Argthtjtr

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?

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.

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.

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.