We're starting a brand-new chapter: Gas Turbines – Ignition Systems — Page 338, Lesson 430

We're starting a brand-new chapter: Gas Turbines – Ignition Systems. Let's get into it. First, the headline fact: every gas turbine engine has a dual ignition system, and they all use what we call high energy, or HE, igniter units for engine starting. Let's unpack that. "Dual" means two of everything—two igniter plugs, two ignition units—so that if one system fails, the other can still light the engine. And "high energy" refers to the electrical punch these units deliver. The output is approximately twelve joules. Now, a joule is a unit of energy, and the book gives us a handy conversion: one watt equals one joule per second. So if you think of a watt as energy per second, twelve joules is a very substantial, powerful spark—enough to reliably ignite the fuel-air mixture in a large turbine. Now, here's an important operational point. It's sometimes necessary to have the igniters selected even when you're not starting the engine. For example, during take-off from a contaminated runway, or when flying through heavy precipitation. Why? To help prevent an engine "flame out"—that's when the combustion flame goes out mid-flight, which is a serious situation. So you'd want the igniters on to relight instantly if the flame dies. But there's a catch. Using the high energy ignition system in these situations erodes the igniter plug very quickly, dramatically shortening its working life. So to minimize that wear, some aircraft engines are fitted with a combination ignition system. This gives you two selections: a low energy mode, which is three to six joules, for continuous operation, and the high energy mode, six to twelve joules, for starting. So you use the big spark to start, and the gentler spark to keep things alive. Now let's look at how these systems are activated. The starting ignition system is activated when the engine start sequence is initiated—either automatically, or by the operation of the HP cock, the start lever, or the fuel and ignition switch. The igniters are then automatically deactivated at some point after the engine reaches self-sustaining speed. That's the point where the engine can keep itself running without the starter motor. Typically, this deactivation is done by a speed switch in the HP rpm indicator. So the system watches the high-pressure compressor speed, and once it's high enough, it cuts the igniters off. Then we have continuous ignition, which is activated by a selection on the engine start panel. This activates the low energy mode of the igniters—that's the three to six joule setting we talked about. Finally, there's automatic ignition. This is a feature on some aircraft. It's typically activated by the aircraft's stall warning system. So if the aircraft detects a stall, it automatically selects continuous ignition to protect against a flame out during that critical maneuver. So to summarize the whole picture: you've got a high-energy starting system for ignition, a low-energy continuous system for in-flight protection, and on some aircraft, an automatic system that ties into the stall warning. Each has its own activation path and its own energy level, and they all work together to keep that flame burning.