One page from the previously posted link. Any thoughts on this?
There are four operational regimes in which carb ice can be a problem—in order of frequency of occurrence, they are:
A) Startup
B) Take-off and climb
C) Cruise
D) Power reduction for landing
Yes, the *least likely* time to encounter carb ice is on the power reduction to land! Let's examine each of these scenarios and discuss why.
A) On the initial startup of the day: You are dealing with a cold engine and cold carburetor body and all of a sudden it's sucking air through the venturi. When the OAT and humidity are right--BINGO, carb ice. As a result, it is important to check that the carb heat is working BEFORE take-off (during the run up). The purpose of this carb heat application is two-fold... First to be sure the mechanism is working and second, to be
certain there is no ice present in the carburetor. If there is no ice present, there will be a drop in power as noted in rpm drop as the heated air enters the carburetor, thereby reducing the number of air molecules available for combustion. If there is ice present from the startup, there will be an increase in rpm as the ice is cleared from the carburetor and proper power is restored.
B) During high power application during take-off, there is a maximum flow of air through the carburetor which creates the greatest temperature drop across the venturi. Carb Ice can develop as a result.
C) During cruise, as the aircraft changes climates, OATs change, humidity changes, etc. and the engine is often WOT, the venturi again is having it's maximum temperature drop for the given situation.
D) During the power reduction to land, there is the least chance of developing carb ice! There is the lowest power being applied and, therefore, the lowest temperature drop across the venturi!
Now, that understood, why do we apply Carb heat as a matter of routine during the least-likely phase of flight?
The answer is simple. Before take-off we have checked the carb heat and know the condition of the carburetor before applying power. During take-off and climb we will know if ice begins to develop as we will see a power loss as the ice builds up. In cruise, we will also see a power loss as ice forms in the carburetor.
It is during the low power condition of preparing to land, that if ice builds up, we will have no way of knowing it. The power is already low and in some cases the engine is not producing any thrust at all. If the carburetor ices up and we need to go around, we will be unpleasantly surprised as we push the throttle forward and we continue to descend!
Soooooo, in the event of an urgent need for power, add throttle, the engine will come to your aid since there is Carb heat ON and no ice present, THEN push the carb heat in (off) to let the engine attain maximum power. It is common to see pilots push the carb heat in, then add power. I think this is not optimal. If the atmospheric condition is conducive to the formation of carb ice, turning the carb heat off, then adding power is a suboptimal idea... remember, the second most frequent time to encounter carb ice is during a high power application!
My recommendation is to add power, then carb heat OFF. That way, if you encounter carb ice as you need to go around, at least you'll already be developing power and know it. That likely scenario is power IN, carb heat off, power loss as ice then develops, then carb heat back ON--you'll make the go-around. If you push the carb heat in first and the conditions are right for ice formation, the likely scenario is Carb heat OFF, power in,
CARB ICE, power loss before you even get any....... and just when you don't need that headache! There is a reasonable argument for doing it either way.
MANY are now using Carb heat in cruise to improve fuel vaporization and improve F:A balance in carbed engines. MANY are able to run smoothly LOP as a result. The side benefit is that those who are doing this end up with a carb temp above freezing where it is not possible to have carb ice at all. Something to ponder......
