CHAPTER 10: IGNITION SYSTEMS - Key Concepts

Magneto Ignition

  • spark-ignition engines require delivering an electrical current to each spark plug at the correct time to ignite the fuel-air mixture.
  • Piston-engine aircraft usually use a magneto ignition because it is independent of the aircraft’s electrical system, ensuring it will continue to operate if the aircraft’s electrical system fails.
  • Aircraft with compression-ignition engines (diesel) do not require an ignition system or spark plugs.

Magneto Ignition: Components and Operation

  • Key components include:
    • magneto
    • distributor
    • high-tension leads
    • spark plugs
    • ignition switch
  • How a magneto works:
    • The magneto is a self-contained unit driven by the engine and generates electric current via electromagnetic induction.
    • Inside the magneto, a magnet creates a magnetic field. A conductor moved across this field induces a high-voltage current.
    • The current is first delivered to a distributor, which routes it to the appropriate spark plug.
    • The distributor also times the spark so it occurs just before the power (compression) stroke.

Case Study: Incorrect Spark Timing

  • Date: 8 August 2008
  • Aircraft: single-engine Cessna 207 in Canada after scheduled maintenance.
  • Issue: magnetos timed incorrectly after refitting.
  • Correct timing: ignitions should occur at 22^\u00b0 before top-dead-center (before TDC).
  • Actual timing after refit: advanced to approximately 50^\u00b0 to 60^\u00b0 before TDC.
  • Consequences: pre-ignition and detonation in cylinders, leading to very high cylinder temperatures and power loss.

High-Tension Leads and Spark Plugs

  • High-tension leads must be checked during preflight; damaged leads may allow high voltage to escape to other parts of the aircraft, creating a serious hazard and risking ignition system failure.
  • Spark plug location: located in the cylinder head at the top of the cylinder.
  • Spark generation: spark occurs when current jumps across the gap between the two electrodes at the plug tip, igniting the fuel-air mixture.
  • Figure 10.1 reference: Basic components of a magneto ignition system.

Dual Ignition and Redundancy

  • Most aircraft have a dual ignition system: two completely independent systems.
  • Each cylinder has two spark plugs, each fed by a different magneto (each with its own magneto and high-tension leads).
  • Benefits:
    • Improved combustion and power by having two simultaneous ignition events.
    • If one system fails, the other keeps the engine running, albeit with some power reduction.
  • Figure 10.1 reference shows the dual ignition arrangement.

Ignition Switch and Operational Checks

  • Ignition switch (Figure 10.2) commonly has five positions: OFF, R (right), L (left), BOTH, START.
    • LEFT: only the left magneto operates.
    • RIGHT: only the right magneto operates.
    • BOTH: both magnetos operate (normal during flight).
    • START: used to start the engine.
  • Preflight and in-flight checks:
    • A common check is to verify each magneto system operates.
    • When switching from BOTH to RIGHT or LEFT, a small rpm drop is expected; the amount is defined in the Pilot Operating Handbook (POH).
    • If the engine stops when one magneto is selected, or there is a large rpm drop, a fault exists and the aircraft should not be flown.
    • No rpm drop at all when switching to a magneto indicates the magneto is still live (on) in the OFF position, which is dangerous if the propeller is moved (risk of engine starting unexpectedly).
    • Example danger: if, when switching to RIGHT, there is no rpm drop, the left magneto may still be live.

Starting the Engine: Starter and Impulse Coupling

  • The magneto system is self-driven when the engine runs, but on start-up a starter system provides initial rotation.
  • Starter system: direct-cranking electric starter turns the engine using electrical power from the battery or an external power unit.
  • Starter operation:
    • Engine turns relatively slowly when starting: about 120\,\text{rpm}.
    • Normal idle speeds are approximately 800\text{–}1000\,\text{rpm}.
  • Impulse coupling:
    • A clever device that helps deliver a spark at the slow starter speeds and also helps time the spark correctly.
    • At low engine speeds, the impulse coupling holds back (retards) the magnet in the magneto and winds up an internal spring.
    • Just before the piston begins the power stroke, the spring releases the magnet, allowing the magneto to generate sufficient current for the spark plugs.
    • The impulse coupling operates during starting but not at normal engine speeds when the magneto operates normally.

Electronic Ignition Systems: Introduction and Benefits

  • Unlike magnetos, electronic ignition systems have fewer moving parts, leading to potentially lower maintenance.
  • Typical electronic ignition components include:
    • a coil (to generate high voltage current)
    • an ignition control unit (the brains of the system)
    • high-tension leads
    • sensors
  • Advantages:
    • Ability to vary spark timing and duration depending on flight conditions, leading to a cleaner burn, improved power output, and better fuel efficiency.
  • Limitations of magnetos historically: fixed timing and duration set on the ground, not adapting in flight conditions.

Types of Electronic Ignition and FADEC Integration

  • Basic electronic ignition systems may replace one or both magnetos.
  • Advanced systems may be integrated with FADEC (Full Authority Digital Engine Control).
  • FADEC characteristics:
    • Digital computer controls engine and propeller management.
    • Numerous sensors monitor engine operation and adjust spark timing, fuel-air mixture, priming, and injector timing.
    • Simplifies cockpit controls to a single lever (e.g., start, cruise power) while the FADEC handles the rest.
    • Benefits include reduced pilot workload, improved efficiency and potential fuel savings.
    • Downside: if FADEC fails, engine control is lost unless backup systems are available.
    • Backup provisions typically include a redundant FADEC and a backup electrical power supply.

Real-World Implications and Safety Case: FADEC Backups

  • 2007 German accident involving a Diamond DA42:
    • The aircraft employed digital engine controls (FADEC) and backup power considerations.
    • During preparation, the battery was found to be flat; the crew started both engines with a ground power unit (GCU).
    • POH procedure stated only one engine should be started with the GCU, with the second engine started using aircraft power (battery-generated).
    • Shortly after take-off, raising the undercarriage caused a very short-term voltage drop to the ECUs.
    • Because the battery was flat, there was insufficient electrical power to support both ECUs, causing them to go offline and both engines to stop.
    • The aircraft performed a successful forced landing in a field next to the runway.
    • Lesson: underscores the importance of backup electrical power and redundant systems to cover FADEC failures or other electrical outages.

Overall Significance and Operational Guidance

  • The ignition system remains highly reliable and has changed little over the years due to its critical role in keeping the engine running.
  • Pilots must stay vigilant for signs of malfunction and should never operate an aircraft with a faulty ignition system.
  • The evolution from magneto to electronic ignition and FADEC reflects a balance between reliability, efficiency, and system complexity; each has trade-offs in maintenance, redundancy, and failure modes.

Key Formulas and Numerical References

  • Spark timing reference in the timing case study:
    • Correct timing: 22^B0 before top-dead-center (before TDC)
    • Incorrect timing in the case study: approximately 50^B0 to 60^B0 before TDC
  • Starter and idle speeds:
    • Starter speed: 120\ ext{rpm}
    • Idle speed: 800\text{--}1000\ \text{rpm}
  • Additional notes:
    • The five-position ignition switch: OFF, R, L, BOTH, START (no numerical values to convert to LaTeX).

Connections to Fundamentals and Real-World Relevance

  • Ignition timing is a critical parameter linking chemistry (combustion), thermodynamics (pressure rise), and mechanical work (piston displacement).
  • Redundancy (dual magnetos) aligns with aviation safety principles: multiple independent systems reduce single-point failures.
  • Understanding FADEC and electronic ignition highlights the trade-off between mechanical simplicity and electrical/electronic system complexity and the need for robust backups.
  • Real-world incidents demonstrate how electrical power management (battery, GCU, ECUs) directly affects engine operability and safety during critical phases of flight.

Ethical, Philosophical, and Practical Implications

  • Dependence on electronic systems raises questions about mission-critical redundancy, failure modes, and human factors in maintaining vigilance against complacency.
  • The balance between reliability (magnetos) and efficiency (FADEC) reflects ongoing challenges in engineering for safety vs. performance.
  • The importance of strict adherence to procedures (e.g., starting procedures with GCU) underscores the ethical responsibility to follow manufacturer guidelines to minimize risk.