If you're on the ground and the environmental conditions don't cause any icing while on the ground, why are carb heat checks often performed in the run-up? Thanks.
2007-01-04 08:59:41 · 7 answers · asked by Anonymous in Cars & Transportation ➔ Aircraft
You are checking the carb heat to ensure that it is working. During the run-up you turn on the carb heat and should have a slight drop in the engine RPM and the an increase in RPMs back to normal when the carb heat is turned off.
2007-01-04 09:03:21 · answer #1 · answered by MIPilot 2 · 2⤊ 0⤋
Carb icing happens when humid air goes through the carburetor. By injecting fuel into the air in the carburetor, evaporation of the fuel causes the temperature to drop. The same thing happens when you sweat. If the air is humid, that temperature drop can be enough to cause ice to form inside the carburetor.
But, conditions on the ground are not the same as conditions at altitude.
The reason is called the adiabatic lapse rate.
As an aircraft gains altitude, the temperature drops.
The dew point (the temperature at which dew forms) also drops, but it drops FASTER than the temperature drops. At some point, the dew point equals the air temperature. That is the level at which you get clouds.
The higher you go, the more humid the air gets.
So, if you go high enough, you WILL hit carb icing conditions.
Testing the carb heat on runup is equivalent to checking the magnetos. Chances are, you're not going to need it.
But it's better to know it works when you don't need it than find out it doesn't when you DO need it.
2007-01-04 22:05:35 · answer #2 · answered by CJR 2 · 0⤊ 0⤋
Good point. Let's say you don't do the carb heat run up because the weather/environmental conditions don't cause any icing while on the ground. You are coming back from a trip somewhere and starting your descent. Recall that a) carb heat requires a carburetor (obviously) which has a venturi inside it. The air that travels through the venturi is cooled as it speeds up. Further, then the liquid fuel is discharged into the throat o f the venturi, it vaporizes, and this change of state drops its temperature. At this point, what do you have? You have cooler (cold?) air. Now, when the temperature of the air is low enough, moisture in the air condenses out and freezes. Here you have carb icing. Do you really want to experience that on a descent? Probably not. That is why you check carb heat during the run-up.
2007-01-04 19:25:38 · answer #3 · answered by barrych209 5 · 0⤊ 1⤋
You'll be glad you checked the carb heat if you experience carb icing conditions later in your flight, won't you?
Same with your mag check . . . . wouldn't you rather find out there's a problem (or NOT) BEFORE you take off?
2007-01-04 21:26:56 · answer #4 · answered by Squiggy 7 · 0⤊ 0⤋
Because you won't be flying at ground altitudes. It can be quite a bit colder higher, and depending on how far you're going, the weather conditions could be different.
Besides, it's on the checklist, and every pilot follows every checklist to the letter, whether it makes sense or not!
And it's part of verifying that everything on the aircraft is working, so that you know the aircraft is airworthy.
2007-01-04 18:02:11 · answer #5 · answered by lowflyer1 5 · 0⤊ 1⤋
To make sure that the carb heat is working properly!
2007-01-04 17:23:30 · answer #6 · answered by Bostonian In MO 7 · 1⤊ 0⤋
Haven't done much work on prop planes, mostly jets and copters. But you want to make sure it works before you get up where you need it and find out then it doesnt work. You'll be able to tell because hot air will change your manifold pressure and show this on the gauge.
2007-01-04 17:06:46 · answer #7 · answered by JET_DOC 2 · 1⤊ 0⤋