One of the most critical engine management decisions an airline transport pilot makes is whether to arm continuous ignition. Unlike a piston engine that relies on a spark plug firing every other revolution, a turbine engine's combustion chamber is normally self-sustaining once lit. That self-sustaining nature, however, is exactly what makes it vulnerable: if the flame is disrupted by water ingestion, ice, or disturbed airflow, the engine can simply go out — a flameout — with little warning. Continuous ignition is the safeguard against that event.
This article explains the mechanics of turbine ignition, the specific conditions that demand its use, how the system works in practice, and the regulatory and operational considerations that every ATP candidate must understand.
How Turbine Ignition Works
A turbine engine uses a high-energy capacitor discharge ignition system. During the start sequence, an exciter box charges a capacitor to a very high voltage — typically in the range of 1 to 2 joules of stored energy — and then discharges it across the igniter plug tip. The resulting spark is far more powerful than a piston magneto spark; it must be, because it has to ignite a fuel-air mixture that is already partially atomized and flowing at high velocity through the combustion liner.
Once the engine reaches self-sustaining speed and combustion is stable, the ignition system is normally switched off. The flame in the combustion chamber is continuous; carefully shaped combustion liners create a recirculation zone near the fuel nozzles that anchors the flame and allows it to re-ignite any temporarily lean pockets of mixture on its own. Under normal, clean-air cruise conditions this recirculation zone is stable, and ignition is not needed.
The igniter plugs themselves are not analogous to spark plugs. They are annular glow plugs or surface-discharge igniters located at specific points in the combustion liner — usually only two per engine — positioned to light the primary combustion zone. Because the spark energy is so high, these plugs erode quickly if used continuously for extended periods, which is one reason manufacturers limit their duty cycle under normal operations.
When Continuous Ignition Is Required
The FAA Aviation Maintenance Technician Powerplant Handbook (FAA-H-8083-32B) identifies the key conditions under which continuous ignition should be selected. The underlying logic is the same in every case: any condition that can disrupt, dilute, or extinguish the combustion flame requires that the ignition system be armed and ready to immediately relight the engine. These conditions include:
- Flight through icing conditions: Ice crystal ingestion — particularly in high-altitude convective environments — can accumulate on compressor stages and fuel nozzles, then shed suddenly into the combustion chamber. The resulting momentary lean condition can extinguish the flame. Additionally, ice on fuel nozzles disrupts atomization, creating locally rich or lean zones that are vulnerable to blowout.
- Heavy rain or precipitation: Ingesting large volumes of water can dilute the fuel-air mixture in the combustion chamber below the lean blowout limit. This is especially relevant during low-altitude operations in heavy thunderstorm precipitation, but also applies to moderate or heavy rain at any altitude.
- Turbulence: Severe turbulence creates rapid fluctuations in airflow through the engine. These fluctuations can transiently move the fuel-air ratio outside the flammability limits of the combustion zone, producing a momentary blowout condition. Continuous ignition ensures instant relight before the event is even detected by the crew.
- Possible fuel contamination: Fuel that is contaminated with water or has unusually low volatility may not atomize or ignite as readily, making the combustion flame less stable.
- Low-power settings: At very low power (idle or near-idle), combustion is less stable because fuel flow is reduced, temperatures are lower, and the flame is more easily disturbed. Some operators require continuous ignition any time the throttle is retarded to idle during flight, such as during a steep descent.
- Crosswind takeoffs and landings: Crosswind can create asymmetric airflow disturbances at the engine inlet, potentially upsetting the compressor airflow and destabilizing combustion.
- Volcanic ash encounters: Ash particles can foul igniters, erode compressor blades, and contaminate combustion chambers — though the primary response to volcanic ash is to exit the area, continuous ignition is part of the immediate action for inadvertent ash encounters.
The Mechanism of Water and Ice Ingestion Flameout
Understanding why water causes flameout helps pilots make better decisions. Water — whether liquid or as ice crystals — acts as a heat sink and a diluent in the combustion zone. When sufficient water is introduced, the heat release rate of combustion drops faster than the air-fuel ratio can compensate, and the flame blows out. The FAA-H-8083-32B notes that modern engines are tested for water ingestion tolerance, but those tolerances have limits; extreme precipitation events can exceed certification test conditions.
Ice crystal icing (ICI) presents a particularly insidious hazard. Unlike supercooled large droplets (SLD) that accrete on airframe surfaces, ice crystals — common above the freezing level inside or near deep convective clouds — pass through the inlet without sticking to the inlet cowl. Because the inlet probes may remain ice-free, the standard ice detection systems may not trigger. Ice crystals instead accumulate on the colder compressor stages, then shed as a mass into the combustion chamber, causing a sudden, dramatic rich excursion or lean blowout. This phenomenon has been linked to several in-service rollbacks and uncommanded engine power losses.
How Continuous Ignition Prevents Flameout
When continuous ignition is selected, the exciter box continuously charges and discharges across the igniter plugs at a rate specified by the manufacturer — typically several times per second. If a partial flame blowout occurs in one region of the combustion liner, the constantly firing igniters provide immediate re-ignition before the event can propagate to a total engine flameout. The pilot and passengers may never even be aware the flame was momentarily disrupted.
It is important to understand that continuous ignition does not prevent the disrupting condition — it does not remove ice or stop water from entering the engine. It simply ensures the engine can immediately recover from any momentary flame instability caused by those conditions.
Why It Matters for ATP Operations
For airline operations under 14 CFR Part 121 and 135, operators are required to develop Operations Specifications and flight manuals (AFM) that specify exactly when crews must select continuous ignition. These requirements are not advisory; failure to use continuous ignition in the specified conditions constitutes a deviation from the AFM and the operator's approved procedures. Examiners on the ATP Practical Test will probe a candidate's understanding of both the theoretical basis and the practical compliance requirements.
The stakes are high. A dual-engine flameout in IMC during an approach in heavy rain, or a rollback during climb through ice crystal icing conditions, can be fatal. History shows that most turbine flameout accidents and incidents involve preventable conditions — conditions that were present, recognized in hindsight, and for which continuous ignition was either not selected or not required by inadequate procedures.
Key Numbers and Rules
- Igniter plug energy: Typically 1–2 joules per discharge; far higher than piston magneto output (millijoule range).
- Number of igniters per engine: Usually two, positioned in the combustion liner primary zone.
- Duty cycle limitation: Manufacturers specify maximum continuous use limits (often 1–3 minutes per hour) for normal ignition; continuous ignition systems use ruggedized igniters rated for extended use — confirm your specific aircraft AFM.
- Ice crystal icing (ICI) altitudes: Most commonly encountered between approximately 20,000 and 40,000 feet MSL in or near deep convective clouds, but can occur at lower altitudes.
- Regulatory basis: 14 CFR Part 121 and 135 operations require compliance with approved AFM ignition procedures; 14 CFR §91.9 prohibits operating an aircraft contrary to its AFM (or markings/placards) limitations for aircraft required to carry an AFM.
- Conditions requiring continuous ignition (general): Icing, heavy precipitation, turbulence, low power settings in flight, volcanic ash, crosswind operations — confirm the specific list in the aircraft AFM, as it varies by type.
Common Test Traps
- Confusing ignition with anti-ice: Continuous ignition does not heat the engine inlet or prevent ice accumulation — engine anti-ice (bleed air to the inlet cowl) serves that function. These are separate systems that are often selected simultaneously but perform entirely different jobs.
- Assuming the engine will always self-relight: In some flameout scenarios — especially at altitude where air density is low — an engine may not relight without special procedures or a descent. Continuous ignition is prophylactic; it is far better than attempting a relight.
- Ignoring ice crystal icing because there is no airframe ice: ICI can occur with no visible moisture on the windshield and no airframe ice accretion. The absence of airframe icing does not mean engine icing is absent.
- Thinking continuous ignition is only needed in visible moisture: Turbulence alone (with no precipitation) is sufficient justification for selecting continuous ignition on most turbine aircraft.
- Overlooking low-power settings as a risk: Many candidates focus only on weather-related conditions and miss that idle descent power can destabilize combustion even in clear air.