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Inspired by this question asking about shutting down a light aircraft engine I realised there is another ritual which I do, but have long forgotten the reason why.

Having parked a light aircraft, and switched off the electrics, with the engine still running I always check left and right magnetos, similar to the magneto drop check done as pre-flight checks.

I don't really pay much attention to the RPM, I do it because I've always done it. It's not (as far as I remember) on my shutdown checklist - and I'm starting to wonder why I do it.

Is this necessary? Something in the back of my mind is saying "prevents mag fouling". What is the real reason to cycle Left & Right mag pre-shutdown?

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    $\begingroup$ Pay attention to the drop and look for roughness. If you didn’t lean enough on your flight, you may have fouled the plugs. It’s better to know that when you shut down rather than when you get to the runup area for your next flight. $\endgroup$
    – JScarry
    Nov 20, 2018 at 16:42
  • $\begingroup$ So called mag fouling normally refers to spark plug fouling, and that is normally addressed during run-up by leaning the mixture to a hot and lean mixture, at a reduced power setting (like 1900 or 2000 RPM) and running with both mags selected for several minutes. Then the mag drops are retested. $\endgroup$
    – mongo
    Nov 20, 2018 at 18:41

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This is a hot-mag check to verify that when you shut the engine down, you are really putting it in a safe(r) state. Think about how the mag-switch works, when you switch to R or L, it grounds the P-Lead on the other side (so switching to R grounds the P-Lead on the Left side). If you don't see the RPM drop, you may have a hot mag on that side.

An un-grounded mag is a dangerous situation in that it can cause the aircraft engine to turn over if you accidentally (or intentionally) move the prop when you are parking the aircraft, or when somebody walks by and runs into your prop.

So during this mag check you are only looking for an RPM drop or a change that tells you that the mag is shut off.

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    $\begingroup$ That’s what I was taught, but really, isn’t testing each mag for a drop doing the same thing? It won’t drop if there’s a ground. I guess that wouldn’t verify no ground in the switch itself when set to OFF. $\endgroup$
    – Greg Taylor
    Nov 20, 2018 at 16:29
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    $\begingroup$ Yes, the reason you do it at shutdown though is that you've just finished flying the aircraft and you are about to move it (possibly push it by placing your hands on the prop). You want to be sure that if you accidentally move the prop it doesn't kick over. You want to make sure that a P-Lead didn't disconnect in flight, the run-up is for checking mag quality and plug fouling, the shutdown is for checking p-lead. $\endgroup$
    – Ron Beyer
    Nov 20, 2018 at 16:36
  • $\begingroup$ Maybe this might be better suited for Electronics, but what is exactly meant by ground? What is the P-lead connecting to when forcing the flow of electricity to be redirected elsewhere? Is it earth's grounding, or the casing of the fusilage? $\endgroup$
    – HeavenlyHarmony
    Mar 28, 2020 at 14:48
  • $\begingroup$ @HeavenlyHarmony It's referenced to the negative side of the battery, which I believe is also grounding the fuselage/airframe (but this probably depends on the aircraft, a composite aircraft wouldn't have a grounded fuselage). $\endgroup$
    – Ron Beyer
    Mar 28, 2020 at 16:23
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You are doing it wrong if you are just cycling left/right/both. It is a hot mag check, not a mag function check like on a runup. You are supposed to cycle them to OFF and then back on to BOTH before shutdown and listen for the sound of combustion stopping in the exhaust. You do it after the engine has had time to cool down and while the engine is at idle to avoid backfires.

Remember that magnetos are disabled (shut off) by grounding out the primary circuit. If the P lead, the ground out wire, to the ignition switch breaks, the mag is prevented from turning off and remains live. You are making sure they are grounding out and really shutting off so you don't have an undetetected live mag after shutdown. You have to do this because shutdown is done with the mixture cutoff and if the one mag is live after being turned off you won't know it, and a surprise could occur if the prop is moved later and the mag fires with residual fuel mixture in the cylinders.

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    $\begingroup$ I don't think you adequately explain why it is wrong to cycle left/right/both. My understanding is that if there's a mag drop on left, and another one on right then it proves both mags are grounding properly as well as turning the key to off. Turning it to off means you get fuel in the exhaust system. $\endgroup$
    – GdD
    Nov 20, 2018 at 16:28
  • $\begingroup$ To get a noticeable mag drop you have to have some power on. This check is done just before shutting down, when at idle. Yes a bit of unburned fuel gets through, but when done at idle after the engine's cooled down it's not sufficient to cause a back fire and when you put the mags back on you're back to normal. You only have them switched off for a quarter to half a second. I've been doing it for 40+ years and have never had a backfire. $\endgroup$
    – John K
    Nov 20, 2018 at 17:54
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This is going to give the same reason as Ron Beyer's answer. I'm only writing another because I found his a little hard to follow and I'm going to try to explain it differently. It's nothing to do with mag fouling at all.

The magnetos are wired up so that a failure of the switch or wiring leaves them live (on), unlike most electrical devices which are wired to be off if they fail. This is normally what you want because if the wiring fails in-flight, you'd like the magnetos to keep working. But it can cause a dangerous state, where the magnetos are stuck on when you think they're off. If you swung the prop in that condition, it could fire when you're not expecting it.

So one at a time, we switch a magneto off, and check that the RPM appears to drop, and that the engine sound changes. The sound is more important - as John K says, the drop is small at the kind of RPM you do your mag drop test, and hard to see unambiguously on the gauge. This way, you know that switching it off really does isolate the magneto, so you know that when you turn both off after stopping the engine, the propeller is safe.

It's not so much for your benefit as whoever will touch or fly the aircraft next. Even so, you should always treat a propeller as live. If you have to swing it (e.g. to inspect it), do so as if you were hand-cranking the engine, keeping well clear and making sure your arm comes out of its path.

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  • $\begingroup$ I am not sure what I meant. I think I was answering a different comment. $\endgroup$
    – Dean F.
    Feb 13, 2020 at 19:35

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