The place for technical discussions about GA and flying.
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By T67M
#1737923
rikur_ wrote:The worse mag fault I've come across was a rental aircraft where the key could be removed in any position. Somewhat defeats the check of keys in pocket before touching the prop.


The worst fault I've come across was on my T67M where the magnetos were grounded the the key in the Off position, but both became live again when the key was removed...! Even our dead cut check (or is it live cut?) couldn't have detected this. This dangerous condition existed unknown for several weeks and was eventually found during the annual check.

Thanks to all for your thoughts - lots of valid views, but the consensus seems to be that the only potential harm is a backfire, and my experience over ten years of operating my shaeroplane is that it NEVER backfires during the check if the throttle is at idle, so I'm inclined to carry on as I have been.
User avatar
By Rob L
#1737943
The two requirements in this regard for EASA aircraft (for the moment :!: ) is for the Bendix key removal SB and the dead-cut check, the latter of which should only be conducted briefly at idle.

The latter is not rocket science, and gives an assurance that the mags will indeed be dead when the switches are in the "off" position. It's a 50-hr check requirement, and a "good thing" for every pilot to do before shut-down to ensure the aircraft is safe for the next person.

Happy and safe flying in 2020!
mick w, Charliesixtysix liked this
By riverrock
#1737977
I've seen a switch fault, where "off" didn't ground both mags, found when checking that off means off before shut down. You don't really get a mag drop at low power settings if just checking L + R separately also.
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By David Wood
#1738212
Flyin'Dutch' wrote:
In 40+ years of flying and being around small piston engines I have yet to encounter either myself or close quarters a magneto that remained 'live' with the switches in the 'off' position.


I’ve certainly experienced it on a Gypsy Major; twice, I think. It’s been down to a disconnected P Lead on that mag.

Back to the OP's point, I’m not sure that I see any value in switching both mags off and on again as part of the pre-shut down. It proves nothing that you haven’t already proved with the 'drop no stop'. On a Gypsy Major, however, the shut down is then done with the mags, simultaneously opening the throttle wide. I’ve never been entirely clear why that helps prevent a kick-back or running on. But it does.
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By Flyin'Dutch'
#1738214
David Wood wrote:I’ve never been entirely clear why that helps prevent a kick-back or running on. But it does.


I suspect as it changes the spark timing whilst at the same time slowing the rotation of the engine.
User avatar
By Rob L
#1738266
David Wood wrote:On a Gypsy Major, however, the shut down is then done with the mags, simultaneously opening the throttle wide. I’ve never been entirely clear why that helps prevent a kick-back or running on. But it does.


Nothing to do with the mags; it's all about butterflies. And vacuum. On such an expensive engine, it's worth learning all about it.

Rob
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By Flyin'Dutch'
#1738268
Go on then, enlighten us, all ears!
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By David Wood
#1738321
Rob L wrote:
Nothing to do with the mags; it's all about butterflies. And vacuum. On such an expensive engine, it's worth learning all about it.

Rob


I’m all ears. I’ve heard various explanations, none of which have really convinced me.
User avatar
By T67M
#1738347
David Wood wrote:Back to the OP's point, I’m not sure that I see any value in switching both mags off and on again as part of the pre-shut down. It proves nothing that you haven’t already proved with the 'drop no stop'.


I don't quite understand this view. I agree that the P-lead wiring is checked adequately in the "drop no stop" check, but the functioning of a key-switch itself in the "off" position is not checked by moving it to the other positions. It would be very different if you have two independent toggle switches though (as per a 'Moth) - then I agree with you completely.
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User avatar
By Rob L
#1738381
David Wood wrote:
Rob L wrote:
Nothing to do with the mags; it's all about butterflies. And vacuum. On such an expensive engine, it's worth learning all about it.

Rob


I’m all ears. I’ve heard various explanations, none of which have really convinced me.


The purpose of opening the throttle after you switch the mags off is to drastically reduce the vacuum on the engine side of the carburettor butterfly. This reduces the amount of fuel drawn into the cylinders, so minimising the risk of running-on. This is also why on engines fitted with a throttle that goes to idle cut-off (ICO), the engine is shut down using that*.

Both methods have the added benefit of cleaning the cylinders of unburnt fuel, which could otherwise wash the oil film off the cylinder walls.

To be honest, I don't know what happens with the accelerator pump (for carbs so fitted) that is often connected to the throttle.

* For engines fitted with ICO, the rpm should just rise (maybe by only 25rpm) immediately before shutting down.
User avatar
By David Wood
#1738417
Rob L wrote:
David Wood wrote:
Rob L wrote:
Nothing to do with the mags; it's all about butterflies. And vacuum. On such an expensive engine, it's worth learning all about it.

Rob


I’m all ears. I’ve heard various explanations, none of which have really convinced me.


The purpose of opening the throttle after you switch the mags off is to drastically reduce the vacuum on the engine side of the carburettor butterfly. This reduces the amount of fuel drawn into the cylinders, so minimising the risk of running-on. This is also why on engines fitted with a throttle that goes to idle cut-off (ICO), the engine is shut down using that*.

Both methods have the added benefit of cleaning the cylinders of unburnt fuel, which could otherwise wash the oil film off the cylinder walls.

To be honest, I don't know what happens with the accelerator pump (for carbs so fitted) that is often connected to the throttle.

* For engines fitted with ICO, the rpm should just rise (maybe by only 25rpm) immediately before shutting down.


Interesting. That sounds a little more convincing than some of the other ‘explanations' I've been offered. Thank you.