What Are Bad Magneto Systems?
Magnetos are an important element of many aircraft engines, serving to create the voltage necessary for fuel combustion. While magnetos are designed to be extremely robust as a result of the intensive environments that they typically operate in, such equipment will still face gradual wear and tear over time, oftentimes leading to losses in performance, operational issues, and eventual failure. As such, when issues arise with the ignition system of the engine, it is important that one addresses things as soon as possible to see if there are symptoms of a bad magneto. As issues can result from a variety of things, it can be useful to have a general understanding of the most common signs of a bad magneto system, and how to conduct general magneto troubleshooting.
In a majority of cases, a warning of a “bad mag check” will actually be a result of a spark plug issue. While the magneto supplies current, the spark plug is the component situated in the combustion chamber assembly where the current arcs and ignites the fuel mixture. The most common issues that spark plugs face include fouling and shorting, making them unable to fire. If there is a sudden drop of RPM by a value of around 250 RPM, then the issue may be the spark plugs. Luckily, when this occurs, no special equipment is needed for cylinder and plug isolation.
The first step of isolation is to shut down the engine and remove the cowling if need be. For safety, one should wait for the engine to completely cool, as even a short amount of operation will lead to immense heat. Once ambient temperatures are reached, the engine should be restarted and turned to the suspected magneto with RPM settings set very high. After the engine has been in operation for a few minutes, the RPM should be reduced to idle while having the mixture set to idle cutoff. Once the magnetos are off as well, you may use your hand to feel each cylinder head fin to check temperatures. With this, it will be easy to tell which is having an issue as the cylinder with a faulty plug should be significantly cooler than the rest.
From there, you can trace the ignition hardness to the spark plug, and with inspection, you should be able to determine whether there is lead fouling, oil fouling, cracks, or other related issues. Once the plug is cleaned and inspected, an Ohmmeter can be used to determine the resistance value from the connection situated at the bottom of the barrel to the center electrode located at the firing end. Most healthy plugs should have a value around 800 to 1200 OHMs, and anything above 5000 OHMs should result in immediate replacement. Spark plug bomb testers are also useful for testing serviceability, and once all readings and replacements have been conducted, everything can be reassembled and tested. If there is no major difference in cylinder temperature and all spark plugs are fine, then the magneto is indeed bad and will need to be replaced or repaired with new components.
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