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I know the POH for the C-152 explicitly states carb heat should be applied before landing. I fly in a tropical country where temperature is at 30°C on average on the ground, twice the standard temp. Airports I frequent have an elevation at 300' the most. Cruising altitude on most days is between 2500-3500'.

Should I still treat the POH as the bible in this situation, or will the 30°C qualify as an exception?

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You should always pull the carb heat when throttling back no matter the conditions for 3 reasons:

  1. Ice forms from moisture (duh!), and there's much more moisture in hot tropical air than cold arctic air, so being in a tropical location does not lower the icing potential. The temperature drop is caused by the Venturi Effect which aerates the fuel in the carburetor, fuel is not frozen in the carb by the outside air temperature, you can get carb icing on a hot day
  2. Habit: it's important to get into good carb heat habits, otherwise you may forget to pull the carb heat when you need it. Make it so automatic it just happens without conscious thought
  3. Workload: Deciding when to use carb heat or not requires thought, you have to consider density altitude, temperature, relative humidity, the make of the carb, and other factors to make the right decision. Why add to your workload when 90% of the time you're probably going to conclude you do need it? Approach is a busy time, you should be spending time looking for traffic, maintaining airspeed and the other critical aspects of flight

It's worth reminding as well that icing can happen in cruise as well, so keep in the habit of applying carb heat regularly.

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    $\begingroup$ Habit: it's important to get into good carb heat habits, otherwise you may forget to pull the carb heat when you need it. Make it so automatic it just happens without conscious thought << This is probably the most important answer to any question in the context of "...it's not needed, should I...". $\endgroup$
    – Shawn
    Jun 20, 2019 at 16:44
  • $\begingroup$ 4. Because the POH says to. It doesn't say "unless OAT is > 30°C". $\endgroup$
    – T.J. Crowder
    Jun 23, 2019 at 12:07
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Carb heat use is not related to OAT, it's related to moisture and how it can turn to ice in the carb, causing the engine to stop. Carb ice can form at any outside air temperature, which is why you will not find any checklist or POH item for an aircraft with normally aspirated engines telling you to check OAT before activating carb heat. You just do it. I learned to fly on a Caribbean island and I got taken to task by the instructor if I ever dropped power to descend and forgot to turn on carb heat. This does not, however, apply to fuel-injected engines, since they do not have carburetors.

Depiction of carburetor icing

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    $\begingroup$ Absolutely right! Warm air holds a lot more moisture than cold. $\endgroup$
    – GdD
    Jun 20, 2019 at 15:11
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    $\begingroup$ "Not related to OAT" is a bit of a stretch - "not solely determined by OAT" would be better. There is a temperature drop in the carb but the drop is from the outside air temperature. I make no claims as to whether the carb used on this particular O-235 is guaranteed not to drop the temperature by more than 30 K. $\endgroup$
    – pericynthion
    Jun 20, 2019 at 16:07
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    $\begingroup$ I learned to fly in Louisiana. So while it was usually unlikely to be anywhere close to cold temperatures, it was ALWAYS very humid. And carb heat is usually used during a phase of flight where you don't really want the engine to sputter or quit. $\endgroup$
    – Shawn
    Jun 20, 2019 at 16:48
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    $\begingroup$ And @pericynthion, I don't know that I'd even say "not solely determined by OAT". I think "OAT shouldn't even be a consideration." is more apt. Don't give a student any reason to think that it's super hot outside so they don't need to worry about carb heat. $\endgroup$
    – Shawn
    Jun 20, 2019 at 16:51
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    $\begingroup$ I agree that the drop in temperature is due to the pressure (and velocity) change in the neck of the venturi, along with latent heat from the fuel evaporation. But my point is that this is a delta-T that is subtracted from the starting temperature of the air, and that latter is highly dependent on the OAT. The delta-T can be large, and that's why you can get carb ice even at OATs well above freezing.bBut it's not correct to say that OAT doesn't matter. After all, when you apply carb heat all you're doing is raising the temp of the incoming air. A sufficiently hot day would be equivalent. $\endgroup$
    – pericynthion
    Jun 21, 2019 at 15:06
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You have mentioned OAT, but remember there are 2 variables for potential icing - the other being Dew Point.

You (roughly) 85F OAT could easily fall into Serious Icing at glide power for a wide range of dew points.

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  • $\begingroup$ This is a good answer. $\endgroup$
    – Douglas Held
    Jun 21, 2019 at 16:31
  • $\begingroup$ It would have been better if the temperatures were in °C, as in the original question. $\endgroup$
    – Zeus
    May 15, 2020 at 2:34
  • $\begingroup$ @Zeus while I agree, at the time this was the only example of this graph I could find, and hence why my commentary converted the original question to F so that accompanying graph could be used $\endgroup$
    – Jamiec ♦
    May 15, 2020 at 8:01
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Thou shalt use Carb Heat.

You said you fly in tropical area where the temperature is 30°C.

  1. Warm tropical air has a lot of moisture as others have mentioned.
  2. The venturi effect can drop the air temperature in your carburetor's venturi by as much as 38.89°C (or 70°F) according to this wikipedia article on Carburetor icing.
  3. Thus if you're flying around in ambient 30°C moist air, and it cools down to -8.89°C, you see that the moisture in the air in the venturi will be at a temperature that is well below freezing. (For American audience water freezes as 0°C).

Note: From experience, when the conditions are just right, the carb ice forms within seconds.

Always use carb heat on landing when you go below the green arc on RPM (or as indicated in the POH).

(I don't have the source handy, but I remember reading that warmer ambient temps are actually more conducive to carb ice than very cold ambient temps. If I find the source I'll link it.)

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    $\begingroup$ The sweet spot is high humidity, 15-20 deg C. If you are flying something with a carb that gets almost no radiated or conducted heat at all from the engine, like a J-3 Cub, flying on a 19C evening with high humidity you will just about have to keep the heat on full time. $\endgroup$
    – John K
    Jun 20, 2019 at 20:37
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Most of the temperature drop is from fuel evaporation downstream of the venturi, causing ice from ambient humidity to form on the throttle plate (injected engines also have venturis and throttle plates for fuel and air metering, but because no fuel is being introduced there, so there is no evaporative cooling, there is no need for carb heat and they just use an alternate air source in case of main intake obstruction from other causes).

The temperature drop itself varies with the amount of fuel being introduced, humidity, and throttle opening and like most natural phenomena like this, it forms a bell curve of temperature/humidity, with the middle of the bell curve (highest risk of carb ice) at around 15-20 deg C. Problem is, that's the middle of the bell curve and the edge of the bell curve can extend to 30 deg C and beyond and you just don't know where you are in it. On top of that, if it's 30C on the ground, it's going to be just over 20C at 4000 ft, getting you closer to the middle of the bell curve.

So it's somewhat less likely when it's that hot outside, but can still happen and unless you have a carb air temperature gauge that you trust, this is something you don't screw around with and you follow the normal carb heat protocol regardless.

There is a similar situation with the engine itself. Lycomings are somewhat less prone to carb ice than Continentals because of the way the carburetor connects to the engine (the Lyc carb gets more conducted and radiated heat from the engine because the intake spider runs through the sump, so its body is slightly warmer than a Cont's), but they can still get ice and you follow the same carb heat procedures on either engine regardless.

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