One of the oldest and most trusted components of aircraft is magneto technology. While you may be unfamiliar with magnetos, for decades they have been an integral part of the aviation industry. An aircraft magneto is a self-contained generator that uses permanent magnets and coils to produce high voltage to start-up the aircraft spark plugs. The magnets spin within close proximity to a coil of wire, which generates a strong magnetic force that is contained by a primary coil. The moment contact points open, the magnetic flow produces a high voltage in the secondary coil, the spark plug is ignited, and the engine starts to run. While this may seem like a straightforward process, there are instances where this operation does not run as smoothly, causing the magneto to fail. For this reason, we will be covering the common faults with magnetos and troubleshooting methods in the case that a magneto fails.
Most general aviation aircraft have two magnetos, the left and the right, and each magneto is responsible for firing a spark plug on each cylinder. More than that, there are two magnetos so that in case one fails, the engine continues to run. Despite these preventative measures, new technology has not been as reliable as the magneto. They have remained the popular choice for aircraft because of their self-contained nature, and the fact that they are not attached to a battery or electrical system. This ensures that the engine will continue to run even if there is a battery or electrical failure. Nonetheless, the magneto has a number of faults that one should be aware of.
There are many reasons a magneto may malfunction, so outlining its various components may provide insight to the location of those failures. One such component is the capacitors, which are short-term storage devices that often fail in older magnetos due to dielectric degradation, often called a short circuit, or conductor breakage, often called an open circuit. A short circuit absorbs all the power, meaning that there is no output or spark. In the case of an open circuit, the device does not have the capacity to absorb the reaction voltage from the coil, resulting in the energy dissipating quickly.
Coil failure may also contribute to malfunctions in the magnetos. Though coils are made to withstand incredibly high voltages and extreme pressure, the insulation is not always perfect, and as a result, a small number of electrons can still travel through the insulation. Additionally, the wire can experience breakage from the constant hot, cold, expansion, contraction, vibration, or mechanical impact the engine faces. While this may not immediately stop the coil from working, the high voltage will extend across the breakage, the gap will widen as the wire ends melt away, and the coil will eventually fail.
Contact points can also cause the magneto to fail. Usually affixed to the end of the armature shaft, contact points can become damaged over time, especially if the capacitor is faulty. The high resistance will reduce both the current flow in the primary winding as well as the magnetic flux, which affects the secondary winding output voltage.
One other failure can be found in the magnet itself. The magnet is typically made from alnico, an alloy made from a combination of aluminum, nickel, cobalt, and iron, as well as copper, titanium, and niobium. Alnico has the tendency to lose its induced magnetic field over time as a result of removing the magnet from its usual location and mechanical shock.
In the case that you are confronted with any of the aforementioned situations, there are a few troubleshooting methods that can prevent your magneto’s complete failure. It begins with being able to identify the origin of the failure. When one magento fails, the pilot might notice that there is a decrease in engine power, a failure to ignite spark plugs, or the internal timing may malfunction. While the pilot can rely on the second magneto until the aircraft reaches a safe destination, if the problem is identified prior to flight, the pilot or a mechanic may take another course of action.
For one, you can shut off the engine, allow it to completely cool, and restart it. Adjust the RPM to allow the engine to run at its roughest for about two minutes then reduce the RPM and shutdown the engine once more. Place your palms over each cylinder to feel for temperature differences, and if it is a spark plug problem, some cylinders will be colder than others. The spark plug may be lead fouled, fuel fouled, oil fouled, or opened through its resistor. Once you’ve identified the faulty plugs, you can clean, test, and reinstall them before you attempt to run the engine.
After completing the cold cylinder test, if you cannot discern a difference, you may have a bad magneto, an issue with the P-lead, or a problem with the ignition switch -- though problems with the ignition switch are more unlikely. In the case of a shorted P-lead, remove the P-lead from the failing magneto, run the engine, and eliminate the mag switch, p-lead wire, and filter capacitor. Keep in mind that the mag is hot when the P-lead is removed.
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