Primarily for general aviation discussion, but other aviation topics are also welcome.
By Bathman
#1809906
A4 Pacific wrote:
I can't see any requirement for such a check in my PA28-161 POH


My Piper PA28 Check List says:

AFTER START

6. Magnetos........... Check for dead cut.


Is that the Piper checklist? because it doesn't say it in the POH.

And do you actually do a dead cut or just check L & R?
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By matthew_w100
#1809908
I've never understood a complete dead cut test. It sounds like a great way to full your silencer with fuel/air mix and let it go bang. What do you learn that you don't learn from turning the mags off one at a time? I'm sure there *is* something - I just don't know what it is.
By A4 Pacific
#1809910
Bathman

My reference is Pooley’s flip style checklist for the PA 28.

From the OP:
During this Lockdown 2 re reading 'Be A Better Pilot' by Alan Bramson', in a section on power checks and specifically mag checks, this got my attention.
"You are not looking for an RPM drop at this stage (although many pilots mistakenly do) but if a magneto is not working there is little point in taxiing out"

He seems to be implying that you're really only checking there's no mag dead cut.


As the “Be A Better Pilot’ reference implies, my understanding is, at this stage you are looking for absence of a dead cut when selecting L & R.

As ‘As I CFIT’ implies, the manufacturer need not mandate everything in their POH that could be considered ‘best practice’?

A useful link?

http://enseinder.com/files/MagCheck.pdf
By OhNoCB
#1809913
Mag checks (maybe mags in general!) seem to cause great confusion/opinion. Especially in my local club scene.

Our club checklist asks for a deadcut check prior to shutdown. The first time I did it during a club checkout the instructor near had a turn and told me never to turn both mags off, and that we do them one at a time. I suggested that this wasn't a dead cut check and he told me he would show me in the POH during debrief. Turns out that the POH doesn't give two hoots about checking the mags for anything after landing, the last mag check is during the run up. Personally I think a mag check is good before stopping the engine, both for ground safety (the original reason for a dead cut check I think? stand to be corrected.) and also to alert to any issue that might otherwise only be discovered on the next flight. One aeroplane I used to fly had a spell of going through mags like no ones business, and it wasn't uncommon to find a duff one at either end of the flight. I do favour checking individually though, to avoid loud noises.

It raises another point however about checklists. The only official 'checklist' is the one in normal and emergency procedures sections of the POH. If Pooley's, or a club or an individual decide to make their own, that is absolutely fine but I personally think it is good to check it against the POH for differences, and if there are differences then find out why there are differences, and decide for yourself if they are warranted/correct.
By riverrock
#1809914
I've seen an issue in the switch, where "off" kept one mag live - found by turning mags to off (very briefly) at the end of the flight (what I call checking for a dead cut).
When starting, no need to check for off - you won't be using that anyway, but checking each Mag is working saves a return trip from the run up area.
By A4 Pacific
#1809916
I do favour checking individually though, to avoid loud noises.

As I understand it, the reason a dead cut check (ie ignition to off) is required, is to ensure the integrity of BOTH ‘P’ leads. If one is not in tact, then what will you ‘detect’ by selecting L & R before shut down?

After shut down you will still have one live magneto. :shock:
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By Dave W
#1809917
A4 Pacific wrote:A useful link?

http://enseinder.com/files/MagCheck.pdf

The author of that PDF claims that:
The dead cut check after start is to ensure that the bendix on the starter motor
disengages fully.

Eh?

The bit later on about a "dead-cut check" prior to shutdown is how I understood things:
The dead cut check just before shutting down is for an entirely different reason. An aircraft ignition system works opposite to a motor vehicle in that when you switch off the key, the wires are connected (earthed). In a motor vehicle, when you turn off the ignition, the wires get disconnected.

The reason it is set up this way in an aircraft is so that, should a wire come loose for some reason, the engine will continue running. In a motor vehicle, it doesn't really matter so they opt for the easy option of merely killing the circuit should a wire come adrift.

Therefore, just before shutting down an aircraft engine, a dead-cut check is done in order ensure that when the key is in the off position, the electrical circuit is dead. (If there were a loose wire the engine would continue running).
By Maxthelion
#1809920
matthew_w100 wrote:I've never understood a complete dead cut test. It sounds like a great way to full your silencer with fuel/air mix and let it go bang. What do you learn that you don't learn from turning the mags off one at a time? I'm sure there *is* something - I just don't know what it is.


If the engine doesn't stop firing when it's got both mags off, you learn that one of the P leads is disconnected, leaving you with a permanently live mag and therefore prop. But you're quite right, it is a great way to wake up the neighbourhood.
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By Charles Hunt
#1809921
Surely it's only a momentary thing. You flick the switch, hear the revs starting to fall and switch it back. 1/4 second maybe?
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By Charles Hunt
#1809922
As a matter of interest has anyone ever found a 'mag' drop on a Jabiru 2200?

I never did on mine.
By Maxthelion
#1809925
Nope, mine neither. I now also have access to a Europa that has the 3300 six in it, and that doesn't have a mag drop to speak of either. It's the modern solid state ignition it has, which gives a much stronger spark at the expense of needing a very healthy battery and starter to get going as the mags don't wake up until something like 600rpm.
By Boxkite
#1809927
Charles Hunt wrote:As a matter of interest has anyone ever found a 'mag' drop on a Jabiru 2200?

I never did on mine.

I have noticed an ever-so-slight difference.

But, in answer to some of the enquiries about a dead-cut test, if you don't detect a mag drop, how do you know the switches are even working??
The answer is a deadcut test by switching off both mags together.
Last edited by Boxkite on Fri Nov 20, 2020 5:07 pm, edited 1 time in total.
By OhNoCB
#1809930
A4 Pacific wrote:
I do favour checking individually though, to avoid loud noises.

As I understand it, the reason a dead cut check (ie ignition to off) is required, is to ensure the integrity of BOTH ‘P’ leads. If one is not in tact, then what will you ‘detect’ by selecting L & R before shut down?

After shut down you will still have one live magneto. :shock:


I listen for a change in engine tone with each one. Not worried about a defined drop but I will listen for a change. If none is present then I will do a proper dead cut.

Does this not achieve the check of the P lead integrity?

As I said, these things seem to cause confusion and debate, and I wasn't elevating myself above that, so I am happy to learn a better way.
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By matthew_w100
#1809947
Maxthelion wrote:If the engine doesn't stop firing when it's got both mags off, you learn that one of the P leads is disconnected, leaving you with a permanently live mag and therefore prop. But you're quite right, it is a great way to wake up the neighbourhood.


But I learned that with the one by one method. Mag left - 50rpm drop. Mag right - no drop. Therefore left mag not shorting out. Inspection under the bonnet confirms broken wire. This actually happened six months ago. We put a big "live mag" sign on the prop till it was fixed.

What can i learn from a dead cut that I *can't* learn one by one?

Charles Hunt wrote:Surely it's only a momentary thing. You flick the switch, hear the revs starting to fall and switch it back. 1/4 second maybe?


At 1200rpm , 1/4 second is 5 revolutions, 10 power strokes in a 4 cylinder engine. That's a lot of fuel/air mix!

Boxkite wrote:But, in answer to some of the enquiries about a dead-cut test, if you don't detect a mag drop, how do you know the switches are even working??
The answer is a deadcut test by switching off both mags together.


If I don't detect a drop at all *something* is wrong and I investigate further. Or rather an engineer does. But if I have detected a drop, no need to do the dead cut.

I do accept that not all engines are the same and some may not show a drop on one mag. So different rules may apply then.