Unlike the small, frequent sparks produced by piston-engine magnetos, turbine engine ignition systems deliver relatively infrequent but enormously powerful electrical discharges. These discharges must reliably ignite a fuel-air mixture inside a combustion chamber that may be subjected to high-altitude cold-soaking, extreme airflow velocities, and widely varying fuel atomization conditions. The two core components that make this possible are the exciter unit — which stores and releases electrical energy — and the igniter plug — which converts that energy into a spark capable of initiating combustion. Understanding how these components work, why they are designed the way they are, and how technicians inspect and maintain them is essential knowledge for any Aviation Maintenance Technician (AMT) working on turbine powerplants.
Role of the Ignition System in Turbine Engines
Turbine engines require ignition only during specific phases: initial ground start, airborne restart (relight) after a flameout, and in some aircraft, continuous ignition during takeoff, heavy precipitation, or icing conditions where combustion could be disrupted. Once the engine is running normally at idle or above, the combustion process is self-sustaining — hot gases continuously ignite incoming fuel-air mixture without external assistance. This intermittent-use philosophy is the opposite of piston engines, where ignition fires on every power stroke throughout engine operation. Because turbine ignition is used sparingly, the components can be engineered to release very high energy without concern for the wear that continuous firing would cause.
The Exciter Unit: Storing and Releasing High Energy
The exciter unit is the electrical heart of the turbine ignition system. Its job is to take a relatively modest supply of aircraft electrical power — typically 24-28 volts DC or 115 volts AC, though the exact input varies by aircraft and exciter design — and convert it into the very high-voltage, high-energy pulses needed to fire the igniter plugs. Most modern exciter units are of the capacitor discharge type, sometimes called a capacitor-type exciter.
Inside the exciter, a transformer steps up the incoming voltage, and a rectifier converts AC to DC if necessary. This high-voltage DC is then used to charge one or more storage capacitors. When the capacitor reaches the designed trigger voltage, the accumulated charge is released almost instantaneously through a discharge circuit and delivered to the igniter plug. The energy released in this single pulse is typically in the range of approximately 1 to 20 joules, depending on whether the system is a low-energy continuous-duty type or a high-energy start/relight type — which is thousands of times greater than the energy in a piston-engine spark. The firing rate is relatively slow — generally on the order of 1 to 2 sparks per second, though exact rates vary by exciter design — because the capacitor requires time to recharge between discharges.
Some aircraft use a dual-exciter system with separate high-energy and low-energy circuits. The high-energy circuit fires during start and relight; the low-energy continuous-duty circuit fires during adverse conditions to guard against flameout. This distinction matters on the written exam: continuous-duty igniters use lower-energy pulses specifically to reduce igniter plug erosion over extended firing periods.
Igniter Plug Design and Construction
Turbine engine igniter plugs bear little resemblance to piston-engine spark plugs. The most common type used in turbine engines is the annular gap (or surface discharge) igniter. In this design, the center electrode and the outer shell are separated by a semiconductive material rather than an open air gap. When the capacitor discharges, current flows across the surface of this semiconductive material, ionizing a thin layer of gas and creating a large, spreading flashover arc rather than a narrow point-to-point spark. This design produces a physically larger spark footprint, making it far more effective at igniting the diffuse fuel-air mixture in a combustion liner under adverse conditions.
A second type is the constrained-gap igniter, sometimes called a recessed-gap igniter. Here, the spark jumps across a small gap that is partially shielded by the plug body, concentrating the discharge energy. Constrained-gap igniters tend to operate at even higher voltages and energies and are used in applications requiring especially reliable ignition.
Because the discharge energy is so high, igniter plugs experience significant erosion at the electrode tip with each firing. For this reason, turbine igniter electrodes are constructed from erosion-resistant materials, often noble metal alloys, which withstand the ablation caused by repeated high-energy discharges far better than the nickel alloys used in piston spark plugs. Even so, igniter plugs have a finite service life measured in total number of firings or hours, and they are inspected and replaced on a scheduled basis per the manufacturer's maintenance manual.
Physical Placement in the Combustion Section
Most turbine engines are commonly equipped with two igniter plugs as a typical design practice, one positioned in each of two combustion cans (in can-type combustors) or in two locations within the annular combustion chamber, though the exact number can vary by engine design. Because turbine combustion chambers use interconnect tubes (or flame tubes) between individual cans, ignition in two cans is sufficient to propagate flame throughout the entire combustor. The igniter plugs are positioned to protrude into the primary combustion zone where fuel-air ratio is near stoichiometric and ignition is most reliable.
Why It Matters: Safety and Airworthiness
A failed ignition system may not ground an aircraft that is already running, but it will prevent a successful start or a relight after a flameout. Flameout at altitude — particularly in adverse weather — represents a serious hazard. The FAA's guidance in the Aviation Maintenance Technician Powerplant Handbook (FAA-H-8083-32) emphasizes that ignition system components must be maintained in accordance with the manufacturer's approved data and that any deviation in spark energy, plug condition, or lead integrity can compromise engine starting reliability across the full envelope of operating conditions including high altitude and cold temperatures.
Another critical safety consideration is the stored charge hazard. Exciter capacitors can retain a lethal charge for a significant period after aircraft power is removed — sometimes minutes or longer depending on the design. Maintenance personnel must follow approved procedures to allow sufficient discharge time or use approved shorting procedures before handling ignition system components. This is a fundamentally different hazard than working on piston-engine magneto systems.
Inspection and Maintenance
Routine igniter plug inspection involves removing the plug and examining the electrode for erosion, cracking, and carbon deposits. Annular-gap igniters often show a characteristic pattern of ablation at the electrode face; excessive erosion beyond the manufacturer's limits requires replacement. Carbon fouling can occur if the engine runs at low power for extended periods, and severely fouled plugs may fail to fire or produce a weak, misdirected discharge.
Technicians also inspect ignition leads (the shielded high-voltage cables connecting the exciter to the igniter plug) for chafing, cracking of the outer shielding braid, and security of connectors. A leaking lead can cause radio frequency interference or, more critically, deliver insufficient energy to the plug. Lead resistance and insulation integrity are checked per manufacturer specifications.
Exciter units themselves are generally line-replaceable units (LRUs) and are tested as a system rather than repaired at line maintenance level. If system output is suspect, the unit is swapped. The output energy of the exciter can be verified using a special ignition system test set approved for the specific engine.
Key Numbers and Rules
- Exciter output energy: typically approximately 1–20 joules per discharge, depending on system design (high-energy vs. low-energy continuous duty).
- Firing rate: approximately 1–2 sparks per second for most capacitor-discharge systems, though exact rates vary by design.
- Number of igniters per engine: most turbine engines commonly use two igniter plugs as a typical design practice, though this can vary.
- Electrode materials: erosion-resistant materials, often noble metal alloys, for erosion resistance.
- Capacitor discharge hazard: lethal voltage may remain after power removal; always follow approved discharge procedures before touching components.
- Service life: igniter plugs have a finite life in total firings or hours; always check the manufacturer's maintenance manual for the applicable limits.
- Continuous ignition: lower-energy circuits used during takeoff, heavy precipitation, and icing to prevent flameout without accelerating igniter erosion.
Common Test Traps
- Confusing ignition duty cycles: Turbine ignition fires intermittently during start and relight only — not continuously during normal operation. Piston engines fire every power stroke. Mixing these up is a classic exam error.
- Energy level misconceptions: Students sometimes assume that because turbine igniters fire less frequently, they use less energy. In fact, each pulse is far more energetic than a piston spark — energy per pulse in joules, not millijoules.
- Surface discharge vs. constrained gap: Know that the annular (surface discharge) igniter fires across a semiconductive surface, not across an open air gap. The FAA exam may present both types and ask which is more common or how each produces its spark.
- Stored charge danger: Forgetting that turbine exciter capacitors retain dangerous voltages after power-off is a safety-critical error. Always allow the manufacturer's specified bleed-down time before handling ignition system components.
- Number of igniters vs. number of combustion cans: Even engines with many combustion cans typically use only two igniters; flame propagates to remaining cans through interconnect tubes. Assuming one igniter per can is incorrect.