Primarily for general aviation discussion, but other aviation topics are also welcome.
By A4 Pacific
#1809965
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.


I entirely agree. I’m here to learn. The problem comes when I’m invited to ‘re-learn’ what I thought I already knew! :lol: I first learned to fly on Chipmunks (about) 40 years ago. I’ve just checked my old FRCs, which call for a dead cut check both after start and immediately prior to shutdown.

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.


Have we established that perhaps some engines won’t produce a noticeable rpm drop when at idle? That certainly seems to tie in with my experience.

Certainly everything I’ve read so far suggests I should continue what I’m doing. Which is a dead cut check, then stopping the engine by starving it of fuel?

Belt and braces. Where is the real danger here?

If an unintended engine start then occurs as someone disturbs the prop, then what more could I possibly have done?

As I said. Every day’s a school day, so happy to be shown a ‘better’ way.
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By Dusty_B
#1809983
There’s a danger of talking apples and oranges here, folks. Instructors and pilots who learned on one type and carry over sayings and fads to other types without understanding why....

The dead cut check is never needed if you have conventional mag switches - as each is independent.

However, barrel/key switches are different. The grounding needs to occur at two positions for each mag, therefore a dead cut check may be needed before shutdown. This should be done at idle RPM (where the flame front will have passed across the whole cylinder before the exhaust valve opens). The dead cut isn’t needed after start, as we are only interested in ensuring that both mags are both a) working and b) grounding independently in case we need to shut one down in flight. Not so much need to shut both down!! It is done before shutdown to prove that the engine is safe.
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By matthew_w100
#1809995
Now that makes more sense! Apart from the flame front bit - if you've cut both mags there won't be a flame front to pass before the exhaust valve opens.
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By Dusty_B
#1810010
matthew_w100 wrote:Now that makes more sense! Apart from the flame front bit - if you've cut both mags there won't be a flame front to pass before the exhaust valve opens.

But the next one with a spark will have!
By A4 Pacific
#1810012
The dead cut check is never needed if you have conventional mag switches - as each is independent.

If there is a lack of continuity in the ‘P’ lead circuit, why is the switching method significant?

Or to put it another way.

If you select the Left mag switch off, and that mag actually remains live, (due to lack of continuity) how can you tell? What have you ‘proven’?

Just curious. Happy to be educated. Sure would appreciate any documented reference for this difference?

As I say, a lifetime ago everyone was doing dead cut checks in Chipmunks without keys!
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By Dusty_B
#1810013
A4 Pacific wrote:
The dead cut check is never needed if you have conventional mag switches - as each is independent.

If there is a lack of continuity in the ‘P’ lead circuit, why is the switching method significant?

Or to put it another way.

If you select the Left mag switch off, and that mag actually remains live, (due to lack of continuity) how can you tell? What have you ‘proven’?


With each mag flicked ‘off’ in turn, you are looking and listening for a drop in RPM. If you don’t get a drop, then there is a problem with either the switch or the P lead. If each switch shows continuity, switching both off at the same time gives you no more information.

With a barrel, there are four (or five) switch positions. Before shut-down you need to check that the ‘both off’ position really is working, as even if the individual off positions show continuity, the barrel switch itself could be knackered.

As I say, a lifetime ago everyone was doing dead cut checks in Chipmunks without keys!

I have no idea why that would have been needed as a check!!
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By Rob L
#1810014
matthew_w100 wrote:At 1200rpm , 1/4 second is 5 revolutions, 10 power strokes in a 4 cylinder engine. That's a lot of fuel/air mix!


It's supposed to be done at idle.
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By A4 Pacific
#1810019
With each mag flicked ‘off’ in turn, you are looking and listening for a drop in RPM.


To be fair, we’re using the wrong term. Of course it’s actually more correctly described as a ‘live magneto check’.

There are a fair number of aircraft, with either switches or keys, in which there is hardly any detectable rpm drop from idle. Good to see there’s no reason to not do it though, and everyone with keys could confirm the safety of their aircraft by so doing.

As I said. It may be ‘belt and braces’ (nothing more important than safety!) but I’ve read nothing to stop me continuing.

It seems this has been discussed before.

https://forums.flyer.co.uk/viewtopic.php?f=40&t=114318
By rdfb
#1810022
As I CFIT wrote:It's such a quick and easy thing to do and it could save you fifteen minutes straight off the bat. Why there's any resistance to doing it, I have no idea.


Let's say it takes ten seconds (edit: well OK, maybe five). If there are ten similar items, the total might be 100 seconds. Each check might allow me to abort my flight early before taxying. Is it worth the trade-off?

What will I really have saved? If I have to abort my flight then I'm going to find myself with plenty of time. I won't be in any hurry so don't care about trying to save that time. Aborting a flight is a rare event anyway.

I do currently still check my mags before taxy. Mainly this is because I believe I should check the reasons very carefully before altering anything, and this is the way I was trained. However, in principle, I would like to be removing unnecessary things from my checklist, not adding them.

An exception might be after maintenance because in my experience abortive flights are much more likely!
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By matthew_w100
#1810027
Rob L wrote:
matthew_w100 wrote:At 1200rpm , 1/4 second is 5 revolutions, 10 power strokes in a 4 cylinder engine. That's a lot of fuel/air mix!


It's supposed to be done at idle.


600 rpm then. 5 cylinders full of explosion. But my little aeroplane doesn't idle at that till it's warmed up thoroughly, by which time I'm ready for power checks. So no early bail out.
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By PeteSpencer
#1810033
There’s no check list as such in the body of our Arrow POH;
it’s a separate entity and the mag check just after start up at 1200 rpm I’ve always taken to be to check intact grounding of each mag in turn .
Then similar R and L checks at 2000rpm will , if grounding is intact, show a 50rpm drop quite clearly.

That is all I need to know : no need for dead cut checks to risk damaging detonation in exhaust system so I don’t do it. :wink:
By Boxkite
#1810055
PeteSpencer wrote:There’s no check list as such in the body of our Arrow POH;
it’s a separate entity and the mag check just after start up at 1200 rpm I’ve always taken to be to check intact grounding of each mag in turn .
Then similar R and L checks at 2000rpm will , if grounding is intact, show a 50rpm drop quite clearly.

That is all I need to know : no need for dead cut checks to risk damaging detonation in exhaust system so I don’t do it. :wink:

Would you do if the 1200rpm check yielded no changes? Or a change on only one of the switches?
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By PeteSpencer
#1810061
Boxkite wrote:
PeteSpencer wrote:There’s no check list as such in the body of our Arrow POH;
it’s a separate entity and the mag check just after start up at 1200 rpm I’ve always taken to be to check intact grounding of each mag in turn .
Then similar R and L checks at 2000rpm will , if grounding is intact, show a 50rpm drop quite clearly.

That is all I need to know : no need for dead cut checks to risk damaging detonation in exhaust system so I don’t do it. :wink:

Would you do if the 1200rpm check yielded no changes? Or a change on only one of the switches?


Depends, as the late Professor Joad would have said, what you mean by changes:

If one mag stopped at 1200, I'd call it a day, if absolutely no change , I'd stay put till engine warmed up fully then check at 2000rpm, then if one mag stopped or no drop I'd call it a day and call the engineers.

If engine ran rough on one mag I'd try to eliminate plug fouling as a cause by full leaning at max revs.

Oh and I also do post flight mag checks at 1200rpm. post shut down.

Ooops edit typo :oops: :oops:
Last edited by PeteSpencer on Sat Nov 21, 2020 11:23 am, edited 1 time in total.
By A4 Pacific
#1810070
Oh and I also do post flight mag checks at 2000rpm. post shut down.

At 2000 rpm? After landing? :shock:

As was helpfully explained earlier, if your Arrow has keys, you are not checking the functionality of your switch barrel in the off position, so why bother at all?