Primarily for general aviation discussion, but other aviation topics are also welcome.
User avatar
By Rob L
#1810267
matthew_w100 wrote:I'm not sure you've got the M&W quote right either.

You may be right; it was probably Tommy Cooper
User avatar
By matthew_w100
#1810268
TopCat wrote: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?


I don't in general - particularly now I know the effect! And I never "slammed" it. "Close it smartly" is what I own up to :-) Doing a glide approach during my first checkout in the Pup was when I first encountered it. And then, ten years later, I was reminded of the effect in my first ever "hour with an instructor" a fortnight ago when we did a few PFLs.
User avatar
By Rob L
#1810270
matthew_w100 wrote:
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?

..snip


You know what I mean.
By TopCat
#1810289
matthew_w100 wrote:And I never "slammed" it. "Close it smartly" is what I own up to

Well, whatever you call it. Why would you?

I can imagine moving the throttle from cruise setting to idle in less than 2-3 seconds - in normal circumstances - to please an instructor or examiner that wanted to see a textbook recovery from a spiral dive, but not for many other reasons, including practising them on my own.

I did it a few times a long time ago when I was measuring my minimum turnback height loss but I'd be a bit more hesitant these days. It's not that I have any reason to think it would do any damage, but I tend to treat my engine as gently as I can.
User avatar
By rohmer
#1810294
thegasguy wrote: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


What carb have you got? My C90-14F has an MA-3SPA which does have idle cut-off.
User avatar
By matthew_w100
#1810297
TopCat wrote:
matthew_w100 wrote:And I never "slammed" it. "Close it smartly" is what I own up to

Well, whatever you call it. Why would you?

I can imagine moving the throttle from cruise setting to idle in less than 2-3 seconds - in normal circumstances - to please an instructor or examiner that wanted to see a textbook recovery from a spiral dive, but not for many other reasons, including practising them on my own.

I did it a few times a long time ago when I was measuring my minimum turnback height loss but I'd be a bit more hesitant these days. It's not that I have any reason to think it would do any damage, but I tend to treat my engine as gently as I can.


Me too. Which is why it was ten years between occurrences. And, in both cases, the instructor pulling the throttle.
By A4 Pacific
#1810343
rohmer wrote:
thegasguy wrote: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


What carb have you got? My C90-14F has an MA-3SPA which does have idle cut-off.


My C85-12F has a Stromberg NA-S3A1 carburettor and works in exactly the same way as thegasguy’s. Selecting mixture to fully lean will not shut the engine down. It does take a while to run down after selecting fuel to off, however I make sure that’s the very first thing I do after coming to a halt, (no park brake!) and before doing a live mag check. Once I’ve completed another couple of bits of niff naff and trivia, it’s just a few seconds before the engine stops.

One question I am curious to answer is, are we all risking burning out valves by starving the engine of fuel whilst it’s still running??
By Bathman
#1817938
I get dragged out of maintaince almost every week (or the aircraft is brought to me). The experts at the flying school say it's tech because there is excessive mag drop 20 seconds after start. It never is.

I have no idea why they are checking the mags at 1200 rpm straight after start. The oil pressure is barely in the the green, the oil temperature is firmly on the stop, and the engine is still coughing and spluttering after being parked outside overnight when it's minus two.

When we do find a mag problem it's invariably lead on the plugs. The fouling of which can be reduced significantly if the engine is shutdown according to the Lycoming's recommendations.

Numerous chats with the CFI have concluded that we don't know what we are talking about and the POH is rubbish. But hey if they want 50 lessons a year cancelled and a bill for 100 hours labour then what's it for me to get involved.
scd975 liked this
By As I CFIT
#1817995
Bathman wrote:I have no idea why they are checking the mags at 1200 rpm straight after start. The oil pressure is barely in the the green, the oil temperature is firmly on the stop, and the engine is still coughing and spluttering after being parked outside overnight when it's minus two.


Inappropriate mag 'drop' checks aside, if a magneto happens to be completely dead then surely the oil pressure and everything else is irrelevant. For my own convenience, I'd prefer to find out that a mag is dead immediately after starting the engine rather than during the run-up. Not that my time is particularly valuable, it just makes sense to me.
Rob L, Human Factor liked this
By OhNoCB
#1818028
I imagine the problem isn't so much about finding a dead mag when the engine isn't warm (obviously, dead is dead), but finding that the RPM drop from 1200 RPM with a freezing engine may not be 'within limits' if expecting it to be the same as when done at higher RPM with a fully warm engine during run up and then declaring the aircraft tech as a result.
User avatar
By T67M
#1818038
If you choose to do a may check shortly after start up, the only limit is "drop, no stop". You should not be looking for the same RPM drop that you would at high RPM on a warm engine, nor even would rough running be a major cause for concern, although personally I might mean the engine slightly harder during the taxi to hopefully start clearing a fouled plug before the "proper" mag check.
By riverrock
#1818040
T67M wrote: I might mean the engine

Hey! What harm has it done to you?
I agree a good diet with not too much food can be good for it during taxi though.
kanga, T67M liked this