From Rans S9's post in the Engineers forum:

The magneto safety check is conducted with the propeller in the high rpm (low pitch) position at approximately 1000 rpm. Move the ignition switch from 'Both' to 'Right' and return to 'Both'; from 'Both' to 'Left' and return to 'Both'; from 'Both' to 'Off' momentarily and return to 'Both'.
When switching from 'Both' to a single magneto position, a slight but noticeable drop in rpm should occur. This indicates that the opposite magneto has been properly grounded out. Complete cutting out of the engine when switching from 'Both' to 'Off' indicates that both magnetos are grounded properly. Failure to obtain any drop while in the single magneto position, or failure of the engine to cut out while switching to 'Off' indicates that one or both ground connections are not secured.
Source: EA-AC 65-12A Airframe & Powerplant Mechanics Powerplant Handbook.
(Note: This is purely a safety check and not an engine run-up mag drop procedure.)

As noted earlier, it is standard procedure to shut a Gipsy engine down using the magnetos (the mixture cannot be leaned to ICO as it is gated by the throttle). What is so different with the Lycoming that prevents using the same procedure?

Shutting down on magnetos would be a better option than dead cutting a running engine.

Dodgy operators
 

The slight drop in RPM's is also a very good indicator that both Mags are giving true indications of working independantly. I found a certain G.A Co in Oz used to wire the good mag to both left and right so the unsuspecting pilot when conducting the mag check would not (unless savvy) determine the airworthiness of both magneto's.

The devil's in the detail!!

What does operating one engine have to do with another, when complying with the manufacturer recommendations? If you propose that shutting down the Gypsy engine should determine how to shut down a Lycoming or Continental powerplant...then why not just reverse it and operate the Gypsy motor as if it were a Lyc or Continental? Different powerplants, different installations, different procedures and practices.

Why do you suggest shutting the Lycoming or Continental powerplant down with the magnetos is better than mixture-killing it?

The ignition/magneto grounding has nothing to do with the plugs, they are not grounded. The off switch grounds to output of the magneto so that no HT is applied to the plugs. The plugs are always "Live " so to speak, its just that the Spark cant get there.

When doing magneto system checks (in a time long long ago) we would fit a specila grounding plug to the maggy then check the switch system.

switchery
 

FWIW I often fly a C90 powered Jodel.......
It has a Stromberg carburettor with no idle cut-off facility.
Therefore, standard shut-down is by using the mag switches........

Talking of Lyco/Conti engines, AFAIK the ignition switch has just one grounding wire coming out of it.

So, the scenario where you get a successful L/R mag test but get a live magneto with the switch in the OFF position, seems exceedingly remote. Is anybody aware of this having ever happened and, if so, any details?

It would need a very strange internal break-up of the contact mechanism, but never having taken one of the switches apart I can't say anything specific.

The RAF method to test mags was this:

At 800rpm, idle:

Drop no stop (Right), BOTH, Drop no stop (Left), Drop and Stop (Off), BOTH.

The last part was done very briefly so that the engine was kept running.

RAF Bulldog, rotary mag switch, 200 bhp Lycoming.

The idle cut-off is pulled to purge fuel from the cylinders so that an ungrounded magneto by itself shouldn't result in a propellor bite if someone turned the motor over compression.

However, as always, the manufacturer's method in the POH is the correct way to do it.

IO540
The 2 magnetos (Left & Right) have what are called P leads connected to the ignition/mag switch. The purpose of the switch is to 'ground' the magnetos via the P lead and thus make them 'dead': so that when you select R mag the L mag is 'grounded' and vice versa. When the mag switch is selected OFF both mags are grounded. Problems can occur: where the P lead attaches to the mag e.g. terminal lug breaks or screw comes undone, the switch itself which may contain metal from constant operation or loss of contact all together, the usually single ground wire at the switch failing. So for all of these reasons it's a good thing to check for a 'dead' cut. This check is not generally a problem if the test is done quickly and with low RPM. And in answer to question, 'has anyone seen this happening' the answer is yes to all these scenarios.

And in answer to question, 'has anyone seen this happening' the answer is yes to all these scenarios.

Your description of how the ignition system works is accurate, and for the sake of determining if all ignition is off a quick check at idle will not damage the engine......

.......however far to many pilots have been taught to do a dead mag check at 1000 RPM or higher every time they shut down in light aircraft.

So how often will you actually find the ignition live with the mags in the both off position?

I have over 30,000 hours and am trying to remember if I ever had that happen........let me think a while longer and if I remember that ever happening I'll get back to you all.:ok:

My procedure (as per checklist) for a Lycoming O-360 (180 hp) is to do a dead mags check at no more than 800 rpm (off, then quickly on again). It's what I've always done.

I have never had the engine go "bang".

I figure somewhere, somehow the boffins at Lycoming and Beech have this all figured out if they publish it in the procedures. That's what the POH is for.

O.K. I quit.

I was trying to point out that there are better ways to check for a live mag than turning both mags off at any RPM above idle.....and as far as someone getting hurt because of a one in hundreds of thousands of shut downs where there is a live mag problem that is a relatively low risk factor.....or do engines now just start up all on their own.

Would it not be better to train people to never touch a prop without taking into consideration the ignition may be on?

We should all do all we can to ensure both mags are dead, 1 in a 1000 or 1 in a million it does not matter. One of my very best friends a very experienced pilot and engineer has recently been injured by a prop to the point of nearly not making it. So consider everything from a non pilot walking by and touching the prop to a tired after a long day engineer doing one last job.
There are not many situations where something like a prop is used or left unguarded

Just out of interest how much movement of the prop is required to cause a problem if the mags are still live? I ask because when training (and after having had it drummed into me to treat all props as live) I was amazed to see experienced pilots manoeuvering their aircraft by pulling on the prop --one hand on each blade eitherside of the spinner.

Well if you hear a metallic clunk, then thats the impulse mechanism on the magneto operating. If the blade doesn't kick over then you've got away with it.

Mind you how often do pilots/engineers pull on props without harm.
And move prop if only to attach tow bar?
Then comes the old question of turning the prop backwards and problems with vac pumps.

I always teach 'keys on top of panel' in full view to try to ensure the mags are off, but thats no guarantee if the key switch is worn.

Was pax on type rating check flight awaiting my turn up front in a Seneca when the pilot being checked out switched off the mags on the live engine on an engine out drill. Very interesting when he switched them back on!

Chuck, I was taught (RAF trained) that the pilot's mag check on shutdown was for the safety benefit of the engineer/mechanic who might need to move the prop before the aircraft flew again.

I have. I lost power and had a lot of shaking and vibration in a Cessna 206 full of passengers once, not long after departure. I pushed the mixture up turned on the boost pump, and switched tanks while I made a turn for a road to land. I had about eight hundred feet of altitude. I checked mags, found one completely dead, and when switching to the other, failed to properly retard the throttle and got a tremendous bang. I was one mag had failed and the other had not, and the improper timing of the failed mag made it appear nearly dead...returning to both restored the good mag and produced a similiar effect to shutting off the ignition and turning it back on again.

If the engine dies, let it die.

If in flight or in a situation in which letting it die isn't practicable or feasible, then retard the power to idle.

I was behind a Cessna 188 AgTruck (like a Cessna 185 with the wing on the bottom, instead of the top) some years ago as we prepared to go spray a field. I saw a BIG fireball erupt out his exhaust as it separated from the airframe, the result of this very conversation's topic.

I've done airborne mag checks for many years in the past because it wasn't practical to do them on the ground on gravel and other loose surfaces with any kind of runup...same thing...you don't need to go to the off position to verify that the mag has grounded out. The OFF position doesn't accomplish anything during your check that the left and right positions haven't already done. The ONLY purpose for the off position is to have a switch position leaving both mags grounded.

When you do a mag check you ground each mag individually, and safely, without any danger of an afterfire or a backfire. If you switch to the left or right mag and both die, you know that you've got one successfully grounding, and the other was already dead. Whatever occurs, if the engine dies or you inadvertantly switch to OFF, let the engine stop completely if at all possible before you do anything. If you're in flight and this occurs, either through your own error or through a problem such as a failed mag, you may not be able to let the engine stop first. In that case, retard to the minimum power possible to minimize any potential damage...but don't make a practice of doing it on a regular basis.

I have seen airplanes set on fire through backfires and after fires, sometimes priming issues, sometimes fuel leaks, and yes, even improper use of the mag switches during a power run. There's just no need to be doing that.

How far does the propeller need to be moved? It may be moved a long distance, or it may not need much movement at all. Treat the propeller as though it could fire at any time. Some years ago I was working on an airplane when a pilot climbed into the cockpit and attempted to start the engine. I was working through a panel on the side of the airplane directly in front of the propeller, and could have been hurt or killed. Simply being by the propller...you just don't know who or what is going on that might cause something unexpected...treat the propeller as though it could turn at any time, and the engine as though it could start any time.

We received a Twin Commander from Central America a few years ago which had received questionable maintenance. We weren't aware until it was delivered, exactly what was wrong. The first time we entered the airplane and turned on the master and the beacon, the left engine started. Bizarre? You bet. Imagine the surprise if someone were standing nearby. How or why the engine may start, and how unexpected it may be...the only sensible solution is to always expect it. It's going to start. The propeller is going to turn. Respect it accordingly, and never rely upon the ignition switch.

Someone mentioned removing the key and resting it on the glareshield. I do that too; I think it's a good practice, in airplanes that use keys. If I'm in the cockpit and someone else is moving the airplane or near the front of the airplane, I have a habit of keeping my hands in plain sight above the glareshield as evidence that I'm not about to do something stupid to hurt them. It's been my practice for many years.

Even if the engine doesn't start, simply being near a point where it might move due to internal compression could be enough to bump the prop and give you a nasty cut or injury. It doesn't even need to fire. Treat it carefully. Never grab the prop in a way that you could be hurt if it rotates, or that it won't slip out of your hand if it rotates, and never stick a part of your body through the prop arc if you can at all help it. What the propeller can do to you, there's not enough time in one lifetime to undo or fix.

Common sense does more to keep you safe than any amount of mag checking or positioning of keys. Do it all and double up on the common sense, and you'll be fine. And so will your engine, if you follow the manufacturer recommendations...all of them...not just what's in the POH or AFM.

I always pull my prop through 8 compressions before starting to prime and also to check compression. I therefore always check for a dead cut when shutting down.

Can't see why it would cause a problem.

ZA

What do you accomplish by pulling it through in the first place? What are you priming? Are you putting prime into the cylinders as raw fuel, and trying to "prime" it by pulling it through (and thus washing any protective oil off the cylinder wall and creating further engine wear on start up)?

I need a differential compression tester to determine engine compression...amazing that you can do it by feel as you "prime" the engine.

A post flight runup, which is recommended by your engine manufacturer, will tell you what you need. Do you think that the engine will suddenly lose compression between the end of the last flight and the start of the next, or that you can't tell if you have a cylinder problem after you fire the engine up?

I've heard a lot of ridiculous ideas for pulling it through...limbering up the oil (no, you're not), priming (not, you're not), checking compression (no, you're not), yada, yada, yada. The only valid reasons for pulling it through are checking for hydraulic lock in a radial engine, or finding the impulse coupling and compression stroke for a hand-propping session...or performing maintenance during a mag timing or differential compression check...specialized things you should leave to your mechanic.

This "dead cut" several have mentioned presumably means they go to off with the magnetos. Fine, if you intend to let the engine come to a complete stop, and then wish to restart. However, it tells you nothing and accomplishes nothing. You already know the mags are grounding out by doing a standard mag check: both-left-both-right-both. No need to go to off. Off doesn't tell you anything that the other positions haven't already told you; the P lead is intact and grounding the mag through the switch.

When you pull that propeller through by hand to "prime" it, are you doing it in the same manner as you would when hand-propping it, and taking the same precautions? Do you pull it through and step back out of the way, as you would when hand propping, or are you staying in the prop arc and simlply pulling it through? Whether you've checked the mags grounding out or not, it can still fire...and should be treated accordingly.