Magneto Checks: Here’s What You’re Monitoring and Why

A typical aircraft spark-ignition engine isn’t at all like the one powering the vehicle you drove to the airport. Unless, maybe, you drove a vintage tractor or motorcycle. There are so many differences, listing them here would take up lots of space, and we only want to focus on what pilots can do and learn about their magnetos from the cockpit.

As we should know from ground school, the magnetos use energy from the operating engine to create electricity. By rotating an armature near a magnet and then sending the resulting energy into a secondary coil, high-voltage electricity is created. The mags, in turn, are connected by a wiring harness to the spark plugs, usually two of which are in each cylinder.

This Article First Appeared in FLYING Magazine

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One reason we have two magnetos and two spark plugs in each cylinder is for redundancy. (Another reason is having two spark plugs slightly increases engine power.) The basic magneto is self-contained and requires no external power to operate. That doesn’t mean it or another component in the system can’t fail, however.

The Run-Up

If you’ve done much flying at all in a typical personal airplane, you’ve done both a run-up and a mag check. The magneto check is done at a specified rpm setting and accomplished by grounding one magneto and allowing the engine to react, and then repeating the process with the other mag.

If you have two or more engines, rinse, repeat. As far as I know, run-ups in all Continental engines are done at 1700 rpm. Lycomings are done at different settings. I have POHs for Lycoming-powered airplanes asking for 1,800 or 2,000 rpm. Use your airplane’s checklist or POH/AFM.

We typically monitor the tachometer when performing the mag check during the run-up. Parameters vary, but they often specify a maximum 150 rpm drop when running on one or the other mag, with a maximum difference between them of 50 rpm. A 100 rpm or less drop on both mags is nominal and, to us, is a sign of a well-maintained ignition system.

A typical keyed magneto switch is pictured with this article. Normally, the switch is used to engage the starter at its rightmost position, then released. Once released, it is spring-loaded to rotate counterclockwise to the “both” position, which is where it should be in normal operation.

Anomalies

But not all magneto checks are successful. A worst-case scenario is the engine quitting on one mag. That’s bad. The natural temptation when that happens is to immediately switch back to the both position. That’s bad, too.

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Ideally, we’d quickly reduce power on the engine before switching back to both. That’s because unburned fuel is likely to accumulate in the exhaust system and ignite explosively—a backfire—when the engine restarts after turning the mags back to both.

Another possible outcome is no rpm drop. This could mean a couple of things, including a bad switch, but typically means a failed P-lead at the magneto. The P-lead is a wire connected to the magneto’s primary coil winding and then to ground. By turning the magneto switch to, say, the “R” position, we’re grounding the left magneto with its P-lead. If there’s no rpm change on the tach, we can’t electrically shut off the magnetos. The airplane isn’t airworthy, and we need a mechanic to diagnose the problem and repair it before flying.

A third abnormal scenario is an engine that runs rough on one magneto, probably accompanied by a much larger rpm drop. This could signify a few things, the most likely of which is a fouled spark plug. The plug can become fouled with engine oil or lead from the fuel. “Burning it off” is a time-honored, old-school practice.

To do so, run the engine up to a higher rpm and slowly lean the mixture. At low power settings, the engine can’t be hurt by leaning it out, so feel free to lean it to the point of stumbling or quitting.  This increases the cylinder’s internal temperature, perhaps high enough to burn off the oil or melt the lead. Run it this way for a minute or so, then reduce rpm to the specified mag-check setting and try again. If that doesn’t work, you’ll need a mechanic.

Beyond the Run-Up

Magneto checks aren’t just for run-ups. Especially if you suspect a problem, but even if you’re just bored and want to see what happens, you can do mag checks while airborne.

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In fact, an airborne, high-power mag check can identify ignition system issues better than the run-up version. This is especially true when operating the engine lean of peak (LOP) exhaust gas temperature (EGT). That’s because LOP operation, by definition, uses a leaner fuel/air mixture that is more difficult to ignite. Just as if the engine quits during the run-up mag check, reduce throttle to minimize any backfiring potential before moving the mag switch back to “both” if that happens in the air.

So far, we’ve talked only about the tachometer as the instrument to use when evaluating the magnetos during a check. We may have other tools, however, most notably multi-probe engine monitor. By sensing each cylinder’s EGT and cylinder head temperature (CHT), we can use a mag check to identify the cylinder exhibiting a lower-than-normal EGT or the one that drops out completely on one mag. We can do this in the run-up block next to the runway or in cruise.

If the engine runs rough on one magneto and the before-takeoff “burning it off” procedure doesn’t fix the problem, we can use the engine monitor to isolate the cylinder, then tell the mechanic which spark plug to clean or change—the one connected to the left mag or the right one.

This column first appeared in the September Issue 962 of the FLYING print edition.