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Carb heat needed?

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jedi

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Can one get carb ice when CAVU and OAT is 10 degrees Far. ?
At that temp air contains very little moisture, further chilling should not produce ice. Any thoughts?
Yes. All CAVU or no weather concerns.

I have had ice at very cold temps. Not sure of actual temps as it was long ago. Likely. Zero to 20 degrees in a Cessna 172. Carb heat was applied but took forever to have an effect.

Also had ice on an ultralight at warm shirt sleeve temperatures probably mid 70s. Ended up in the river and was not cold.

Engine quit on a J 3 with carb heat on and OAT probably between 25 and 35 degrees F.
 

Pops

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Have had carb ice 4 times. One time with a Lyc engine.
 

speedracer

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Only once for me. We were descending into Jackpot, Nevada out of 17,500' in my angle valve (GO 435 cylinders) 290 powered LongEZ. At about 15K' I added a little power and nothing happened (uh oh). I pulled the carb heat lever and headed over to the Twin Falls airport with a 7K' runway. I said to my wife "We have a problem". Wife: "Well you better fix our problem cuz I gotta pee". I found that with full throttle and the mixture near idle cut off the engine would do 1,500 RPM. Then the ice started melting and making the engine unhappy. It took 15-20 minutes to melt all the ice. Then we did a 40 mile, 15 minute descent and landed at Jackpot. Wife sprinted to the restroom.
 

Dana

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Rotax has it on some 912 models - the wife's RANS-S6 on an S-LSA certificate. It is a 100W electric element in each intake manifold. I suspect that it is there because of LSA rules, but I do not know for sure. Flight Manual is quite clear that it is only to be used if you have carb ice symptoms, and it is to be turned back off once the symptoms go away. From watching the voltmeter, I suspect that we drawing more power than the charging system can sustain. Oh, and we all doubt that 100W will make much of a dent in any significant carb ice.
I did some back of the envelope calculations when somebody was talking about putting a 25W light bulb inside the air filter for heat. Bottom line, fuggetaboutit.

FAR 23 says a carb heat system should give a 90°F temperature rise. Assuming a 2000cc engine turning 1000 rpm, that would require a little over 1000W. That's assuming 1 atm manifold pressure (29.92"), so in reality it would be somewhat less... but more at higher rpm. 100W ain't gonna cut it.

Though, to be fair,I believe the Rotax system heats the carb itself, not the air. As such, it doesn't prevent ice from forming, but it may prevent it from adhering to and blocking the carburetor throat.
 
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Dan Thomas

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Only once for me. We were descending into Jackpot, Nevada out of 17,500' in my angle valve (GO 435 cylinders) 290 powered LongEZ. At about 15K' I added a little power and nothing happened (uh oh). I pulled the carb heat lever and headed over to the Twin Falls airport with a 7K' runway. I said to my wife "We have a problem". Wife: "Well you better fix our problem cuz I gotta pee". I found that with full throttle and the mixture near idle cut off the engine would do 1,500 RPM. Then the ice started melting and making the engine unhappy. It took 15-20 minutes to melt all the ice. Then we did a 40 mile, 15 minute descent and landed at Jackpot. Wife sprinted to the restroom.
Carb ice is far more common than most pilots realize. They often have it and don't know it. Ice can be forming slowly, causing a gradual reduction in RPM, and what do they do? They open the throttle to get the RPM back. Under some conditions that will sublimate the ice out and they never know they had it. This can happen repeatedly until one day the conditions are just right and opening the throttle doesn't work anymore. By the time they pull the carb heat the exhaust system is cold and a forced landing is mandatory.

Too many pilots think that carb ice is a cold-weather thing. It's not. I wish groundschool theory was a lot more robust. There are far too many crashes due to common misconceptions about carb ice and stall speeds. It's sad.
 

Wingding

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Olympia, WA USA
I would put the tomato juice can on the exhaust and run some tubes to a box and put a flapper valve control in the cockpit.

I plan on doing so for my Revmaster when I get to that point on a VP build.

I have had carb ice on a clear day, full power climb out at 35 degrees f in a C-170 with an O-300 A, the temperature dewpoint spread was 4 degrees C. I was new to the C-170 and Continental, and did all the procedures, was still within glide distance to the airport, turned back and landed opposite direction with minimum power.

After landing, during the taxi in everything was normal. Called a friend, he laughed and said carb ice. We un-cowled the engine the next day, all was normal. Now, regardless of temp. I leave it on after runup until right before takeoff, and then put it on every thirty minutes or so during cruise. Always during power back and descent. It doesn't cost anything so why not? If I notice any decrease in RPM, it comes on. More times than I can count after using it the RPM is higher than when I pulled it on.
 

mcrae0104

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Can one get carb ice when CAVU and OAT is 10 degrees Far. ?
Without a doubt, yes.

Relative humidity, not visibility, is the decisive factor. At 10 F, the air has little capacity to store water vapor, but, if we suppose that it has 75% relative humidity, it only takes a temperature drop of a few degrees at the venturi to reach saturation, where ice will form. Here is a snip from a psychrometric chart to demonstrate (I wish it was Farenheit, but the charts usually cut off above 20 F because they're usually geared toward the HVAC crowd).

Look here for the whole psychrometric chart, where you can examine any combination of temperature and RH to find its dewpoint. Find the dry bulb temperature (OAT) along the scale at the bottom. Trace it straight up until it intercepts the relative humidity curve, then go straight left until you hit the 100% RH curve. That is the dewpoint, the temperature at which the vapor condenses (and freezes, if low enough). Wikipedia says that the venturi can reduce temperature by 70 deg F. If you dig into the psychrometric chart, you will quickly find that carb ice exists in a much wider variety of circumstances than seems intuitive.

1600915230472.png
 

Dan Thomas

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Without a doubt, yes.

Relative humidity, not visibility, is the decisive factor. At 10 F, the air has little capacity to store water vapor, but, if we suppose that it has 75% relative humidity, it only takes a temperature drop of a few degrees at the venturi to reach saturation, where ice will form. Here is a snip from a psychrometric chart to demonstrate (I wish it was Farenheit, but the charts usually cut off above 20 F because they're usually geared toward the HVAC crowd).

Look here for the whole psychrometric chart, where you can examine any combination of temperature and RH to find its dewpoint. Find the dry bulb temperature (OAT) along the scale at the bottom. Trace it straight up until it intercepts the relative humidity curve, then go straight left until you hit the 100% RH curve. That is the dewpoint, the temperature at which the vapor condenses (and freezes, if low enough). Wikipedia says that the venturi can reduce temperature by 70 deg F. If you dig into the psychrometric chart, you will quickly find that carb ice exists in a much wider variety of circumstances than seems intuitive.

View attachment 102111
Good chart. Thanks.
 
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