How does a cessna 150 carburetor freeze during decent?

Question

How does a cessna 150 carburetor freeze during decent?

Answer 1

The venturi effect cools the air as it rushes through the carburetor barrel, even when it is 80 degrees outside.  If there is enough moisture in the air, it will freeze on the interior surfaces of the carburetor.  If there is enough freezing, it can constrict the carburetor venturi.

Answer 2

Actually, carb icing can occur at any throttle setting and almost any temp, but most often occurs between an OAT of 26 - 34 Deg F and when the throttle is less than WOT.  So higher humidity, temps about freezing during descent (idle or low power cetting) are perfect.  And you might not notice it until you advance the throttle, as in the case of a missed approach.

Answer 3

Carb ice can form anywhere between 20 and 80 degrees F if the humidity is above 80%. During decents, your throttle would normally be half closed, reducing pressure, like the other guy said, but also, with a lower fuel flow, the engine would produce less heat, also encouraging carb ice.

Answer 4

Another nuance of Bernoulli's equation in fluid dynamics.  As the velocity of a fluid (air in this case) increases the relative temperature of the fluid drops.  It really has absolutely NOTHING to do with the ambient temperature.  I have seen carb ice in the middle of summer in a test cell at 80 degrees F!!

This is the principle (beside expansion) the is the basis of the vapor cycle cooling system in your car and many GA aircraft up to and including the Citation CJ series.

Answer 5

jle4044 answered this well...

Answer 6

It also freezes quite sneakily.

I've experienced carb icing in a '152'.

DO use that carb heat.

Answer 7

It freezes quite nicely...

Seriously, carb ice forms when the air that speeds up in the carb, due to the venturi effect, has a pressure drop enough to lower the air temp to the point that the moisture in the air condenses and forms ice on the surrounding parts of the carb, mostly around the venturi and the butterfly valve. With the butterly valve partially closed, like when you're doing a low power descent, the temp drop is greater because of the decreased pressure in the carb.

Not all carbs experience that. Cessna recommends applying carb heat whenever you go below 2000 RPM, I believe, whereas Piper says to use it if/when necessary.