I have about 200 flight hours in a fuel-injected C172. I have just jointly bought a C152 with a carburetor and I am getting used to using the carb heat. I am scared whenever I hear stories about engine failures due to carb icing.

What I know is to turn CH on when:

  • RPM lower than 2000
  • When visible moisture is in the air and when it's raining

But should I leave CH on when taking off in the rain? Can you give me some ideas about other phases and conditions of the flight when I should turn on the carb heat?

  • $\begingroup$ You usually have some indication of carb ice before the engine shuts down, when it starts running rough turn heat on and leave it on. I would not use it on take off because of the low powered engine that the 152 has. $\endgroup$ – Ron Beyer Mar 23 '17 at 21:20
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    $\begingroup$ The short answer is "do whatever it says in your POH"; it should tell you whether to have it on or off for takeoff, for example. We already have a few related questions that might help you: here, here. $\endgroup$ – Pondlife Mar 23 '17 at 21:22
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    $\begingroup$ One "gotcha" that some people don't get is that turning carb heat on can, in some circumstances, make ice accretion more likely. Air well below freezing can still have a moisture content but it's already frozen and won't stick to the venturi. Warm it up to just above freezing and it's now very cold moisture that will freeze on the venturi. As Pondlife says, POH. $\endgroup$ – Simon Mar 23 '17 at 21:37

Carburettor icing is rare at high RPMs, as there will not be much of a venturi effect. To have carburettor icing in such a scenario, it would have to be very cold and very humid/wet, in which case you would probably also encounter airframe icing, and might want to reconsider your flight.

Applying carby heat has two downsides:

  1. The air will be unfiltered, meaning dirt or other contaminants can be sucked into the engine;
  2. More importantly, the warm air means you won't be able to reach your usual maximum RPM.

These are two undesirable traits for takeoff.

The POH in a Cessna 172 I fly says to have carburettor heat off for takeoff and landing, but only really says it should be on when you suspect icing. I was taught to be much more conservative - prevention is better than cure. I apply carby heat whenever the RPM is below the 'green range', unless it is especially hot and dry outside. However, I remove it when I'm about 30 seconds from landing, in case I need that extra power for a go-around. You also shouldn't apply it 'half way'. Either it's all in or all out.

Boldmethod has a good article on why it's important, and you should pay attention to the conditions where it is most likely to occur.

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  • $\begingroup$ Why is there less of a Venturi effect at high RPMs? Higher RPM means faster airflow through the carburetor which means lower pressure which means lower temperature. Right? $\endgroup$ – Maxpm Sep 19 '18 at 5:26
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    $\begingroup$ @Maxpm the throttle butterfly valve setting is what largely dictates the RPM - if you close the valve, resulting in low engine rpm, you still have more or less the same airflow hitting the throttle (prop speed probably influences that too but I digress). Anyway, the venturi effect we're referring to is that caused by the butterfly valve. When it is nearly shut but not quite, bernoulli tells us that the air will accelerate through the small gap, lowering pressure and temperature. Make sense? $\endgroup$ – Ben Sep 19 '18 at 7:35
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    $\begingroup$ Yes, thank you. $\endgroup$ – Maxpm Sep 19 '18 at 18:33

You should refer to your POH for the official word but according to this copy of the 152 POH,

For a rough engine

A gradual loss of RPM and eventual engine roughness may result from the formation of carburetor ice. To clear the ice, apply full throttle and pull the carburetor heat knob full out until the engine runs smoothly; then remove carburetor heat and readjust the throttle. If conditions require the continued use of carburetor heat in cruise flight, use the minimum amount of heat necessary to prevent ice from forming and lean the mixture slightly for smoothest engine operation

Similar notes are made in the cruise section

Carburetor ice, as evidenced by an unexplained drop in RPM, can be removed by application of full carburetor heat. Upon regaining the original RPM (with heat off), use the minimum amount of heat (by trial and error) to prevent ice from forming. Since the heated air causes a richer mixture, readjust the mixture setting when carburetor heat is to be used continuously in cruise flight.

And also

The use of full carburetor heat is recommended during flight in very heavy rain to avoid the possibility of engine stoppage due to excessive water ingestion. The mixture setting should be readjusted for smoothest operation.

for a cold start without preheat,

The procedure for starting without preheat is the same as with preheat except the engine should be primed an additional three strokes just prior to pulling the propeller through by hand. Carburetor heat should be applied after the engine starts. Leave the carburetor heat on until the engine run smoothly.

Generally you should run carb heat any time you SUSPECT carb icing. You can take a look at this AOPA brief on carb ice.

You can also check out this brief from the NTSB.

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When I was taught to fly in an Aeronica Champ in 1966 my instructor would always tell me to apply carb heat just before reducing the power for landing. Leave the carb heat on until you are going to reply the power for a go around. As others have stated if roughness was detected ad carb heat. My instructor taught Sully how to fly in 1967. :-)

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