Mag Check

Mag SwitchIf 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

JPI EDM 830What 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?

MagnetoThe “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!

MooneyThe 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.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).


  1. “Most aircraft these days have a digital engine monitor”? I have yet to see a small aircraft come through my home airport with one. There are hundreds of flying videos on youtube that show planes without a digital engine monitors. You may be seeing this explosion in digital engine monitors where you are but i’m not seeing that at all. Although I’d love to have them installed in our club’s planes its, in my opinion, far from the norm to see one installed in a small, pre 1990, aircraft

    • A different Dan here…
      I just installed a used EDM-700 in my 1960 Debonair… first time I have had this luxury! Still learning how to use it, but it’s already providing great insight. Highly recommended.
      A lot of upscale guys are upgrading their “old” units to fancier models, so systems are available used. Mine cost $800 on eBay including all probes and fuel flow. Installed it myself (with “supervision” from my A&P).

  2. Excellent advice, as always.

  3. Great Article – Thank you.

  4. “In my view, this archaic procedure makes little sense for aircraft that are equipped with a digital engine monitor (as most are these days)”

    I know in my 1966 Cherokee with original instrumentation, I’m ALL OVER that digital EGT (just like all my other non One-Percenter pilot friends).

  5. As an A&P, I always push owners to install an engine monitor before any other fancy gadgets they might want. If you don’t have one, you are flying blindly behind an engine that may be running in ways that could shorten its TBO. One friend just installed a JPI 830 and is finding that his #3 cylinder CHT has been easily hitting 425 degrees during climb – and this before the heat of summer has arrived. Seeing this temp caused him to lower the nose and reduce power to keep it under control. He previously had a single cylinder monitor on #4, which showed normal temps. For the price of a couple new cylinders, you can have the visibility needed to run the engine properly.

  6. I’d been taught the magneto check is a performance check of the engine. Is the engine good for taking off? Does it make enough rpm on one magneto to give the static rpm as defined in the TC. It is not meant for troubleshooting your engine. That is a different concept and does not belong in the pre-flight check procedure.

    One should keep these two tests procedures apart Mike. Especially when talking to the young pilots.

    • You are making Mike’s point. Most people do not understand what the “mag check” is actually checking, thus they do not know how to use it to troubleshoot the engine when the check fails. If you do not understand, by all means take the plane over to your mechanic and pull out their wallet. Those that understand can do their own troubleshooting and save a few bucks. Spark plug maintenance is permitted by owner pilots.

      Mike did not mention that the on-ground ignition check can be done withe the mixture leaned. As long as you are not making 70% power you cannot hurt a NA engine by running it lean on the ground or in the air.

  7. An inflight, LOP ignition system check just prior to Top of Descent is good insurance. This way you will know if you have a magneto or plug problem prior to your next flight.
    What Mike did not mention is what to do if things get rough or silent when switching magnetos in flight.
    The normal reaction is to put the mags back to both. This would introduce ignition after a few cycles of fuel-air with no spark.
    I would suggest having a plan before you do the inflight ignition test. Brief yourself that if things go quiet, take a deep breath, pull the mixture to cutoff, THEN restore the mags to both before slowly restoring the mixture to the previous LOP fuel flow.

  8. Robert Collins

    March 29, 2018 at 9:41 pm

    Thank you very much. I had never thought about a in flight mag check but it’s a perfect place to do it.

  9. I have an EDM-700 in my airplane. Awesome. Worth every cent. I do as Mike suggests and can tell subtle issues long before they become problems. Great article.

  10. Claudio Friederich

    March 30, 2018 at 3:57 am

    For the 90% of us who do not have engine monitors: One way you can check if a problem is the mag or the spark plug(s) is the speed of the drop. If you have an excessive, but slow and gradually increasing rpm drop, then it is likely the mag. But if the excessive drop is sudden and “all at once” then it is likely the spark plugs.

  11. I’ve been a follower and SavvyAnalysis subscriber of Mike Busch since we installed our EI monitor in our 1978 Cardinal about two years ago. In flight LOP mag check just prior to annual revealed loss of power (severe miss) with comparatively much high(er) EGT on #1 at 1600+ degrees before #1 quit producing power.

    SavvyAnalysis recommended a check of the lower plug on #1. The plug bench checked as normal with about 200 hours SN on all plugs. What we did find, thanks to the EI monitor, was an air induction leak around the intake gasket where the intake header bolts to #1 cylinder. Prior installation of new style Tanis heat element (replaceing one of two regular fasteners) had not been torqued properly thus causing air induction leak.

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