Primarily for general aviation discussion, but other aviation topics are also welcome.
User avatar
By Miscellaneous
#1810110
matthew_w100 wrote:Huge inertia of the prop is what I was guessing. Much more than the flywheel on a car engine.

Even if the inertia is sufficient to turn the engine over there's still no method of igniting the mixture. The compression ratio is not sufficient, unlike diesels that depend on the higher compression ratio.

I'm confused as to @johnm's thinking. :?
matthew_w100 wrote:Rotaxes do stop violently - it feels quit horrid.

Ain't that the truth. :shock:
By Boxkite
#1810111
matthew_w100 wrote:Lycoming O320 (in a Beagle Pup) always cut off the fuel to shut down.

Rotax 912S (in a P&M Blade flexwing) switch off the mags to shut down. There is no mixture control, so no option!

What do you mean "cut off the fuel" in the Pup? How?
By As I CFIT
#1810114
rdfb wrote:
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.


It doesn't even take five seconds. About these ten similar items, what would they be? Bearing in mind that at this point, you will have already operated the flight controls, the flaps, the trim, the fuel pump and you've obviously got the engine going - how many other no-go items could potentially be unearthed prior to doing the power checks?

You may consider the post-start 'two live mags' check to be unnecessary but that's a personal thing, as is my habit of doing it whilst feeling that it's a good use of my time.
By johnm
#1810121
In my experience if you switch off mags too early hot bits cause random explosive fuel burn sometimes called a backfire
User avatar
By skydriller
#1810129
matthew_w100 wrote:
skydriller wrote:
johnm wrote:An aeroplane engine will spin down slowly and with the mags off fuel will simply explode in the cylinders and exhaust which is not a great idea.


How without the spark from the plugs if the magneto is off... diesel is different as its compression , hence the pull to stop on a tractor... what obvious thing am I missing?


Huge inertia of the prop is what I was guessing. Much more than the flywheel on a car engine.


Yeah, but inertia doesnt provide a spark, its petrol, not a diesel...?

Regards, SD..
By Pilot Pete
#1810147
Let's recap.
A mag check early on can save a long warm up and a long taxi only to find an issue at the run-up. If that doesn't apply to you, don't bother.
I can struggle to cross wet grass to our runway only to find an issue and have to do the same back to a tricky parking spot. A check is a very good idea for me.

We quit an average Lyco or conti using the mixture cut-off position because in theory no fuel is left in the system to allow an unintended start which could happen if a) fuel was there and b) it's a bad day and a mag is live because something has been disturbed in the wiring, gone faulty, or someone has left it on.

I do a pre shutdown dead cut. I do it extremely quickly. The engine responds very quickly but you can definitely tell it's quitting if you're paying attention.
It's so quick that if you didn't know I was doing it you'd just notice something and look at me, but only that. That's all that's needed.
I've done it on 0320, 0360 lyco and TIO520 Conti for years. Never ever once heard a backfire.

Never flown with singular mag switches where it's not necessary.

I once flew a club c152 at night. Checked the ignition before moving the prop. Had to move the prop a couple of times.
Got in to fly and saw the previous guy had removed the key in the on-both position.
Glad he shut down using the mixture.
As I CFIT liked this
User avatar
By Rob L
#1810148
Pilot Pete wrote:...

I once flew a club c152 at night. Checked the ignition before moving the prop. Had to move the prop a couple of times.
Got in to fly ....and saw the previous guy had removed the key in the on-both position.
Glad he shut down using the mixture.


(my very heavy snip)

That's two different ADs.

Rob
By thegasguy
#1810188
It's not often that I can post an absolutely definitive answer to a point raised here - but this time I can, and provide evidence to boot.
My jodel has a Continental C 90 -14 F and has the sort of carburettor with a the mixture control that won't provide idle cut-off; just mixture leaning at altitude
So, the only practical way to stop the engine is to turn the mags off via the switch.
Turning off the fuel results in a very very long wait indeed; pipe and gascolator have a fair bit of juice in them.
It's been this way for ever, with the previous owner as well, and has operated in some very hot and some very cold areas of Europe, if you think that's a factor.
I can definitely put the "exhaust explodes" myth to bed.
It doesn't.
I've 'dead cut, tested' hundreds of times - zero result.

I'm very amused by people saying ' it's not in the checklist !' - come on, really !
I have a mental checklist for going out the door to work every day ( phone, wallet, keys, lunch, if you are interested)
but it's just an aide-memoir, not the gospel.
Myself, I take the same view of check lists for aircraft.

But then I've noticed that there are two sorts of flyers- those that get their thrills from learning lists by rote and memorising intricate procedures and sticking to them blindly , and those that like to fly places and do things.

The second group have a much more exciting life - but possibly shorter with a very positive full stop at the end.
You pays your money and takes your choice, but I think the first lot tend to give up flying after a couple of years.

Is it like ice skating to them? By this I mean, it's only fun when you can't do it very well - after you get proficient it's just endlessly going round and round in circles.

By the way, despite lots of advice to ' save the battery and starter motor' I never prime and pull blades through by hand any more on mine.
I never want what I call a "Luscombe surprise "; seems like a bit of fuel or fumes can still cause that even when the engine has been stopped the conventional way.

No Luscome pilots were harmed in the making of this post- don't troll me!
As I CFIT, Flying_john liked this
By johnm
#1810211
The TB 20 POH contains this little section

WARNING
THE TEST HEREAFTER MUST BE IMPERATIVELY
CARRIED OUT WITH ENGINE POWER LOWER
THAN 1000 RPM; THE FAILURE TO OBSERVE THIS
RULE MAY LEAD TO EXHAUST SYSTEM DAMAGE

Magnetos cut-off test (*) OFF, then BOTH
Throttle 1200 RPM
Mixture IDLE CUT -OFF

(*) Depending on the kind of operation, it is not
necessary to perform this test more than once a day,
but just before securing the airplane,

The normal shutdown is to check for mag drop on each mag in turn at 1200 rpm
User avatar
By matthew_w100
#1810249
While we're on the theme, why do i get a loud exhaust pop when I close the throttle smartly in the cruise? I can't think why that would cause unburned mixture to pass through the engine.
User avatar
By Rob L
#1810254
matthew_w100 wrote:While we're on the theme, why do i get a loud exhaust pop when I close the throttle smartly in the cruise? I can't think why that would cause unburned mixture to pass through the engine.


Because it's not unburned, and it passes through a hot exhaust, which causes it to burn. So don't "close the throttle smartly in the cruise" :roll:

It reminds me of a Morecamb & Wise sketch: "Why does it hurt so much when I slap myself on my face?" (or words to that effect)
User avatar
By matthew_w100
#1810258
Rob L wrote:
matthew_w100 wrote:While we're on the theme, why do i get a loud exhaust pop when I close the throttle smartly in the cruise? I can't think why that would cause unburned mixture to pass through the engine.


Because it's not unburned, and it passes through a hot exhaust, which causes it to burn. So don't "close the throttle smartly in the cruise" :roll:

It reminds me of a Morecamb & Wise sketch: "Why does it hurt so much when I slap myself on my face?" (or words to that effect)


Eh? "It's NOT UNburned"? Am I being thick or is that a typo? My question is why. Closing the throttle reduces the amount of mixture in the cylinder but there is still a spark, so it should burn in the cylinder. Like it does at idle. But some leaks through unburned and goes bang in the exhaust. I say again, why?

I'm not sure you've got the M&W quote right either.
By TopCat
#1810263
matthew_w100 wrote:Eh? "It's NOT UNburned"? Am I being thick or is that a typo? My question is why. Closing the throttle reduces the amount of mixture in the cylinder but there is still a spark, so it should burn in the cylinder. Like it does at idle. But some leaks through unburned and goes bang in the exhaust. I say again, why?

You're not being thick.

That was an example of a variant of Muphry's law, which I'll call Topcat's law (more an aphorism, really) ... which goes along the lines of:

"Be careful when using the ' :roll: ' smiley, lest you post something that might itself deserve one".

In fairness, though, although I don't know the answer to your perfectly reasonable question, why would you slam the throttle shut in the cruise?