Unless you’re lucky enough to fly a turbine or an engine with an electronic ignition system, the internal combustion engines you fly behind or between likely operate using a simple system involving a magnet, coil, condenser and contact points. As the spinny thing in the accessory case turns the magneto drive shaft, the magnet rotates and imparts electrical energy to the coil. When the points close, a jolt of electricity is sent from the coil to the respective spark plug wire to the attached spark plug and, kaboom, the cylinder fires. At least that’s the way it’s supposed to work.
Meanwhile, a second magneto does the same thing, preferably at the same time, to a second spark plug in the same cylinder. Between two magnetos and two spark plugs operating in sync, the engine will deliver power with a bit more efficiency than a conventional engine with only one spark plug per cylinder, everything else being equal. There’s also a so-called “dual” magneto, which uses only one drive shaft but fits two ignition harnesses. The design can simplify installation and maintenance but at the expense of some redundancy. And redundancy is always good.
The trusty magneto has been with us pretty much as long as internal combustion engines have been around, and the basic technology hasn’t change much in a century or so. With magnetos, spark-ignition engines don’t need an aircraft electrical system. Instead, they generate the electrical energy needed for ignition as long as they’re within specification and correctly synchronized (timed) internally and to the engine. They’re typically painted black and are mounted to the accessory case at the top rear of the engine. They should be easy to identify; they’re the accessories with the ignition harness wires leading off to the spark plugs.
Magnetos are a relatively simple electro-mechanical system, but they are subject to wear and tear, plus the stress of creating and releasing thousands of volts each minute. The nice thing about having a pair of magnetos is that one can fail and the engine still runs, just not quite as well. I know this because I have had the experience of a magneto failure, which ended with a safe landing at my home airport.
SIMPLE AND SOMETIMES FICKLE
Magnetos are mechanically driven by the engine, so there are bearings and couplings to worry about. For the electrical energy to jump from the coil to the spark plug wire, two pieces of metal—the contact points—have to make contact as the shaft rotates to convey the electricity. More opportunities for wear. The failure of the spark to make it across the contact points might be due to a bad capacitor, one with an open- or short-circuit failure. Due to age, the coil also may not be delivering adequate voltage. Worn contact points is another failure mode, perhaps due to arcing, which can result from the capacitor or coil being out of spec.
The beauty of a magneto is its simplicity. When one fails, it’s not dramatic, and will not result in a catastrophic engine failure unless both mags shoot craps at the same time. If that happens, the engine will stop running or might run rough (missing on one or more cylinders). Losing a magneto will not cause the engine to blow up or send a valve through the engine or blow a cylinder head. The engine just won’t make as much power. If you’re lucky, you may not even notice the failure until the next mag check.
That said, the problem with magneto failure you will only have one system igniting the spark. With a failed magneto, you will make slightly less power, but you are now 100-percent dependent on the single remaining magneto in the system, one that just demonstrated to you that it isn’t always reliable.
A logical person would consider the fact that chances are both magnetos were put on the plane at the same time. They very likely even share a common history of either being from the same manufacturer or rebuild shop, and were installed by the same mechanic. Their common heritage makes it prudent to complete the flight or return to base, and to be mindful of alternative off-airport landing options along the way.
Once one magneto fails in flight, it behooves you to keep altitude to maximize potential gliding distance. Also keep your power setting changes to a minimum (engine failures occur most often during major power changes). If your engine is humming along on the remaining mag, leave it alone. When making power changes for approach and landing, make them smoothly and slowly.
DECLARE AN EMERGENCY?
When a magneto fails, should you declare an emergency? Probably. If the other magneto goes out, you will wish you had. If you are in busy airspace, controllers will give you priority handling and, if there are numerous planes in the pattern at your intended airport, they will yield their spot in the pattern to you, as they should. Anything that helps you beat the odds and land safely should be used to your advantage. That’s what the E-word was invented for.
Should you perform a precautionary off-field landing? I wouldn’t, unless in addition to the magneto failure you also have a rough engine. With a single failed mag, chances are the engine is still running strong, just not as efficiently. If it is very rough on the remaining mag, a precautionary landing at the nearest airport makes a lot of sense. If it is running smoothly, but on only one mag, I personally would cautiously proceed to my destination airport as long as it’s not hours away.
ALWAYS CHECK MAGS?
I always check my mags, but not always on the ground. When operating on gravel and dirt strips, a high-rpm runup can cause unwanted damage to the aircraft, including the prop and tail. Once in the air, you can verify the mags are both working. This should not be done in a climb or while developing full power, but rather at a reduced power setting that results in less engine strain.
There are exceptions. I heard a case where the mag failure was due to a gear in the accessory case failing. The mag check is what presented the failure, which resulted in metal components in the interior of the engine dropping to the bottom where they would be chewed up. Had the pilot proceeded to fly after the failed mag check, the engine would have likely suffered a catastrophic failure due to the accessory case internals bouncing around. Fortunately, the failed mag check resulted in an immediate grounding of the plane and disaster was averted. In this case, the failed mag check wasn’t actually due to the failure of the magneto itself, but rather the internal components deeper in the engine that turned the magnetos.
P-LEADS AND HOT MAGS
When magnetos are switched to the “Off” position, the P-leads to the magneto’s primary coil are grounded. A magneto with a grounded P-lead will not operate. If the P-lead is not grounded, either by switching them on or by a failure of a P-lead, the engine has a “hot” mag, one that can and will discharge its electrical energy if it is rotated so that the points close. This means the engine may start, even if the keys are in your pocket. See the sidebar above for a tragic example.
I own a Piper J-3 Cub with no electrical system. The only way to start the engine is to hand-prop the airplane, an experience that has taught me one thing: A prop is a hair-triggered son of a bitch ready to kill you. While I have occasionally winded myself trying to start persnickety engine, I have learned to not flood the carburetor and figured out the secret sauce it takes to make the engine turn over.
Cued with the right stoichiometry, a single spark will bring the engine to life with the lightest, most innocuous turn of the prop. If the prop is in the right position and the impulse coupling is ready to click, a few ounces of pressure or a few millimeters of motion can be enough to send electrical energy through the ignition harness to a spark plug, and that can be more than enough to start the engine.
To avoid accidental hot props, it is a good idea to do a mag check at the end of each flight. Slowly rotate the key from left, to right, to the “Off” position to verify whether “Off” is truly “Off.”