Carb Icing Question....

[dbakeg00]

Ok...im preparing for my commercial multi and MEI Oral (to be done pretty much back to back). I'm looking at the subject of carb icing and came up with this question that I cannot answer. If carb icing is caused by (1) a decrease in air pressure (which supercools the air) + fuel vaporization (which also cools the air, maybe b/c of evaporation...i dont know). And this cooling of the air (which contains moisture) allows in to contain less moisture..b/c its cooled, therefore it deposits the moisture on the venturi throat and the throttle body. And here is the question....Why does the P.H.A.K. say that Carb icing is most likely to happen with reduced power settings in a descent. If the power is reduced...isnt there less air flowing through the venturi...and during a descent, isnt the temp of the air getting warmer usually? I just cant seem to figure this one out. Any help would be appreciated...thanks alot.

[averyrm]

It relates to your MP question. With the throttle closed, there is more vacuum, hence more pressure drop, hence colder.

[GaTechKid]

[ QUOTE ]
fuel vaporization (which also cools the air, maybe b/c of evaporation...i dont know).

[/ QUOTE ]

State changes always require an exchange of energy, ie. heat. In order to change a liquid to a gas it takes heat. The vaporization of fuel takes the heat from the surrounding air, thereby cooling it.

[B767Driver]

Originally Posted by dbakeg00

And here is the question....Why does the P.H.A.K. say that Carb icing is most likely to happen with reduced power settings in a descent. If the power is reduced...isnt there less air flowing through the venturi...and during a descent, isnt the temp of the air getting warmer usually? I just cant seem to figure this one out. Any help would be appreciated...thanks alot.

I always thought it was because the engine compartment runs cooler at low power settings (+ the airspeed is higher causing more cooling). At high power settings in a climb...the engine runs hotter and the airspeed is slower...causing the carburetor to warm by conduction.

[USMCmech]

The main reason that carb ice is more likely at lower power settings is the position of the throttle valve itself. Not airflow or temps or anything else.

If ice forms on the throttle plate and the perimiter of the venturi, then closeing the throttle partway could inadvertantly choke off all airflow to the engine.

[BrettInLJ]

Originally Posted by USMCmech

The main reason that carb ice is more likely at lower power settings is the position of the throttle valve itself. Not airflow or temps or anything else.

If ice forms on the throttle plate and the perimiter of the venturi, then closeing the throttle partway could inadvertantly choke off all airflow to the engine.

So does the fact that the engine is at a lower power setting and producing less heat around it (but at higher power settings, producing enough ambient heat outside the engine block reaching the carb), add anything to the equation?

[E_Dawg]

you're really not supposed to ask questions like that. just pull the black knob when the white needle isn't in the green arc.

[BrettInLJ]

Originally Posted by E_Dawg

you're really not supposed to ask questions like that. just pull the black knob when the white needle isn't in the green arc.

Would you have added the sarcasm tag to that statement if it still existed?

Asking questions like this comes down to the understanding of systems and how they interact. That is what can save your butt when unusual scenarios happen and the checklists become useless.

[USMCmech]

Originally Posted by BrettInLJ

So does the fact that the engine is at a lower power setting and producing less heat around it (but at higher power settings, producing enough ambient heat outside the engine block reaching the carb), add anything to the equation?

Not really enough to matter.

Some engines have the carb located where it recives more heat from the engine or the oil sump which helps retard carb ice, but power settings have a negligable effect.

Rember that the carb gets it's airflow from outside the cowling, so the temp inside won't make much difference.

[E_Dawg]

Originally Posted by BrettInLJ

Would you have added the sarcasm tag to that statement if it still existed?

Asking questions like this comes down to the understanding of systems and how they interact. That is what can save your butt when unusual scenarios happen and the checklists become useless.

I said it half jokingly... But not sarcastically.

I think it's easy to get too involved in the exact workings of many systems when what you can do from the cockpit is very limited.

Especially when you consider that in this particular case you have a single knob, to be pulled in two situations: tach less than the green arc, and if it runs rough.