> Although having the system on permanently might be a good idea I have heard
> of cases (on other aircraft) in very cold OAT conditions of carb heat
> raising the carb body temp into the icing range. Remember that there is a
> lower limit to the range of temperatures where icing is a risk. The band is
> about zero to -10C. If it is possible to get icing with an OAT of say 10C (
Icing can occur as high as 20C, since the temperature drop at the venturi
is quite high. Indeed, as our friends in Florida have seen, the worst
conditions for icing can be a very humid day 60F to 70F (15C to 20C).
> the air is cooled further by the pressure drop in the venturi and by the
> fuel vapourising), then if the OAT was -5C, the air in the carb could be
> as low as -15C. If this was the case heating the carb could bring it into
> the icing range. Leaving the system on all the time is only a good idea if
> you are sure that the heat is sufficient to raise the carb temp above zero
> in all conditions.
Carb icing becomes less critical at lower temperatures simply because cold
air can hold less moisture. At an OAT of -5C the air can hold very little
moisture and so can deposit very little moisture. (Note that this is not
true if there is visible moisture in the form of snow or freezing sleet,
but in that case you need anti-icing on the airframe and prop as well as
the carb!)
Brian
> Nigel Charles
>
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