Introduction
Igniting a turbine engine is a fundamentally different challenge from firing a piston engine. Rather than igniting a nearly homogeneous air-fuel mixture in a sealed cylinder, a turbine combustor must light off a continuous spray of atomized jet fuel in a large, open annular or can-type combustion chamber — often at altitude where air is thin and temperatures are extreme. The capacitor discharge ignition (CDI) system was engineered specifically to meet this challenge. By storing electrical energy in a capacitor and then dumping it almost instantaneously into an igniter plug, the CDI system produces a spark of enormous intensity and duration — far exceeding what a conventional induction coil system can achieve. Every certificated aviation maintenance technician working on turbine powerplants must understand how CDI systems work, why they are designed the way they are, and how to maintain them safely.
How a CDI System Works
The fundamental operating principle of a capacitor discharge ignition system is energy storage and rapid release. Incoming aircraft electrical power — typically 28 V DC or 115 V AC — is first converted and stepped up by a transformer or DC-to-DC converter within the ignition exciter unit. This raises the voltage to a level high enough to charge a storage capacitor to several hundred or even several thousand volts, depending on the system design. Because a capacitor stores energy electrostatically (as charge separated across a dielectric), it can accumulate energy relatively slowly from the aircraft bus and then release that energy almost instantaneously when a discharge circuit is triggered.
Once the capacitor reaches its designed voltage threshold, a triggering device — historically a triggered spark gap, but in modern designs often a solid-state switching element — fires and connects the capacitor directly to the igniter plug through a high-tension lead. The result is a very brief but extremely energetic electrical discharge. Typical CDI systems used on turbine engines deliver spark energies in the range of 1 to 4 joules per discharge (some high-energy systems used in specific applications can be higher), compared to fractions of a joule for conventional reciprocating-engine ignition. This intense spark is capable of vaporizing and igniting fuel that a weaker spark simply could not light.
The Ignition Exciter
The ignition exciter unit is the heart of the CDI system. It is a self-contained box that accepts low-voltage aircraft power, houses the step-up circuitry, the storage capacitor(s), the triggering device, and the output terminals. Many turbine engines use two exciter units — one per igniter — creating a fully redundant system, though this configuration is not universal; some smaller turbine engines and APUs use a single exciter feeding a single igniter. Exciters are rated by their output energy per spark and their spark repetition rate. A typical unit fires approximately 1 spark per second during the ignition cycle, with some designs firing slightly faster or slower. This relatively low repetition rate is acceptable because turbine ignition is only needed for engine start and certain in-flight relighting conditions; once combustion is established, the ignition system is normally switched off to extend igniter life.
High-Tension Ignition Leads
The high-voltage pulse traveling from the exciter to the igniter plug requires a specially constructed ignition lead. These cables use heavily insulated, low-capacitance coaxial construction to handle peak voltages that can exceed 20,000 volts without arcing to the surrounding structure. The outer shielding also suppresses the intense electromagnetic interference (EMI) generated by each discharge, protecting avionics and other aircraft systems. Maintenance technicians must inspect ignition leads for cracking, chafing, moisture ingress, and secure attachment; a compromised lead can misfire, arc externally, or create EMI that disrupts navigation and communication equipment.
Turbine Igniter Plugs
Turbine igniter plugs are not the fine-wire spark plugs used in reciprocating engines. Instead, most turbine engines use surface discharge igniters (sometimes called annular gap or surface gap igniters). In a surface discharge igniter, the spark travels across a semiconductor material bridging the center electrode and the ground electrode, rather than jumping across an open air gap. The semiconductor material allows the capacitor's stored charge to ionize a path along the surface, producing a large-diameter, intense plasma discharge that sweeps across the face of the plug. This design is extremely effective at igniting fuel-air mixtures that are not perfectly homogeneous and is highly resistant to fouling by carbon deposits — a critical advantage in a combustion environment.
Because of the enormous energy delivered per spark, turbine igniters erode relatively quickly. Manufacturers specify service life in terms of the number of starts or total ignition-on time, and technicians must track and replace igniters accordingly. Unlike piston-engine plugs, turbine igniters are not routinely cleaned and regapped; they are inspected visually for cracking, erosion, and carbon bridging and replaced at the manufacturer's prescribed interval or when a fault is found.
Why CDI Systems Are Used in Turbine Engines
The choice of CDI technology for turbine engines is driven by the demands of the operating environment. At high altitude, air density drops dramatically, and the reduced dielectric strength of the thinner air makes it much harder for any spark to jump a gap reliably. The high-energy plasma produced by a CDI system is large enough and hot enough to initiate combustion even in marginal conditions. During windmill relights — where the crew attempts to restart a flamed-out engine in flight with the compressor spinning in the airstream — the system must reliably ignite a relatively cold, poorly atomized fuel spray. The stored-energy approach of CDI provides the extra margin needed in these challenging scenarios.
Furthermore, the CDI system's ability to fire from aircraft battery power alone (without a running engine) is essential for initial ground starts. The system draws power, charges its capacitor, and fires without relying on an engine-driven magneto as piston engines do. This architecture also means that turbine ignition systems require rigorous safety protocols during maintenance: a fully charged exciter capacitor stores enough energy to cause a serious or fatal electrical shock, and the system must be properly de-energized and discharged before any work on igniters or leads begins.
Key Numbers and Rules
- Spark energy: CDI systems typically deliver about 1–4 joules per spark; some high-energy applications may exceed this depending on design.
- Spark repetition rate: Most exciters fire at approximately 1 spark per second during the start cycle, with some variation by design.
- Peak voltage: Ignition lead insulation must withstand pulse voltages commonly exceeding 20,000 V.
- Igniter service life: Tracked by number of starts or hours of ignition-on time per the manufacturer's Aircraft Maintenance Manual (AMM); there is no standard universal interval.
- Redundancy: Many certificated turbine engines use two independent ignition circuits (two exciters, two igniter plugs) for reliability, though some smaller turbine engines and APUs use a single exciter/igniter configuration.
- Safety discharge requirement: Technicians must allow the exciter capacitor to discharge — following the specific wait time and procedure stated in the manufacturer's AMM for that unit, since discharge times vary by design — or use a proper shorting device before touching igniters or leads.
- Regulatory basis: 14 CFR Part 33, Subpart E contains the airworthiness standards applicable to turbine engine ignition and starting systems, including requirements addressed through engine type certification testing; technicians should consult the specific applicable sections rather than a single generalized clause.
Common Test Traps
- Confusing energy with voltage: A CDI system's effectiveness is primarily measured in joules of stored energy, not simply peak voltage. High voltage is a means to an end — driving current through the plug — but energy (joules) is the figure that determines ignition capability.
- Assuming igniters work like spark plugs: Surface discharge igniters do not have a conventional open air gap and are not serviced by cleaning and regapping. They are replaced based on erosion, condition, and manufacturer-specified life limits.
- Leaving ignition on during normal operation: Unlike magneto-fired piston engines, turbine ignition systems are switched off after a successful light-off. Continuous use rapidly erodes the igniter electrodes. Ignition is only active during start sequences and specific in-flight conditions (icing, heavy precipitation, certain approach conditions per airline SOPs).
- Overlooking capacitor discharge hazard: The stored charge in an exciter capacitor is lethal. Failing to follow the manufacturer-specified wait time or approved discharge procedure before handling igniters or leads is an extremely dangerous error — and a common written-exam topic.
- Treating both igniter plugs as a single-point system: Many turbine CDI installations are dual-channel; each igniter has its own exciter. If one exciter fails, the remaining channel should still support a start. Understanding this redundancy — and knowing that both channels are tested individually during maintenance inspections — is frequently tested on the AMT Powerplant knowledge exam.
Practical Maintenance Considerations
When performing scheduled maintenance on a CDI ignition system, technicians work from the engine manufacturer's AMM and any applicable airworthiness directives (ADs). Typical tasks include inspecting ignition leads for insulation condition and secure attachment at both the exciter and igniter ends, checking igniter plugs for electrode erosion and ceramic cracking, verifying exciter output with approved test equipment, and confirming that the ignition system fires within the time limits specified for the start sequence. Any evidence of external arcing — scorch marks on the lead or the surrounding nacelle structure — must be investigated immediately, as external arcing can cause fires or structural damage. Technicians should also verify that igniter reach (the distance the igniter tip protrudes into the combustion liner) is within limits, because incorrect reach changes the spark's position relative to the fuel spray pattern and can impair ignition reliability.