Originally Posted by
Memphis_bell
Yes it's true that if you apply full or partial carb heat, you neglect the full potential of engine output. A good example is on final approach - you descend at an aplied rate (say a 3 degree descent) with full carb heat set HOT, but before touch down (approx 100 feet) carb heat MUST be set to COLD. The reason for this is simples = if you are forced to do a go around, full engine output will be required to ascend, hence the reason for turning carb heat to cold - because carb heat set to HOT reduces output.
Simples :-p
If it is so "Simples" how come
No Grumman or Piper or Cessna POH requires you to select Carb heat off before touchdown ? What do you know that the flight test engineers at all the major light aircraft companies don't know ?
Since the optimum conditions for carb ice formation is high velocity air into the carb throat with a idle throttle setting, precisely the conditions on short final; what is stopping the formation of ice between the time you select carb heat off and then for what ever reason have to go around ?
And it should be noted that under the right conditions significant amounts of ice can develop in seconds
The fact is that the "requirement" to select carb heat off before touchdown at some arbitrary height is a unique to UK flying schools urban myth mindlessly passed down from Instructors who got it from their Instructors. Certainly it is not common practice in North America, where 70 % of the worlds light aircraft are operated.
The certification of light aircraft engines requires that the makers prove they can be run at full power with full carb heat on under any normal circumstances and so there is no reason to prevent you, in the event of a go around, from simply applying full throttle and then selecting carb heat off. If conditions warrant carb heat on final then I firmly believe it should stay on until you have landed and vacated the runway.