Why Do Jet Engines Have Two Igniter Plugs? Understanding Aircraft Engine Ignition Systems
Introduction
One of the most common questions asked by aviation enthusiasts is whether jet engines have spark plugs like automobile engines. The answer is yes, but they are known as igniter plugs rather than spark plugs.
Although they perform a similar function—producing a high-energy spark to ignite a fuel-air mixture—their operation is very different from that of automotive spark plugs. Unlike a car engine, where spark plugs fire continuously during every power stroke, a jet engine normally uses its igniter plugs only during engine start and under specific operating conditions.
This article explains why modern jet engines typically use two igniter plugs, how they work, and when they are required.
What Is an Igniter Plug?
An igniter plug is a high-energy electrical device installed in the combustion chamber of a gas turbine engine. Its purpose is to generate a powerful spark capable of igniting the fuel-air mixture during engine start.
Modern commercial turbofan engines usually have two igniter plugs, positioned at different locations around the combustor to ensure reliable ignition.
Unlike automotive spark plugs, aircraft igniters are designed to operate in an extremely demanding environment with:
High temperatures
High pressures
Intense vibration
Continuous airflow
Fuel spray from multiple fuel nozzles
Why Are Two Igniter Plugs Used?
Most modern turbofan engines are equipped with two independent igniter plugs.
Using two igniters provides several important advantages.
1. Improved Starting Reliability
During engine start, fuel must ignite quickly and evenly inside the combustion chamber. Two sparks increase the probability of successful ignition, especially in adverse weather or at high-altitude airports.
2. System Redundancy
Aircraft systems are designed with redundancy wherever practical.
If one igniter becomes unserviceable, the second can often still provide sufficient ignition for engine starting or in-flight relight, depending on the engine and operating procedures.
3. Faster Flame Propagation
The combustion chamber contains multiple fuel nozzles arranged around its circumference.
Two ignition points help the flame spread more rapidly through the combustor, producing a smoother and more reliable engine start.
How Does a Jet Engine Start?
A simplified engine start sequence is as follows:
Step 1 – Engine Rotation
An air turbine starter or electric starter rotates the engine compressor to the required starting speed.
Step 2 – Airflow
As the compressor turns, it supplies compressed air to the combustion chamber.
Step 3 – Ignition
The ignition system energises one or both igniter plugs, producing high-energy sparks.
Step 4 – Fuel Introduction
Fuel is sprayed into the combustion chamber through the fuel nozzles.
Step 5 – Combustion
The igniter sparks ignite the fuel-air mixture, establishing a stable flame.
Step 6 – Self-Sustaining Operation
Once combustion becomes stable, the turbine produces enough power to keep the compressor rotating without assistance from the starter.
The engine is now self-sustaining.
Do Igniter Plugs Operate Continuously?
No.
Unlike the spark plugs in a petrol engine, jet engine igniter plugs are not normally energised throughout the flight.
Once combustion is established, the flame is continuously sustained by:
Constant airflow from the compressor
Continuous fuel supply
High combustion temperatures
The combustion process becomes self-sustaining, so repeated spark generation is unnecessary.
When Is Continuous Ignition Used?
Although igniters are usually switched off after engine start, pilots may select continuous ignition in certain situations.
Typical examples include:
Heavy rain
Severe turbulence
Icing conditions
Volcanic ash avoidance procedures
Windshear
Engine anti-ice operation (on some aircraft)
During take-off and landing on some aircraft types
As required by aircraft operating procedures
Continuous ignition helps prevent flameout if combustion is temporarily disturbed.
What Happens During an In-Flight Flameout?
A flameout occurs when combustion inside the engine is extinguished while the engine is still rotating.
Possible causes include:
Heavy precipitation
Severe icing
Fuel interruption
Compressor stall
Volcanic ash ingestion
Bird strike
Extremely disturbed airflow
If engine rotation is maintained, the ignition system can assist in relighting the engine once fuel flow is restored.
Components of the Ignition System
A typical aircraft engine ignition system consists of:
Ignition Exciter
The exciter converts aircraft electrical power into high-voltage pulses.
Igniter Leads
Shielded high-voltage cables carry electrical energy from the exciter to the igniter plugs.
Igniter Plugs
The igniter plugs produce the high-energy spark required for combustion.
Engine Electronic Control
Modern FADEC systems control ignition sequencing during engine start and relight, ensuring the ignition system operates only when required.
Types of Igniter Plugs
Most turbine engines use one of two igniter designs:
High-Tension Igniters
Produce a high-voltage spark across a small gap.
Similar in principle to automotive spark plugs but much more robust.
Low-Tension (Surface Discharge) Igniters
Generate a powerful surface-discharge spark.
Better suited to the high-pressure, high-temperature environment inside modern gas turbine combustors.
Commonly used in contemporary commercial turbofan engines.
Igniter Plug Maintenance
Although igniter plugs operate only intermittently, they are inspected regularly during scheduled maintenance.
Maintenance activities include:
Visual inspection
Cleaning carbon deposits
Measuring electrode wear
Checking insulation condition
Verifying spark quality
Replacing worn igniters as required
Reliable ignition is essential for safe engine starting and relight capability.
Jet Engine Igniters vs. Automotive Spark Plugs
| Feature | Jet Engine Igniter | Automotive Spark Plug |
|---|---|---|
| Purpose | Initiates combustion during start and relight | Ignites the air-fuel mixture every engine cycle |
| Number | Usually two per engine | One or more per cylinder |
| Operating Time | Intermittent or continuous only when selected | Continuous during engine operation |
| Spark Energy | Very high | Lower |
| Operating Environment | High pressure, high temperature, turbine combustor | Lower pressure, piston engine cylinder |
| Controlled By | FADEC or ignition control system | Engine ignition system |
Conclusion
Modern high-bypass turbofan engines generally use two igniter plugs to provide reliable engine starting and in-flight relight capability. These igniters generate powerful electrical sparks that ignite the fuel-air mixture during engine start, after which combustion becomes self-sustaining.
Unlike automotive spark plugs, aircraft igniters do not fire continuously during normal operation. They are activated only when needed, such as during engine start, continuous ignition in adverse weather, or an in-flight relight following a flameout. Their robust design, high-energy output, and built-in redundancy contribute significantly to the safety and reliability of modern gas turbine engines.
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