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By Guest
#130753
During a recent solo qualifying cross country (I'm a student pilot) I managed to forget to reset the carb left to cold after a FREDA check and didn't notice this for what must have been several minutes. I made the same mistake on several other occasions during the trip, again after FREDA checks and after descents.

The obvious remedy is to REMEMBER THE BLOODY CARB HEAT FOOL, which I seem to be getting better at.

But I wanted to know what the likely damage could be if the carb heat was left on for a prolonged period, and whether any of you have experienced such damage?

Yours,

Mr Guest
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By Timothy
#130758
In the air there is unlikely to be any damage, just lower power and engine efficiency.

On the ground there is the extra risk of rubbish getting into the carb because the air is unfiltered, so you could get a blocked jet.
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By David Williams
#130788
I make this mistake all the time - it's not really much to worry about. The way I try to make myself remember is to apply the carb heat and hold onto the knob (stop giggling at the back, please). That way, if I get distracted then sooner or later I'll wonder why I'm still holding onto it.

Indeed, there are those who swear by using permanent carb heat to allow you to lean past peak EGT. On most carburettor engines this isn't possible because of the poor vaporisation of the fuel, resulting in uneven mixture distribution. The droplets of fuel that didn't quite manage to vaporise tend to travel further along the inlet manifold due to their momentum, so the front cylinders get a richer mixture. Carb heat improves the fuel vaporisation and can make the distribution even enough to allow lean-of-peak leaning. This has been the subject of l-o-o-o-o-ng discussions elsewhere.
By Guest
#131042
ok - it's good to know I wasn't a) ruining the engine b) putting myself in danger

I'm getting better an remembering this anyway!
By ROG
#131130
Count 10 (or whatever )then push it back.
By Me
#131604
There is a risk if you leave the Carb heat on for a prolonged period that Carb ice will build up even with the heat on. Then you will find that you have nothing left to take the carb ice away with. Pick a field!
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By leiafee
FLYER Club Member  FLYER Club Member
#131618
Me wrote:There is a risk if you leave the Carb heat on for a prolonged period that Carb ice will build up even with the heat on. Then you will find that you have nothing left to take the carb ice away with. Pick a field!


Okay, I can't grasp the physics of that one. How can it be hot enough to melt ice if you switched it on from cold but not hot enough to stop it forming if it was left on? :scratch:
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By rlthomas
#131653
Hi Leia,

The problem with Carburettor Ice is that it doesn't necessarily form in freezing conditions.

Indeed it usually forms when one has a moist atmosphere (ie water vapour) and can form in high temperatures, around 20 degrees C isn't uncommon. This is why Carburettor Ice is a big problem in the summer months as well.

So you're flying along with the Carburettor Heat off. You notice the engine starts to sound a little rough so glance at the RPM Tachometer and see that the RPM has dropped from the PA38s cruise of 2200 down to say 1900. You apply Carburettor Heat which warms up the Carburettor, melting the build up of ice thus the engine starts to sound healthy again and the RPM returns to 2200 RPM. Then the Carburettor Heat is turned off.

So then... if you have the Carburettor Heat on all the time, you're going through moisture etc and you have taken the ambient temperture into the icing critical range? The carburettor heat is on, the ice is building up and the engine is loosing power...

I'd be picking a field right about NOW, if I don't already have one in mind that is!!!

I'm sure someone on here can type something a little more technical than myself.

All the best.

Richard.
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By PhilE
#131714
And our survey said.......Bzzzzt :-) Take a read here:

http://www.flyontrack.co.uk/carbice2004.htm

Just a bit of selective quoting from said missive:

...the DHC Chipmunk which, when in military use, had its carburettor heat wired permanently on.


permanently applied carburettor heat is to be found in the 912 versions of the Rotax engine


This article states a couple of known contra-indications for wiring the carb heat permanently on, like high compression engines and engines which are a bit low on power output in the first place, but just having the carburettor heat permanently on will not, in itself, result in ice build up.
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By AndyR
FLYER Club Member  FLYER Club Member
#131722
I stand to be corrected if wrong, but would dispute the above l2kphil. The engines you refer to I believe have the equivalent of either an electric "jacket" or have hot water from the cooling system bypassed around them, rather than carb heat in it's normal form.

I believe (but can't find the reference at present) that carb ice can form when the carb heat is selected on, subject to the humidity (rather than air temperature) being sufficient to cause it.
By rlthomas
#131724
I forgot to mention Humidity is a major factor, thanks to Andy for pointing that out.

Sorry!

I really need to know this stuff! Maybe I should go back to ppl training instead of my atpl studies. :lol:
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By Pilot H
FLYER Club Member  FLYER Club Member
#131725
Unlikely that properly adjusted carb heat control would result in icing if left full on. Biggest danger is the possibility of a rich cut when going from idle to full power - which can happen on a hot day with, perhaps, an already slightly over-rich adjusted carb.

Partial carb heat can raise the temperature into the icing zone, so should never be used unless a carb temperature gauge is fitted, in which case it can be useful for cruise flight in icing conditions (The partial carb heat can then be used to raise the temp enough to prevent icing as verified by the Temperature gauge, but does not cut back power as much as full heat would)

H
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By AndyR
FLYER Club Member  FLYER Club Member
#131728
My thanks to Pilot H, having found the text I was searching for, it does refer to partial carb heat being as dangerous as none at all in certain situations/temperatures etc.

It does mention that leaving carb heat on can cause detonation, particularly in high compression engines.
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By Rob P
#132160
ROG wrote:Count 10 (or whatever )then push it back.


My understanding is that '30' would be better

Rob P
By bookworm
#132462
David Williams wrote:I make this mistake all the time - it's not really much to worry about.


Ah, the comforting, homely smell of puppies roasting on an open, heated, alternate induction system... :wink: