Magnetos---best way to check

[RansS9]

I have heard differing advice on the best ways to test the mags and in particular to check they are not still live after shut-down.

Any advice from the forum greatly received.

[Der absolute Hammer]

SHUT DOWN CHECKS?
On stand.
Away from fuel pumps.
Park brake on.
Radios off.
Throttle set. IDLE.
Magneto switch.....
To Left... back to ON.
To Right...back to ON.
To OFF...back to ON.
(Deadcut check).
SHUT DOWN.
If leaving key in aircraft (if it has a key) do not leave in place. Hang it on a knob.
Always believe Magnetos ae live.
After shut down and all systems off, there is not a way to check. Only you could swing the propeller, but that would be quite stupid.

[FlyingForFun]

There was a recent CAA publication (GASIL, possibly) in which they quoted the authority from another country - from memory, I think it was the New Zealand authority. The quote included the suggestion that the mags are switched briefly to Off, then back to both, in order to check they are not live after shut-down. However, in a side-bar, there was a note that the CAA do not agree with the New Zealand authorities, and that they think it better to go to each mag in turn then back to both.

Personally, I prefer each mag and then back to both, but that's probably only because that's the way I was taught. I don't know if going to Off would actually damage the engine - I'm guessing not, if the New Zealand authorities suggest it - but it certainly doesn't sound very pleasant when you do it. The only advantage of going to Off that I can think of is the small possibility that there is in internal fault in the switch itself which wouldn't be detected by checking each mag in turn.

I've searched briefly for the reference, but can't find it. Does anyone remember the article I'm thinking of, and where it was?

FFF
----------------

[Johnm]

Switching to both off and then on isn't likely to be that good for an engine I would have thought. You can usually detect a slight drop at tick over by switching each on off in turn and that's a clear indication that they're working properly.

Of course if one mag has failed the engine will try to stop and it will pop and bang when you switch back to the other, trust me I know:uhoh:

[IO540]

One should read the POH on this one.

The action of switching off BOTH mags checks for the very rare magneto switch failure case where individually they check out but together there is a problem. I cannot imagine how (mechanically) this is possible but I suppose one cannot completely rule it out.

My (TB20) POH says you can do this but only below 1000rpm otherwise you risk exhaust system damage.

I never do it. Did it once a few years ago and got a big bang.

[BackPacker]

There's two types of magentos checks.

When checking the magentos during the runup (briefly to L, both, R, both) what you are checking is that all spark plugs are functioning fully. You ground the R circuit so the engine only runs on the L magneto. If any of the magnets, or leads, or spark plugs is in the L circuitry is duff, you are not firing on all cylinders (literally) so you lose 1/4 or 1/6 of the performance of the engine (depending on the number of cylinders of course) and this translates into a massive RPM loss and rough running (trust me, you'll notice: I had this on my first solo). In contrast, if everything is allright the only thing that happens is that the flame front is ignited slower so the RPM drop is small.

When doing a dead-cut check you're not checking the leads, spark plugs and everything, but you are specifically checking that the grounding circuit is in place for all magnets. If you were to do this one magneto at a time (L, both, R, both) a normal drop in RPM only suggests that some of the grounding circuitry is in place but you cannot be sure that all circuitry works as designed. Think about it: suppose you switch to L but the grounding circuit for one of the plugs in R is still active. Three cylinders are now running on the L spark plug alone and the fourth is still running on L+R. All cylinders are firing and you are not going to be able to distinguish this RPM drop from what it should have been. But when you switch the magnetos to "Off" you are still left with one live spark plug in R.

Now my main question is: how likely/possible is it that the grounding circuit fails partially. In other words: that some of the spark plugs are alive in a certain grounded circuit, and other are not? If the only failure mode of a magneto is a complete, 100% failure, then of course you can detect whether the grounding circuit works by switching L/Both/R/Both. But if you have a magneto system (or electronic ignition, or whatever) that can fail in such a way that some spark plugs in a circuit are left "live" while others are grounded, then a dead-cut check is the only way to find out.

Disadvantage of a dead cut check is of course that you blow unburned fuel into the (potentially hot) exhaust (+ turbo if you have one) and eventually in the atmosphere. This might be dangerous depending on exhaust design and how hot the exhaust really is. The other disadvantage is that you could do the dead-cut test too long and thereby stop the engine, with air/fuel mixture still in the cylinders. Maybe not the way it was designed, although for instance the Rotax 912/914 is intended to be stopped by grounding the magnetos, instead of idle-cut-off.

What surprises me is that the authorities are playing a role in this. Isn't this something that's simply written in the POH? A dead-cut test might be completely sensible for an engine with a direct drive, heavy non-adjustable prop (lots of momentum), no turbo, traditional magnetos and straight pipes as exhaust, but completely inappropriate for a geared drive engine with a light CS prop, turbo, electronic ignition and a silencing exhaust system.

[FlyingForFun]

I have a feeling (don't have anything to hand to quote, though) that many POHs simply say "Magnetos.... check" at this point in the checklist, and don't give specific advice on how to do the check (except, during the power checks, for giving limits for the allowable rpm drop).

My comments on this thread only relate the case where there is no specific advice in the POH. If the POH contains more details, then this should override anything you read on these forums.

FFF
-------------

[GonTek]

You will find that if you go from " mags off and back to on /both "you will hear a loud bang as you ignite unburnt fuel and possibly blow the exhaust off.
I have inadvertantly done this once with no problems and very carefull not to do it again.

Safe flying

Gontek

[BackPacker]

many POHs simply say "Magnetos.... check" at this point in the checklist

If you browse a little further into the POH there should also be an amplified/expanded checklist with procedures on how to perform these checks, what the failure parameters are and so forth.

Having said that, I just checked the PA-28 Cadet POH which I have here and there's no mention of a magneto check at shutdown at all.

[RansS9]

The crux of the problem....... is L-both R-both good enough or is All-both needed? And if so does it run the risk of damaging the engine? Is that risk dependant on the exact prop engine exhaust installation?

It never ceases to amaze me how such basic procedures end up shrouded in mystery; with most of us doing "just by rote" what we were taught by people who were also "just by rote" doing what they were taught!

[Chuck Ellsworth]

Wow!!!

I lose faith in the human race when I read these discussions.

Why would anyone want to chance damaging their engine or exhaust system by turning the ignition off at 1000 RPM and letting the fuel air mixture race into the system then turn the ignition back on just to see if there was a live mag?

Checking each mag separately will determine if there is a live mag....if you do not get a slight drop in RPM when turning off each mag separately then you obviously have a live mag.

If you are truly concerned that there may be a one in a million chance that there is a live mag and are determined to turn the switch to both off then do so at idle and allow the engine to stop......

Where do all these weird ideas come from?

Sort of reminds me of how many times I watch people lean out of their airplane doors and call " Clear Prop " and then hit the starter with a time frame so short that even super man would not have had time to move away from the prop had he been standing near it.

The monkey see monkey do mentality is truly at work in society.....and best observed by hanging around your local airport and observing the habits of a lot of pilots.

[T10]

As an operator of an aircraft powered by a Gipsy Major 10-1/1 the method of shutting down is move throttle to idle and switch off both mags and as the rpm decays open the throttle fully.
This proves that it is the switching off of both mag switches that has caused the engine to stop and not a lean cut.
Bottom line treat all props/engines as live no matter what!!
Just my thoughts

T10

[Brian Abraham]

This the article FlyingForFun?
http://www.caa.govt.nz/Publications/Vector/Vector_Articles/Magneto_Check_Basics_MarApr08.pdf

[stevef]

Speaking as an engineer, it's standard to perform a 'dead cut' check at low rpm (idle) to ensure that the grounding circuit is functioning correctly. I've never had any backfires whilst doing this (it only takes a second to switch to Both Off and back on again and it's obvious to the ears when the ignition cuts).
I once heard someone inadvertantly 'dead cut' a Dakota at max rpm and switch the mags back on again. The resultant explosion of unburnt fuel in the exhausts could easily have been heard a mile or two away!

[Leezyjet]

Where I trained in South Africa, it was always L,both,R,both,off,both. In the Uk however, it seems to be the norm to not bother with the "off" bit.

I was also taught not to do mag checks near the fuel pumps in SA, but my CPL instructor in the UK said I had to follow what the check list said regardless of where I was parked !!.

Another thing I was taught in SA was to do a mixture check during the run up, by leaning out the mixture until it runs rough, then winding in 3 full turns, then back to fully rich. When I did this in the UK, (whilst using my SA check list) the Instructor nearly had a heart attack.

The UK check list also just states "mixture rich", however try taking off at a high altitude airfield on a hot day with a fully rich mixture and you will be lucky to get off the ground - probably not a major problem in the UK, but if one goes off touring overseas it could be. In SA it is "mixture - Rich for sea level" so you know to set it depending on the requirements of the airfield elevation/temp on the day.

Now I just do what each club wants depending on where I'm flying, it's their toy after all.

:\

[jxk]

Talking Chipmunks; I recently encountered one which had a live mag and the method for shutting down this particular aircraft was to turn both mags off. As you might expect the engine just ran on - the only way to shut it down was to turn off the fuel (NB this aircraft not fitted with ICO). Now, turning the mags off to shut down the engine seems like a good way of checking the the mags are dead in L, R and OFF configurations without causing a big bang in the exhaust system. The danger of a live mag is not from pilots who know the routine but spectators who inadvertently lean on the prop. On most other aircraft it's another good reason to leave mixture in ICO.

[FlyingForFun]

Brian,

No, the article I'm thinking of was published by the UK CAA. The one you've linked to may be the one the CAA quote, though, I'm not sure.

The reason I'm looking specifically for that article is because it highlights how you get different ideas from different countries and different authorities.

FFF
------------------

[SNS3Guppy]

Now my main question is: how likely/possible is it that the grounding circuit fails partially. In other words: that some of the spark plugs are alive in a certain grounded circuit, and other are not? If the only failure mode of a magneto is a complete, 100% failure, then of course you can detect whether the grounding circuit works by switching L/Both/R/Both. But if you have a magneto system (or electronic ignition, or whatever) that can fail in such a way that some spark plugs in a circuit are left "live" while others are grounded, then a dead-cut check is the only way to find out.


Where one isn't grounding plus or leads, but instead grounding the magneto, one can't have a "partial grounding." An engine may continue to run when hot particles in the engine keep it running...it does happen that engines continue sometimes to "diesel" or chug on after they've been shut down, but that's another matter not connected to the mag.

If your mag is inadvertantly switched to off, let it die, restart, and perform your check again.

Why the need to determine if the mag is grounding out? Yes, you can do a mag check to determine that the mags are grounding and that the p-leads are intact and doing their job...but the prop should be treated as though it's hot all the time...whether the check is sucessful or otherwise. Much like handling a firearm; the safety isn't the mechanical safety on the gun...it's the shooter's straight finger, and his brain. The safety for the propeller isn't the grounding of the mag. It's the common sense respect accorded the propeller. One should always assume the engine can and will fire if the prop is rotated, and treat it accordingly.

What the mag check really does, during a post flight run-up, is act as part of the post-flight inspection and is really for maintenance purposes. Just like finding a problem on the post-flight walk-around, a post flight run-up gives you a last chance to catch a problem and get it fixed.

You'll find that Continental and Lycoming both give detailed descriptions of what should be done, and how it should be done, for a post flight run-up...which includes the idle mixture check you should be doing each time. As others noted, it's probably not being passed along, and too many pilots simply go through the motions because they've seen others do it.

Not everything you need to know is in your pilot operating handbook or aircraft flight manual. Remember, that manual was put out by the airframe manufacturer...publications are also available from the engine manufacturer which give valuable insight into properly operating that engine.

[DB6]

Had a live mag a couple of weeks ago, found it on a drop check and confirmed by switching both off and the engine ran on. Can't see any real reason for switching off both mags, but having said that we used to do that at school I used to teach at, always at idle though and for a fraction of a second, just enough to hear the engine miss a beat.
The problem with the dead cut check - in a training environment anyway - is that students tend to be a little ham-fisted and it was not unusual for them to get the key stuck in the 'off' position and wrestle to try and get it back to 'both'. You had to jump in quickly then and tell them to leave it or you got the aforementioned POP as the unburnt fuel went off (this was much worse if the engine wasn't at idle). Better not to in my book, unless to confirm a suspected live mag.

[ericferret]

One thing for sure don't turn both mags to off and then back on again if it's a helicopter.
You may well damage the drive train.

The Bell 47 has shear bolts in the main transmission, the sudden shock load is enough to damage them.

I suspect that geared engines probably don't like it much either.

[stevef]

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.)

[jabberwok]

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.

[greenslopes]

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!!

[SNS3Guppy]

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.


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?

[Avtrician]

Quote:
Now my main question is: how likely/possible is it that the grounding circuit fails partially. In other words: that some of the spark plugs are alive in a certain grounded circuit, and other are not? If the only failure mode of a magneto is a complete, 100% failure, then of course you can detect whether the grounding circuit works by switching L/Both/R/Both. But if you have a magneto system (or electronic ignition, or whatever) that can fail in such a way that some spark plugs in a circuit are left "live" while others are grounded, then a dead-cut check is the only way to find out.

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.

[Them thar hills]

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........

[IO540]

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.

[ShyTorque]

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.

[jxk]

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.

[Chuck Ellsworth]

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:

[BeechNut]

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.

[jxk]

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

It doesn't matter how often this check is done with negative results; I had a live mag a couple of weeks ago and if saves anyone from being injured it must be worthwhile.:)

[Chuck Ellsworth]

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?

[T10]

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

[RansS9]

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.

[BigEndBob]

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!

[ShyTorque]

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

[SNS3Guppy]

I have never had the engine go "bang".


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.

[Zulu Alpha]

There are not many situations where something like a prop is used or left unguarded

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

[SNS3Guppy]

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.

[ShyTorque]

The Bulldog had (has) a rotary mag switch, which controlled both mags, rather than two separate ON/OFF switches.

I can remember one of the Standards Squadron examiners telling me that it was possible for the metal contacts in the switch itself to malfunction such that at "OFF" the mags, or at least one of them, wasn't grounded.

The mags could still be grounded separately as normal, because the switch used different contacts inside the switch body to ground them at the L/R positions.

Unless a ['Both mags OFF / "Drop & Stop"] check was done, you wouldn't know the switch contact had failed.

[Chuck Ellsworth]

We can end this discussion by having everyone go back and not only read ..but try and understand SNS3Guppy's last two posts......

Good posts SNS3Guppy.......

[SNS3Guppy]

Good Lord. I've entered some kind of time warp.

[jxk]

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.
This is not true. If you've ever had a rotary type mag switch apart you will know that the OFF position is internally a different terminal for the contact to 'make'. So, testing L & R is not testing the OFF position within the switch. And yes I have seen this problem and that was reason I took the switch apart. This may be the reason why the DEAD cut check appears in some check-lists.

[ShyTorque]

Exactly, which is why we should all obey the POH and not rely on advice given elsewhere if it contradicts.

Individual magneto switches present a different situation to a rotary, combined switch. In an aircraft fitted with individual mag switches, I would certainly agree that a dead cut check is unnecessary.

Personally I would prefer individual mag switches as I see them as a safer way of doing things.

I know of one case where one of our RAF Bulldog aircraft developed a badly rough running engine and was unable to maintain height. My colleague who was on an instructional sortie then discovered the rotary mag switch was the problem and it was intermittently grounding out the mags. Going to "L" and "R" didn't cure the fault. He was able to recover the aircraft by holding the switch in a certain position just away from the "BOTH" setting while his student flew the aircraft to the overhead where they cut their losses and flew a forced landing pattern to the grass.

[Chuck Ellsworth]

Good Lord. I've entered some kind of time warp.

Why SNS3Guppy, is there something wrong with me agreeing with you? :E

[SNS3Guppy]

Wrong, no. Just feel a little light headed, that's all.

[jxk]

Individual magneto switches present a different situation to a rotary, combined switch. In an aircraft fitted with individual mag switches, I would certainly agree that a dead cut check is unnecessary.
Not necessarily true. If you take a Chipmunk as a case with individual switches although you have tested left and right it's very difficult to detect a RPM drop so all you've done is proved that both mags are working. You've haven't proved whether one mag is permanently live or not. I know I'm a pedantic old fool - sorry. But this very situation occured to my knowledge recently and the only way to eventually stop the engine was by turning the fuel off, which of course took some time. Certainly worried the pilot who thought he was never going to be able to stop the engine. No ICO you see!

[ShyTorque]

Doesn't matter what theory or rule is quoted - we all know that someone will beg to differ.

In that case, the dead cut check is when you switch off both mags :ouch:

If you want three opinions, put two pilots together ;)

What does the POH say?