How do Magnetos work? If you are asking what a magneto is, you’re probably in the wrong Club. Most of the time, it would be better just to say magic and fairy dust than the answers I’ve heard. I would imagine all of you understand what a Magneto is. But do you really understand what a magneto is? And of course the follow-up question is, why should you care.
In this day and age, we take starting engines for granted. In one of my Jeeps, I press a button and magically, it starts. Airplane engines are waaaay behind the times. Our vehicles are so far advanced that most of our airplanes have a kind of machine envy. This is especially true for the airplanes that sit by the fence just tormented by all those pretty cars right outside the fence. When we get in our trusty airplane to “slip the surly bonds of earth” we don’t think twice about what goes into keeping your engine running. Hello Magneto. Many of you are probably familiar with an alternator. After all, you have one in your car, unless of course you have one of those electrical contraptions. The alternator provides your engine with the sparks it needs to generate the combustion which is, of course, required for your combustion engine. What happens though, when that alternator decides it is taking the day off? No spark = No combustion = Hello, AAA?
The magnetos help with this problem by ensuring that as long as the engine is actually turning, the magnetos are also turning and, in a healthy magneto, creating a sufficient spark required for combustion. When your engine fires up, the permanent magnet (hence the term magneto) inside the magneto rotates around a high output electrical coil, which then sends a spark to the spark plugs. Each magneto is wired to a spark plug in each cylinder of your engine. This is a slight oversimplification to the inner workings of the magneto but it gives you the basic concept. The magneto is basically an alternator. It generates alternating current as the poles of the magnet rotate around the coil.
The vast majority of pilots rarely take the time to really understand that big metal box they trust their’s and their families lives to.
When you move the prop, say to push the plane back into its spot, sometimes you hear a “click” from inside the engine compartment. Have you ever wondered what that was? If you were to get in your car, start it up, and heard the cry of a chupacabra, would you be at all curious as to what may be in your engine compartment? Of course you would.
The impulse coupling is a clever little device really. Its main job is to “charge” up the magnetos to be able to provide just a bit more energy during the start sequence. Think of it this way. Magnetos have been around for a LOOOONG time. The theory of electromagnetic induction was discovered in 1831. By 1914, magnetos were all the rage on Facebook and Twitter. That’s 104 years of pretty-much the same technology. Your magnetos are tired. They just want to retire already, collect social security and travel the world in jet-powered airplanes, laughing at how they started all this flying rage. Until then, they need help starting. I would too at 104. This is where our trusty Impulse Coupling comes to the rescue. When the airplane is first attempting to start, the engine is turning very slowly. This is problematic for the magentos which need a faster rotation to be effective. The impulse coupling helps the start process by allowing the magneto shaft to build up energy, which is then released at the proper time to create that 20-30,000 Volt spark you need to get those 180+ horses working. Ever have those wind-up toys as a kid that creates a cool looking spark when you release it and it moves all around the floor? That is basically the same principle of the impulse coupling. The next time you hear that click, you know what it is.
The Grounding Check
What is the grounding check for and why do we do it? For some, it is because their instructor told them to. Fair enough, but do you know why you are actually doing it and what you are checking for? There have been several incidents (including some very local) where someone went to move a prop for one reason or another and the airplane started up, instantly killing the person. The grounding check attempts to ensure that, when you power down the airplane, the chance of the engine restarting is nil. When you turn the key from BOTH to OFF, you are grounding the P-Leads on both magnetos to ensure that they will not allow the engine to start once you pull the mixture to kill the engine. Do you ever check grounding of the individual mags? Can you? While I grab a cup of coffee as you think about this question, think about your grounding check.
When you are doing your runup, what are you checking? You are checking each individual magneto to ensure that it will run independent of the other one should one of your magnetos decide on early retirement. The only way for this to occur is for you to ground the other one. So, when you turn your key from BOTH to L or R, you are grounding the other magneto. The switch tells you what magneto you are actually running on, the opposite one being grounded.
Putting it all together
Now that you have all of this amazing information, can you do anything with it aside from impressing your friends? Let’s say you have a rough running engine on the ground. Can you tell if it is a spark plug fouling or the entire magneto. If you have an engine analyzer, it’s actually pretty easy. When performing your run-up check and watching your EGT, look for a specific cylinder that has a lower EGT than the rest which will indicate a fouled plug. This is the most common cause of a failed “mag check.” If you have an entire magneto failing, you will likely see all of the cylinder’s EGT will be affected, not just one.
Mike Busch, A & P extraordinaire put out a great article on the Mag Check and you can dive more into some common troubleshooting that can be done if you experience some of these issues. For now though, the next time someone asks you what a Magneto is, you’ll have a good answer!