The conditions in which carburetor icing can occur encompass more than visible moisture and visible moisture alone isn’t sufficient for carburetor icing. In any case, radial engines are apparently handled a bit differently because they need super cold air for best performance. Rob Randazzo’s advice (based on 10 years experience flying radials) is not to touch the carb heat unless and until a drop in manifold pressure is experienced. This seems like sound advice to me since the build up of carburetor ice isn’t likely to be so fast that it can’t be dealt with if and when it occurs. I’d rather keep the engines performing at their peak than run around with the carburetor heat turned on to prevent icing that might not even happen.
Here is what Rob said in one of his posts on the PMDG forum:
I can see how you are getting confused.
This is described in some detail in the documentation, but let me give you a simplified rundown in hopes it will help you understand it in a way that will help you operate:
The best thing you can give carbs is super cold air. Super cold air is highly dense, and reduces the amount of work that must be done to compress the fuel air charge. You will generally only find this at very cold temperatures found at high altitude.
When the visualizer is blue, you don’t have a problem. It is merely advising you to keep an eye on engine performance because you are in a temperature range that is conducive to carburetor icing. If you have those indications of icing, then add a tiny bit of carb heat to eliminate the indication of trouble. It is important to point out here that you generally only use carb heat with radial engines under extreme icing conditions in the climb or moderate indications in cruise. It’s use is actually quite rare and you should follow the guidance provided in the FOM in that regard.
You shouldn’t provide carb heat unless there is a compelling reason to do so- and a “blue dot” isn’t compelling unless accompanied by a loss of power affiliated with icing.
Hope that helps.
The POH does talk about using carburetor heat to prevent the formation of ice in icing conditions, but the rub is that carburetor icing can occur on a warm, sunny day if the humidity is high enough. I read this to say that icing conditions are identified by a drop in manifold pressure that occurs when carburetor temperatures are within icing range.
- CARBURETOR HEAT
Carburetor icing is apt to occur when the ambient air is highly humid, or
when the carburetor air temperature is between-5°C (23°F) and 15°C (59°F).
To prevent the formation of ice, carburetor air temperature should be kept
above 15°C (59°F) and below 40°C (104°F) (detonation limit). A drop in
manifold pressure is an indication of carburetor icing.
During icing conditions, where carburetor heat is used, apply only enough
heat to prevent the formation of ice, and apply it continuously rather than in
short applications. The amount of heat available is proportional to the power
output of the engine; reduction of power in a descent is at the expense of available carburetor heat.
Under extremely cold ambient air conditions, applications of carburetor heat
may cause the formation of ice in the induction system as a result of bringing
the carburetor intake temperature upward within the icing range.
Do not take-off or land with carburetor heat applied.
The emergency procedures and troubleshooting section of the POH is a bit clearer and it notes that the application of carburetor heat as soon as a manifold pressure drop is experienced is generally sufficient to deice the carburetor. Alcohol is only needed if that doesn’t work.
- CARBURETOR ICING
If manifold pressure and fuel flow unaccountably drops, carburetor icing may
be the cause. Apply preheat to the carburetor for a short period. This will result
in further manifold pressure reduction, but if a subsequent slow rise is noted in
manifold pressure, ice is present and is melting. In this event, leave the carburetor heat on continuously until icing conditions no longer prevail. Manifold
pressure and engine temperatures should be watched closely during this period.
Return the carburetor temperature to desired limits when the ice has been
cleared.
Use preheat before carburetor icing becomes critical, as, if the ice accretion
is allowed to progress to a critical extent, the loss of engine power may make it
impossible to generate sufficient heat to clear the engine. If the preheat capacity is sufficient and if remedial action is not delayed, the elimination of ice will
only be a matter of seconds.
If carburetor heat fails to remove ice formation, use the carburetor alcohol
anti-icing system until the malfunctioning engine is operating properly.