Unlike a piston engine, jet engines are powered by self-sustaining combustion. The igniters in jet engines are used relatively infrequently, but these are 7 times you'll use continuous ignition...

But First... What Exactly Is Continuous Ignition?

Jet engines, which are also called gas turbines, work by sucking air into the front of the engine using a fan. From there, the engine compresses the air, mixes fuel with it, ignites the fuel/air mixture, and shoots it out the back of the engine, creating thrust.

In a jet engine, the combustor is where the fire happens. As air exits the compressor and enters the combustor, it is mixed with fuel, and ignited. The igniter is very similar to the spark plugs in your car or piston-engine airplane. Once the igniter lights the fire, it is self-sustaining, and the igniter is turned off.

When jet engines experience a temporary disturbance of constant airflow, they can flame out. The fuel to air mixture needs to remain within close parameters for a self-sustaining flame. Continuous ignition is when the igniter is turned on, which provides continuous sparks to ensure the engine doesn't flame out.

So when are the igniters turned on? Here are 7 times...

1) During Engine Start

As described above, the igniters come on during engine start to light the initial fuel air mixture. Once the flame is lit and the starter cuts off, the igniters are powered down.

2) Icing Conditions Or When Engine Anti-Ice Is Selected "ON"

Before entering icing conditions, or during an inadvertent icing encounter, the igniters are turned on to ensure the ice doesn't interfere with airflow through the engine, which could result in a flameout.

3) Heavy Precipitation

When entering moderate, heavy, or severe precipitation, turning on igniters provides a backup for large quantities of water interfering with the combustion process. For example, the POH for the Cessna 208 Caravan, which is equipped with a PT6 Turboprop Engine, recommends this procedure.

4) Wet Or Contaminated Runways

Foreign object debris (FOD), slush, snow, ice, and water, can provide a hazard to jet engines during takeoff.

5) Moderate Or Greater Turbulence

In areas of turbulence, it may be challenging for an engine to receive a constant rate of airflow. Ignition is can be turned on to provide a backup in case internal combustion flames are suddenly extinguished.

6) Low Fuel Pressure Or Quantity

If pilots receive a low fuel pressure or low fuel quantity, they're already having a bad day. Turning on the igniters gives the engine a better chance of continually running and providing thrust for its final few minutes. As fuel being sprayed into the engine by fuel nozzles becomes sporadic during low fuel level, igniters help keep the flames burning.

7) Or, When FADEC Automatically Selects "Ignition ON"...

New jets are often equipped with a system called FADEC, or Full Authority Digital Engine Control. It's essentially a computer system that controls specific engine parameters.

FADEC will turn on ignition automatically in many of the cases listed above, taking some workload off the pilots. It also adds the benefit of turning on the ignition during stall warnings, when takeoff thrust is set, during approach idle, or when the engine experiences a surge.

So, Why Aren't Igniters Used All The Time?

Simply put, igniters don't need to be used all the time in jet engines. Continuous ignition adds undue cost and wear to the critical ignition system. It's rare for a jet engine to flameout in the first place, and pilots are trained to turn ignition on in situations when there could be a lack of sufficient airflow or fuel into an engine.