Ewfflyer

Ok, we're taking this story out of context. This situations has the following conditions

Temps around freezing, can be at or below.
Moisture is present, most likely frozen particulate, but doesn't have to be.(Compare it to Rime Icing, it's liquid until it contacts something)
Carb-heat is on

What happens is as the air is induced to the carb, it was already frozen, which is a good thing, it won't stick to anything. Now, we add carb-heat and melt the precip at it's most critical point, prior to going into the carb, and then re-freezing through the Venturi. This is where you can create the hazard yourself. I always taught to not use carb-heat at all 32 and below, unless you do start getting a rough engine.

Hope this clears some things up, but who knows

Planespotta

Grandpa D still teaches the same thing in 144 . . . carb heat only from 20 F to 90 F

PGTx

Here is the graph:

Zayghami

i was always told to apply carb heat when Descending and when there is a power change in 20-70 degree temp and relatively high humididity days...

Mitragorz

Maybe I'm totally misunderstanding what you're saying, but based on what you just said, wouldn't that WORSEN the problem?

If you're already icing up while taking in frozen particulate, wouldn't *turning on the carb heat, melting the incoming particles, and having it re-freeze* (paraphrasing what you said... I think) over the existing ice be the last thing you'd want to do?

I mean, if turning ON carb heat below 32F in frozen particulate causes ice, how can turning the carb heat ON in the same conditions get rid of ice that's already formed? If you can turn it on to get rid of ice, I would think that leaving it on would prevent the ice from forming in the first place

Zayghami

lol thats what I though also....thats why im getting confused with this whole thing

withthatsaid182

in an archer the only time you need carb heat is if you actually run into carb ice. i would also let it run on the taxi out on rainy days or when it was cold and wet out...rarely on a warm humid day will you get carb ice in a piper.

carburated cessnas also seemed more apt to carb ice the pipers were always good about it.

Planespotta

I think what he's saying is that you won't be icing up in those conditions. From "The Killing Zone:"

When the air accelerates through the venturi, the air's temperature drops because the molecules speed up and string out. This "stringing out" is called expansion. When the molecules are farther apart, there is less friction and therefore less heat. Acceleration makes the air cooler. When the air cools, it may very well reach the dew point, and fog will form inside the carburetor. A fog is a mass of liquid water droplets. So now air, fuel, and liquid water is traveling through the carburetor. When the air reaches the throttle valve, the air must once again accelerate to make it around the valve. As the air accelerates once more the temperature will drop even further, sometimes to a temperature below freezing. This is where ice can form, even when it is a warm and sunny day outside.

If it is cold enough outside (i.e., below freezing), the liquid mass will already consist of tiny ice crystals that will not refreeze on the throttle valve or throat of the venturi. "Because the moisture is already frozen, applying carb heat may cause it to melt and freeze at a point too far for carb heat to help" (my manual).

Hope this helped.

Mitragorz

That didn't help too much... you stated what causes carb ice to form, which I already understand. Venturi causes air to cool, causing moisture to freeze... pre-private stuff.

I understand that theory. My question now is: Why would he apply carb heat in that situation (-32F w/ frozen particulate), even when experiencing engine roughness? According to the "freeze-melt-refreeze" theory that you two are presenting, applying carb heat during engine roughness will cause ice to form, WORSENING the problem. Even if you're not sure if the particles are frozen it just seems like (by EWF's theory) you'd be putting yourself in a bad situation by turning that carb heat on, potentially creating ice that you won't be able to get rid of.

Regardless of what's causing the engine roughness... according to that theory, applying carb heat will cause ice to form! Bad news bears!

EWF: If applying carb heat below 32F could potentially cause carb ice to form, why would you apply it during engine roughness? Doesn't that leave open the possibility of worsening the situation?

I'd like to hear EWF's explanation. Not for a personal attack, just because he's the one who gave that example and can probably go more in-depth about what he meant.

Subpilot

How hot would carb heat make the induction air? I am thinking the ambient air temp would be at least 100 degrees hotter, thus making any chance of carb ice from forming unrealistic.