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Magnetos---best way to check

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Magnetos---best way to check

Old 7th Sep 2008, 06:57
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Magnetos---best way to check

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.
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Old 7th Sep 2008, 07:58
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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).
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.

Last edited by Der absolute Hammer; 7th Sep 2008 at 18:51. Reason: Error
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Old 7th Sep 2008, 08:36
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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?

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Old 7th Sep 2008, 08:47
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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
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Old 7th Sep 2008, 09:03
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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.
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Old 7th Sep 2008, 09:19
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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.

Last edited by BackPacker; 7th Sep 2008 at 11:26.
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Old 7th Sep 2008, 09:29
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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.

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Old 7th Sep 2008, 09:40
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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

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Old 7th Sep 2008, 09:46
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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.
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Old 7th Sep 2008, 11:59
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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!
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Old 7th Sep 2008, 15:15
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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.
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Old 7th Sep 2008, 15:55
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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

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Old 7th Sep 2008, 16:54
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This the article FlyingForFun?
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Old 7th Sep 2008, 17:53
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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!
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Old 7th Sep 2008, 18:03
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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.

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Old 7th Sep 2008, 18:21
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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.
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Old 7th Sep 2008, 18:29
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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.

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Old 8th Sep 2008, 08:30
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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.
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Old 8th Sep 2008, 09:12
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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.
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Old 8th Sep 2008, 09:27
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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.
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