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So, if there's evidence of carb icing, it's full on. How long? It's a judgement call, but longer than just the time it takes to make the symptom go away. The symptom may go away while there is residual ice on the carb throat and throttle plate, so add, say, 10 seconds or so. It's not cast in stone, because every airplane is different and you have no data to go on. 10 seconds, 15 seconds, it's not that big a deal, and a little too long is better than not long enough. Just don't get carried away.

So, if there's evidence of carb icing, it's full on. How long? It's a judgement call, but longer than just the time it takes to make the symptom go away. The symptom may go away while there is residual ice on the carb throat and throttle plate, so add, say, 10 seconds or so. It's not cast in stone, because every airplane is different and you have no data to go on. 10 seconds, 15 seconds, it's not that big of a deal, and a little too long is better than not long enough. Just don't get carried away.

With unfiltered air, you don't want the airplane running with carb heat on any longer than necessary, because the unfiltered air means the engine is breathing dust, which outdoors means silica particles, which gets into the oil and makes it abrasive. Over the very long term it increases engine wear, so operation on unfiltered air is generally minimized, especially at low altitude. In WWII desert operations, piston engine fighters, that normally had no air filters, had to use very bukly "desert" air filters to keep the engines from being worn out in no time due to fine sand, which turns the engine oil into lapping compound after a while (some of what the engine breaths always ends up in the oil as particles settle on the cylinder walls).

With unfiltered air, you don't want the airplane running with carb heat on any longer than necessary, because the unfiltered air means the engine is breathing dust, which outdoors means silica particles, which gets into the oil and makes it abrasive. Over the very long term it increases engine wear, so operation on unfiltered air is generally minimized, especially at low altitude. In WWII desert operations, piston engine fighters, that normally had no air filters, had to use very bulky "desert" air filters to keep the engines from being worn out in no time due to fine sand, which turns the engine oil into lapping compound after a while (some of what the engine breathes always ends up in the oil as particles settle on the cylinder walls).

With unfiltered air, you don't want the airplane running with carb heat on any longer than necessary, because the unfiltered air means the engine is breathing dust, which outdoors means silica particles, which gets into the oil and makes it abrasive. Over the very long term it increases engine wear, so operation on unfiltered air is generally minimized, especially at low altitude. In WWII desert operations, piston engine fighters that normally had no air filters, had to use very bukly air filters to keep the engines from being worn out in no time, due to fine sand, which turns the engine oil into lapping compound after a while (some of what the engine breaths ends up in the oil).

Because don't know the actual air temperature, it becomes a binary choice: either full hot or full cold. If you apply partial carb heat, you may only be bringing the air temperature into the ideal range for icing without realizing it.

In cruise, it's normal practice to just add full carb heat periodically for 10 seconds, around every 15 minutes, or so just in case ice is beginning to form.

With unfiltered air, you don't want the airplane running with carb heat on any longer than necessary, because the unfiltered air means the engine is breathing dust, which outdoors means silica particles, which gets into the oil and makes it abrasive. Over the very long term it increases engine wear, so operation on unfiltered air is generally minimized, especially at low altitude. In WWII desert operations, piston engine fighters, that normally had no air filters, had to use very bukly "desert" air filters to keep the engines from being worn out in no time due to fine sand, which turns the engine oil into lapping compound after a while (some of what the engine breaths always ends up in the oil as particles settle on the cylinder walls).

Because you don't know the actual air temperature, it becomes a binary choice: either full hot or full cold. If you apply partial carb heat, you may only be bringing the air temperature into the ideal range for icing without realizing it.

In cruise, it's normal practice to just add full carb heat periodically for 10 seconds, around every 15 minutes or so, just in case ice is beginning to form.