Piston Engines - Ignition — Page 72, Lesson 91

I want to walk you through the ignition system of a piston engine — specifically the magneto circuit, the ignition switch, and the checks we run to make sure the whole thing is safe. This is the heart of how we control when the spark fires, and it's also where a lot of the pre-flight and run-up discipline comes from. Let's start with the ignition switch. The ignition switch gives you complete control of the engine's magneto circuit. The key idea here is that the magneto is made inoperative by earthing the primary circuit. Let me unpack that. A magneto generates its own electricity — it doesn't need a battery. The primary circuit is the low-voltage side that builds up a magnetic field, and the contact breaker points are the mechanical switch that opens and closes that primary circuit to fire the spark. When you earth the primary circuit, you're giving the electricity a path to ground, which stops the magneto from producing a spark. That's how you switch it off. Now, here's the clever part about the switch positions. In the 'OFF' position, the switch is closed. That means it short-circuits the contact breaker points. So even though the engine is turning and the points are trying to open and close, the switch has bypassed them — the primary circuit stays shorted, no spark, engine off. In the 'ON' position, the switch is open. That means the primary circuit is now controlled by the action of the contact breaker — the points can do their job, make and break the circuit, and the magneto fires normally. Next, the grounding wire. This is the physical wire that connects the switch to the magneto to earth the primary circuit. If that grounding wire breaks while the engine is running, you'd expect something to happen, but actually there will be no apparent change in engine performance — the magneto just keeps working because the circuit isn't earthed anymore. But here's the dangerous case: if the grounding wire breaks and touches the engine-body or the airframe, that's the equivalent of grounding the primary circuit. The magneto is switched off instantly. That's exactly why we have the magneto checks — to catch a broken or shorted grounding wire before it bites you in flight. So let's look at those checks. The first one is the Dead Cut Check, carried out at slow running — idle. The purpose is to ensure the pilot has control of the ignition before we go on to any higher-speed checks. The rule is absolute: RPM MUST DROP BUT ENGINE MUST NOT STOP while switching one magneto off at a time. If you switch one magneto off and the engine dies, you've just proven you don't have control — the other magneto isn't doing its job. Now, why is that so critical? Consider a situation where the engine is running and the pilot is unaware that only one magneto is actually working. If that live magneto is switched 'OFF' during a high-rpm magneto check, the engine would die. The pilot's automatic reaction would be to slam the ignition switch back to 'BOTH'. The engine suddenly bursts back into life with the throttle still at the check position. That sets up a high torque reaction between the airframe and the engine — possibly causing extensive damage. So the Dead Cut Check is your early warning system. There's also the Live Magneto Check, but it's not normally required. Why? Because evidence of a live magneto — a magneto that's firing when it shouldn't be — is usually found at the Dead Cut Check simply by observing a change in rpm as the switch is operated. If you see an rpm change when you shouldn't, you've found your problem. Then we have the Magneto rpm Drop Check, carried out at approximately 75% of maximum engine speed. This checks that the magneto and the sparking plugs are functioning correctly. As each magneto is switched off in turn, you check for a drop in rpm, and that drop must be within the limits laid down by the manufacturers. Now, why does the rpm drop at all? Because when you switch off one magneto, you're also switching off one sparking plug in each cylinder. The mixture takes longer to burn — the fall in rpm is due to the increased time taken for the mixture to burn in the cylinders. That's the physics behind the drop. Finally, let's talk about auxiliary starting devices. During starting, most aero-engines are cranked at about 25 rpm. At that low speed, the magneto will not produce a spark with adequate energy to ignite the petrol/air mixture. So we need auxiliary methods of spark augmentation to ease starting. The main one is the High Tension (HT) Booster Coil. It supplies a succession of high voltage electrical impulses to the trailing, starting, or retarded brush — that's the electrode — of the main distributor rotor. This is the "shower of sparks" system. It's switched 'ON' for starting. So the whole picture: the ignition switch controls the magneto by earthing the primary circuit, the grounding wire is the weak link we check for, the Dead Cut Check proves control at idle, the rpm Drop Check proves the magnetos and plugs work at 75% power, and the HT booster coil gets you started when the magneto can't. That's the ignition system in full.