Hi Bob/Richard

Recently, a sentence from your book confused me for a while:"During a glide there there would be no symptoms of ice formation since the throttle is fully closed." Could you explain it for me please?
What i found on website was that throttle ice is formed at, or near, a partly closed throttle butterfly, so why are there no symptoms of ice formation since the fully closed?
Also why is the carburettor more susceptible to icing when closed ?!?(which was opposite to what your book said) I hope it's not too obvious, or a silly question... but it's been puzzling me. I thought the major contibutor to icing was due to the venturii effect - so how is this related (if it is) a closed throttle and ice !? Or is it simply due to a cooler engine at low RPM. Many books state this without explaining !
I understand that"carburettor heat should be applied before commencing a glide descent and returned to the "off" position before power is restored," which means there would be no ice formation on the inlet side of throttle, but what about engine side at this moment? what I thought is that because of throttle closed, the heat could not go to engine side, so engine stops running, become cooler and may be icing(not sure)...

Please explain little bit more. Thanks so much

Chao

Firstly let's consider the word 'symptoms'. Saying that there are no symptoms is not the same as saying that there is no ice. The symptoms of ice formation include loss of engine power, rough running and dropping RPM).

If ice forms during a power-off glide, you will not notice a loss of engine power because you have no power applied. You would not notice a drop in RPM or rough running because the prop would be windmilling and the passing airflow would be maintaining the RPM - usually at about 1000 RPM.

Also, when the throttle is completely closed, the pressure in the inlet manifold ahead of the throttle would be about 30" (at sea level) while the pressure behind the throttle would be about 10" at idle. That means the air suffers a large drop in pressure as it passes across the throttle. When the pressure of a gas drops, the temperature also drops (a fact of physics known as the gas laws). This is known as 'adiabatic cooling' and it adds to the icing risk. When the throttle is closed there is still some fuel being added to the air so evaporation is still occurring and adding to the cooling effect.

All of these effects increase the likelihood of carburettor icing during a power-off glide while the pilot may have no warning that it is happening - no symptoms.

Bob

Thanks Bob, appreciate!