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By Rob L
#1738427
David Wood wrote:Interesting. That sounds a little more convincing than some of the other ‘explanations' I've been offered. Thank you.


David: would you care to share the other explanations you've received? Sometimes, different advice has the same intended result, depending upon the understanding of both the sender and the recipient (me, in this case!).
For example: although I have worked on Gypsy engines, I can't recall if they have accelerator pumps, as VRB_20kt rightly asks. Do Gypsy engines have them?

Rob
(always willing to learn)
User avatar
By PaulSS
#1738477
On a Gypsy Major, however, the shut down is then done with the mags, simultaneously opening the throttle wide.


I honestly don't remember doing that with the military Chipmunks. I've only got 150hrs on them and it was some time ago, so my atrophied brain has probably deleted the memory. I seem to recall just switching off the mags and then shutting off the fuel. Of course, the rear mags would still be on if flying solo unless the ground crew turned them off or the pilot did (if it was a weekend and gliders were being towed into the air....when no ground crew were around).

Pushing the throttle forward would have allowed the mixture to be leaned but I really don't think that was part of the routine.
User avatar
By David Wood
#1738546
It’s certainly part of the Tiger Moth handling.

The various explanations I’ve heard are: 1) that it “flushes cold air through the cylinders” allegedly reducing the risk of the engine running on; or 2) that it advances the mags thereby reducing the risk of running on; or 3) that that’s just how it’s done.

The GM has no accelerator pump so the suggested reasoning that in dramatically reducing the manifold pressure by opening the throttle after cutting the mags one reduces the mixture is the most plausible that I’ve heard.

In any event it works, whereas just cutting the mags produces a lumpy, kicky stop,; and just turning off the fuel takes a while to have effect and produces a rather coughing stop. One of the reasons I don’t like just turning off the fuel is that it opens up the possibility of then forgetting to turn off the mags.
By TopCat
#1738584
I understand the sense in assuming that any prop can leap into life at any time, and I always do, but this is a question about the actual - rather than hypothetical - risks of a live mag.

I've always shut down using the POH method - mixture to full lean, wait till the engine stops, then mags to off.

I've always understood the rationale for this as ensuring that there's then no fuel that could ignite, even if the mags aren't really off.

So it seems to me that the only way of having one of the notoriously unpleasant accidents is if the mixture is subsequently set to rich, and then the prop is hand-swung by someone assuming that the mags are off when they aren't.

Is there any other route to an accident?

I'm thinking of ground crew, repositioning the prop a bit to make it easier to move the aeroplane about, or perhaps turning the engine over a few times in cold weather (also recommended in my POH) - but if the mixture hasn't been returned to rich, is there any way at all that it could fire with live mags?
User avatar
By Rob L
#1738588
Yes, TopCat...you are quite right for your POH. What engine are you flying? Injected or carburettor? (loaded question)

It's not so much about the likelihood of a prop suddenly swinging into life, but about the consequences of it doing so (risk management 101!)

As regard "actual vs hypothetical" : who knows who has been fiddling with the prop since you last saw it? Or what corrosion has set in to the mag switch & the P-leads, etc since the last time it was used....
There are too many "unanswerables", but magnetos and the associated ignition circuitry are designed to fail "live" rather than fail "safe" for very good reason!

There was a recent report in the the last quarter CHIRP report of someone getting rapped on the knuckles by a live prop. (or perhaps it was AAIB; I can't recall). I do know someone who got his knuckles knocked (ouch!) about 20 years ago. He was disturbed by a passer-by during his pre-flight, and inadvertently left the mags on during his subsequent resuming of the pre-flight. It took a few months for his hand to recover!

You are right to ask questions, as is everyone, about this important issue.

By the way, a lot of hand-swung engines don't have mixture control al all.
User avatar
By Rob L
#1738591
TopCat wrote:...perhaps turning the engine over a few times in cold weather (also recommended in my POH)


That's another quandary which may well spark controversy! Could you scan & post here a copy of the section of your POH that says that?

Rob
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By TopCat
#1738650
Rob L wrote:
TopCat wrote:...perhaps turning the engine over a few times in cold weather (also recommended in my POH)


That's another quandary which may well spark controversy! Could you scan & post here a copy of the section of your POH that says that?

Yes I thought it might. I'll see what I can dig out. Maybe for another thread though. No point in having just one majorly controversial topic when we can have several in parallel. I've got questions about Sky Echo too...
By TopCat
#1738651
Just to bring the thread back on topic though...

I'm genuinely interested in any mechanism that could cause the engine to fire (taking off someone's hand, or head, or whatever) given that..

... it was shut down by pulling the mixture to full lean before switching the mags to off....
... but that the mags were then not actually off
... and then someone moved the prop.

I get that if someone sets the mixture rich, and pulls the prop through, when the mags aren't really off, then the engine could start, with all the gory consequences we all pray we'll never experience or witness.... but is there any other way?
User avatar
By Paul_Sengupta
#1738660
TopCat wrote:... it was shut down by pulling the mixture to full lean before switching the mags to off....


Rob L has hinted at it above. If you're flying with an injected engine, after shut down the heat from the engine expands the fuel in the lines, flooding the manifold. Thus a "hot" start with an injected engine is actually a flooded start. So depending on where in the "engine gets more fuel, engine floods, fuel evaporates creating ideal mixture, fuel has evaporated leaving the manifold too lean" cycle you are, it could easily start.
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User avatar
By Trent772
#1738686
flybymike wrote:I spent fourteen years crying, wailing, praying and begging on bended knees trying to start a hot turbocharged fuel injected C206 Continental 520. :evil:


That's because you didn't get in touch with me sooner......

10 years of starting a hot continental on our parachuting 206 was easy :pirat:
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