Carburetor Temp Indication

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Jon Wilder

Pre-takeoff checklist
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Jwylde
In cruise flight with carburetor heat off, can someone tell me whether carburetor temp should be above or below outside air temperature?
 
Today's reading will be from the Pilot's Handbook of Aeronautical Knowledge, Chapter 7: Aircraft Systems.
 
It depends. Yes, the carburetor is cooled as it atomizes fuel. However, the carb does not live in the OAT environment; it is typically in the lower cowl, and the air there is heated by the exhaust and cooling fins. This air temperature is greater than OAT; hence we don't place our OAT probe in the engine bay. This additional heat may be high enough that carb temp is greater than OAT temp even the carb is being actively cooled.
 
Kevin Holbrook said:
It depends. Yes, the carburetor is cooled as it atomizes fuel. However, the carb does not live in the OAT environment; it is typically in the lower cowl, and the air there is heated by the exhaust and cooling fins. This air temperature is greater than OAT; hence we don't place our OAT probe in the engine bay. This additional heat may be high enough that carb temp is greater than OAT temp even the carb is being actively cooled.
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Baloney. That carb is a tiny refrigerator. There's a pressure drop in it, and anytime you lower air's pressure you also lower its temperature. Then the injection of the fuel and consequent evaporation of it will reduce the temperature further. The sum of the two factors can easily lower the carb temp by as much as 70° or a bit more, and the heat inside the cowl cannot keep up with that. Nor can the heat from a Lycoming's sump. The extraction of heat from the carb body is too aggressive and too fast.

That's the reason you can get carb ice on nice warm days, if the humidity is high enough. Carb ice is NOT limited to wintertime; in fact, ice is less of a problem at very low temps. The air is too dry.

Misunderstanding carb ice is a prime cause of power failures and accidents. It outweighs fuel starvation as a cause, for instance.
 
We had a carb temperature guage on one of our planes.
The difference from OAT was astounding at first. Even taxiing in on a hot day, hot engine, the carb was chill.

As Dan Thomas has pointed out, there are two strong refrigerating factors hard at work. I have experienced carb ice while taxiing on a hot day, and very high humidity.

Long taxi, runup carb heat check, the engine slowed, then went rough for many seconds, then smoothed out at normal carb test RPM. Ice had melted loose and been eaten. I had been taught to keep carb heat on for at least 5 seconds, and after that, I went to 10 seconds on humid days.
 
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