If you fly a piston-powered aircraft, you undoubtedly were taught to perform a “mag check” during the pre-takeoff runup. But do you know how to do it correctly, what to look for, and how to interpret the results? Surprisingly, many pilots don’t.
To begin with, most POHs instruct you to note the RPM drop when you switch from both mags to just one, and give some maximum acceptable drop. In my view, this archaic procedure makes little sense for aircraft that are equipped with a digital engine monitor (as most are these days).
EGT rise is a far better indicator of proper ignition performance than RPM drop. Watching EGT on the engine monitor during the pre-flight mag check tells you exactly which spark plug and cylinder is having a problem. So my advice is to focus primarily on the engine monitor, not the tachometer, when performing the mag check.
What to look for
What you should be looking for is all EGT bars rising and none falling when you switch from both mags to one mag. The EGT rise will typically be 50 to 100 degrees F, but the exact amount of rise is not critical. It’s perfectly normal for the rise to be a bit different for odd- and even-numbered cylinders.
You should also be looking for smooth engine operation and stable EGT values when operating on each magneto individually. A falling or erratic EGT bar or rough engine constitutes a “bad mag check” and warrants further troubleshooting of the ignition system before flying.
Bad mag or bad plug?
The “mag check” is poorly named, because because the vast majority of “bad mag checks” are caused by spark plug problems, not magneto problems. We really should be calling it an “ignition system check” but the “mag check” terminology is deeply entrenched in pilot lingo, so I’m not going to try to fight that battle.
How can you tell if the culprit is the plugs or the mags? Simple: A faulty spark plug affects only one cylinder (and one EGT bar on your engine monitor), while a faulty magneto affects all cylinders (and all EGT bars).
If you get an excessive RPM drop when you switch to one mag, but the EGTs all rise and the engine runs smooth, chances are that it’s not a bad mag but rather retarded ignition timing. This is often caused by mechanic error in timing the mags during maintenance, although it is possible for ignition timing to drift out of spec due to cam follower wear or some other internal magneto issue. Retarded ignition timing also results in higher-than-usual EGT indications.
Conversely, advanced ignition timing results in lower-than-usual EGT indications, and also higher-than-usual CHT indications. Advanced timing is a much more serious condition because it can lead to detonation, pre-ignition, and serious engine damage. If you observe low EGTs and high CHTs after an aircraft comes out of maintenance, do not fly until you’ve had the ignition timing re-checked. Advanced timing is easily detected with an engine monitor, but you won’t be able to detect it if you’re just looking for RPM drop.
Do it aloft!
The usual pre-flight mag check is a relatively non-demanding test, and will only detect gross defects in the ignition system. To make sure your engine’s ignition is in tip-top shape, I strongly recommend performing an in-flight mag check every few flights.
The in-flight mag check is performed at normal cruise power and normal lean mixture (preferably LOP). Run the engine on each individual mag for at least 15 or 20 seconds. Ensure that all EGTs rise, that they are stable, and that the engine runs smoothly on each mag individually. If you see a falling or unstable EGT, write down which cylinder and which mag, otherwise you’ll probably forget which plug is the culprit by the time you land.
Because a lean mixture is much harder to ignite than a rich one, an in-flight LOP mag check is the most demanding and discriminating way to test your ignition system, and will expose subtle flaws and marginal ignition performance that are undetectable during the usual on-the-ground pre-flight mag check. It’s by far the best way to detect ignition system problems early, before they reach the point of delaying your departure or soiling your underwear.