The magnetos we’ve been relying on to fire our aircraft’s spark plugs may be the trailing edge of technology, yet if cared for appropriately, they are remarkably reliable. It’s the cared for part of the equation that matters—because they do require regular maintenance, otherwise some failure modes can mean engine stoppage or even catastrophic engine damage in a matter of seconds. The good news is that keeping your mags healthy usually costs less than two dollars per hour of engine operation per mag.
The purpose of a magneto is to provide enough electrical energy, at the right time, to fire a spark plug and ignite the fuel/air mixture in a cylinder. It operates on the principle that a rotating magnet will generate alternating electrical current in a coil of wire. By wrapping another coil of wire, with many more turns, around the primary coil, the voltage can be stepped up dramatically. The high-voltage current in the secondary coil—20,000 to 30,000 volts—is then mechanically directed to the appropriate spark plug as its piston reaches 20 to 24 degrees before top dead center (TDC). We’ll take a brief tour through the major components of a magneto, discussing what they do, how they can go bad, what happens when they do so and how to prolong their useful lives.