies
    Spark Plug Anomalies
  • Pilot Handbook of Aeronautical Knowledge, Normal vs. Explosive Combustion
    Pilot Handbook of Aeronautical Knowledge,
    Normal vs. Explosive Combustion
  • Abnormal combustion can result from poor fuel, bad mixture, or fauly ignition parts
  • Detonation:

    • Detonation is an uncontrolled, explosive ignition of the fuel/air mixture within the cylinder's combustion chamber
    • Caused by hot engine temperatures; or using a lower than recommended fuel grade
    • It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves
    • In less severe cases, detonation causes engine overheating, roughness, or loss of power
    • Characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings
    • Common Detonation Causes:

      • Use of a lower fuel grade than that specified by the aircraft manufacturer
      • Operation of the engine with extremely high manifold pressures in conjunction with low rpm
      • Operation of the engine at high power settings with an excessively lean mixture
      • Maintaining extended ground operations or steep climbs in which cylinder cooling is reduced
    • Detonation Avoidance:

      • Make sure the proper grade of fuel is used
      • Keep the cowl flaps (if available) in the full-open position while on the ground to provide the maximum airflow through the cowling
      • Use an enriched fuel mixture, as well as a shallower climb angle to increase cylinder cooling during takeoff and initial climb
      • Avoid extended, high power, steep climbs
      • Develop the habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer
  • Pre-Ignition:

    • Pre-ignition occurs when the fuel/air mixture ignites prior to the engine's normal ignition event
    • Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel/air charge
    • Pre-ignition causes the engine to lose power, and produces high operating temperature
    • As with detonation, pre-ignition may also cause severe engine damage, because the expanding gases exert excessive pressure on the piston while still on its compression stroke
  • Detonation and pre-ignition often occur simultaneously and one may cause the other
  • Since either condition causes high engine temperature accompanied by a decrease in engine performance, it is often difficult to distinguish between the two
  • Using the recommended grade of fuel and operating the engine within its proper temperature, pressure, and rpm ranges reduce the chance of detonation or pre-ignition
  • Pilot Handbook of Aeronautical Knowledge, Spark Plug Anomalies
    Spark Plug Anomalies
  • Pilot Handbook of Aeronautical Knowledge, Normal vs. Explosive Combustion
    Pilot Handbook of Aeronautical Knowledge,
    Normal vs. Explosive Combustion


  • As part of the shut down checklist, you'll want to check your primary leads (p-leads) to your magnetos:
    • This is accomplished by quickly moving the ignition key from BOTH to OFF (or L then R as per procedures) to ensure the engine begins to cut
    • If the engine does not cut, it means the magneto is not grounding out and is therefore "hot"
    • Such a condition can result in an aircraft starting with only the movement of the prop, regardless of a key in the ignition

Full Authority Digital Engine Control (FADEC):

  • FADEC is a system consisting of a digital computer and ancillary components that control an aircraft's engine and propeller
  • First used in turbine-powered aircraft, and referred to as full authority digital electronic control, these sophisticated control systems are increasingly being used in piston powered aircraft
  • In a spark ignition reciprocating engine the FADEC uses speed, temperature, and pressure sensors to monitor the status of each cylinder
  • A digital computer calculates the ideal pulse for each injector and adjusts ignition timing as necessary to achieve optimal performance
  • In a compression ignition engine the FADEC operates similarly and performs all of the same functions, excluding those specifically related to the spark ignition process
  • FADEC systems eliminate the need for magnetos, carburetor heat, mixture controls, and engine priming
    • A single throttle lever is characteristic of an aircraft equipped with a FADEC system
  • The pilot simply positions the throttle lever to a desired detent such as start, idle, cruise power, or max power, and the FADEC system adjusts the engine and propeller automatically for the mode selected
  • During aircraft starting, the FADEC primes the cylinders, adjusts the mixture, and positions the throttle based on engine temperature and ambient pressure
  • During cruise flight, the FADEC constantly monitors the engine and adjusts fuel flow, and ignition timing individually in each cylinder
    • This precise control of the combustion process often results in decreased fuel consumption and increased horsepower
  • There must be a backup electrical source available because failure of a FADEC system could result in a complete loss of engine thrust
  • To prevent loss of thrust, two separate and identical digital channels are incorporated for redundancy, each channel capable of providing all engine and propeller functions without limitations

Ignition Case Studies:


  • Pivotal to the ignition system's operation is a source of electricity
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