Tue Dec 31, 2019 4:39 pm
#1737915
I'm glad I only have one mag to worry about.
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.
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.
David Wood wrote:I’ve never been entirely clear why that helps prevent a kick-back or running on. But it does.
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.
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
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'.
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.
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.