Turbine engines operate across an enormous range of conditions — from ground-level start-up on a hot desert afternoon to high-altitude relight attempts in near-freezing, thin air. Under all of these circumstances, the ignition system must deliver a spark that is energetic enough to reliably ignite a fuel-air mixture that may be lean, poorly atomized, or buffeted by turbulent airflow inside the combustion liner. Conventional magneto-based ignition systems, well-suited to reciprocating engines, simply cannot produce the raw spark energy needed. That is why virtually every certificated turbine engine uses a capacitor discharge ignition (CDI) system — a purpose-built design that stores electrical energy and dumps it all at once into the igniter plug, creating a spark that is orders of magnitude more powerful than anything a magneto can generate.
Understanding how capacitor discharge ignition systems work, why they are designed the way they are, and how they differ from reciprocating-engine ignition systems is essential knowledge for any Aviation Maintenance Technician (AMT) working on turbine powerplants. The FAA's Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32) addresses these systems thoroughly, and the topic appears regularly on the Powerplant knowledge test.
How the System Works
A turbine ignition system has three main sections: the exciter unit (the power supply and energy-storage assembly), the ignition leads (high-tension cables carrying the discharge to the plugs), and the igniter plugs (the turbine equivalent of spark plugs). Each element is engineered to handle the extraordinary voltages and currents involved.
The Exciter Unit
Aircraft electrical power — typically 28 volts DC or 115 volts AC — enters the exciter unit at relatively low energy. Inside the exciter, a DC-to-AC inverter or vibrator circuit converts the input into a pulsating current that can be stepped up by a transformer. The transformer raises the voltage to several thousand volts. That high voltage is then rectified back into DC and used to charge one or more capacitors (also called condensers) to a very high potential, typically in the range of 1,000 to 2,000 volts or more, depending on the system design.
Once the capacitor reaches its trigger voltage, a discharge gap or triggering device (often a spark gap or thyratron-type device) fires, allowing the capacitor to release all of its stored energy almost instantaneously through the ignition lead to the igniter plug. The rate at which the capacitor charges and discharges determines the spark repetition rate, which is typically between 1 and 2 sparks per second during normal ignition operation. Each individual spark lasts only a fraction of a millisecond, but the energy delivered during that brief pulse can exceed 1 to 4 joules per spark — dramatically higher than the few millijoules a typical magneto spark provides.
High-Energy vs. Low-Energy Systems
Turbine ignition systems are often categorized by their output energy level. High-energy systems deliver approximately 4 joules per spark and are used for main engine ignition — starting and in-flight relight. Low-energy systems, sometimes called continuous ignition systems, produce roughly 1 joule or less per spark and are used during conditions where flame-out is a risk: heavy rain, icing, turbulence, or flight near the engine's lean blowout limits. Many modern engines are equipped with both: a high-energy mode for starting and a low-energy continuous mode for precautionary operation.
Igniter Plugs
Igniter plugs used in turbine engines look nothing like reciprocating-engine spark plugs. They are typically of the annular gap or constrained gap design, built to handle very high-energy discharges repeatedly without eroding rapidly. Because the discharge is so energetic, the spark actually jumps across the surface of a semiconducting material rather than across an air gap alone — this is called a surface-discharge or shunted-gap design. The discharge erodes the igniter tip over time, so igniters have a defined service life measured in operating hours or number of starts, and must be inspected and replaced on schedule. Unlike spark plugs, igniters typically fire only during starting and specific abnormal conditions, so their service life is not as short as one might expect.
Why Capacitor Discharge Ignition Matters
The core reason turbine engines use capacitor discharge ignition is the need for reliable ignition under extreme conditions. At high altitude, atmospheric pressure is low, air density is reduced, and fuel atomization changes. Under these conditions, a weak spark would fail to ignite the mixture before it is swept away by the high-velocity airflow inside the combustor. The high-energy spark from a CDI system creates a large, intensely hot plasma kernel that overwhelms these adverse factors and reliably initiates combustion.
The system's design also reflects important safety considerations. Because the exciter charges a capacitor to lethal voltage levels, turbine ignition systems are classified as extremely hazardous — the stored charge can kill even after power is removed. FAA guidance and manufacturer maintenance manuals specify mandatory discharge procedures before any work is performed on ignition components. Technicians must wait a prescribed period (typically at least 5 minutes after power removal) and use approved shorting devices to bleed off residual charge. This is not optional; failure to do so has caused fatalities.
Key Numbers and Rules
- Operating voltage: Capacitors in turbine ignition exciters may be charged to 1,000–2,000 volts or higher — always treat the system as live unless positively discharged.
- Spark energy: High-energy systems deliver approximately 4 joules per spark; low-energy continuous ignition systems deliver approximately 1 joule or less.
- Spark repetition rate: Typically 1–2 sparks per second during ignition operation.
- Discharge wait time: Most manufacturers require a minimum of 5 minutes after power removal before opening ignition system components; always follow the specific aircraft maintenance manual (AMM).
- Igniter plugs: Have a finite service life (often listed as total starts or operating hours); must be replaced per the manufacturer's time limits.
- Number of igniters: Most turbine engines use two igniter plugs per combustor — typically positioned at opposing locations within the annular combustion chamber, or one per can in a can-type combustor, to ensure reliable light-around.
- Continuous ignition: Selected by the flight crew or automatically activated during conditions such as anti-icing operation, heavy precipitation, or approach and landing in turbine-powered aircraft where flame-out risk is elevated.
System Components and Maintenance Considerations
The ignition leads connecting the exciter to the igniter plugs carry very high voltage pulses and must be carefully maintained. They are shielded with metal braid to prevent electromagnetic interference (EMI) with avionics and other aircraft systems. Leads must be inspected for chafing, cracking of the insulation, and secure attachment at both ends. A damaged lead can arc internally, waste discharge energy, or create a fire hazard. Leads also have specified replacement intervals.
The exciter unit itself is usually a sealed, non-repairable line-replaceable unit (LRU). If internal testing reveals a fault, the entire exciter is replaced rather than repaired in the field. However, technicians must still inspect the unit's mounting, electrical connectors, and any bonding straps for corrosion or security.
During a normal engine start sequence, the ignition system is activated along with fuel flow, and the igniters fire continuously at their repetition rate until light-off is confirmed and the starter cuts out. The ignition system is then de-energized for normal operation because, unlike a reciprocating engine, a turbine's combustion is self-sustaining once established — it does not need a spark to maintain combustion.
Common Test Traps
- Confusing joules and volts: The FAA test may ask about spark energy (joules) versus exciter voltage (volts). Remember, high voltage is how the capacitor is charged, but spark energy in joules is what determines ignition effectiveness. Both are large numbers for turbine systems.
- Assuming continuous operation: Students familiar with magneto ignition may assume turbine igniters fire continuously in flight. They do not under normal conditions — ignition is needed only for starting and precautionary continuous-ignition situations.
- Discharge time requirement: A common test question involves the hazard of turbine ignition exciters. The correct answer always emphasizes that the capacitor retains a dangerous charge after power removal and must be allowed to discharge (or be manually discharged) before maintenance.
- Igniter vs. spark plug terminology: The test will use the term igniter plug for turbine engines. Calling them spark plugs is technically incorrect and can indicate knowledge confusion to the examiner.
- Two igniters per engine: Questions sometimes probe whether students know that most turbine engines use two igniter plugs (not one, not four) for reliable combustor light-around. Verify the specific engine in any given scenario, but two is the standard configuration for the written test.