Carb Heat in rain

geezer said:
It was surprising how much RPM came back from over rich when I ran heat continuously and reset the mixture in near dew point cloud.
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Near dew point cloud? The definition of a cloud is saturated air. Dewpoint and temp the same, not "near."

geezer said:
I have flown in rain so heavy that the plane slowed 3 knots (No convective action), and went to full heat just to be sure no ice occurred. The outside air was saturated, for sure.
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The outside air wasn't saturated if there was no fog or cloud, but it would be close.
 
Know your airplane. My C-180 isn't prone to ice. I rarely use carb heat. A friend's 180 is an effective ice maker, and he applies momentary full heat about every two minutes when the weather is scuddy. Why some planes make ice and others don't is a mystery.

I never use partial heat. My carb heat comes directly off the muffler and is quite warm. I've always had a carb temp gauge and have played with different temps. All or nothing for carb heat is my preference.
 
Dana said:
The manual for my Lycoming basically says, use carb heat when needed, don't use it on landing approach unless icing conditions are known or suspected, and don't use partial heat unless you have a carb temp gauge.
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Yes, that's for a Lyc in something like a PA-28. Airframe manufacturers will vary from what the engine manufacturer recommends if their testing finds a risk. I have had Lycoming ice up on nice days.

"Known or suspected" implies that a pilot has read the METARs and knows the carb ice risk based on temp and dewpoint. In my teaching I found many pilots that had NO CLUE about this stuff, and were often the ones that thought carb ice was, like airframe ice, a winter thing.

AOPA had an article years ago that I can't find anymore that outlined the causes of power-loss accidents. Carb ice was at the top of the list and outweighed all other causes by a wide margin. Many times an accident investigation report will state something like this: No defects were found in the engine or fuel system or controls that would cause power loss. Carburetor ice is suspected, as the temperature and dewpoint at the time of the accident were conducive to carb icing."

Needless accidents due to ignorance.
 
With temps 40-50F, light mist, damp taxiway, close temp/dewpoint spread, do you use carb heat on the ground? For takeoff and climb out? Does little Continental or Lycoming matter? My previous employer required engine heat on any time there was visible moisture and temperature was below 10C (granted that was for turbine operations).
 
Stewartb said:
Know your airplane. My C-180 isn't prone to ice. I rarely use carb heat. A friend's 180 is an effective ice maker, and he applies momentary full heat about every two minutes when the weather is scuddy. Why some planes make ice and others don't is a mystery.

I never use partial heat. My carb heat comes directly off the muffler and is quite warm. I've always had a carb temp gauge and have played with different temps. All or nothing for carb heat is my preference.
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Different models of 180s can have slightly different carb heat sources and plumbing. We had one that would make so much carb heat that it would stumble and barf when we pulled it. Worn parts in carb airboxes will also make a difference, allowing cold air past the edges of the flapper valve when the carb heat is pulled, reducing the carb heat's effectiveness. It can also allow the engine to be breathing some heated air all the time.

We had a Citabria 7GCBC with a Lycoming O-320-A2B that was an awful icemaker. Totally unlike most Lycomings. We could never figure out what made the difference. It had the same sump-mounted carb, same MS carb, same induction system and airbox idea as a 172, but it made ice like mad and we really had to watch it.
 
farmrjohn said:
With temps 40-50F, light mist, damp taxiway, close temp/dewpoint spread, do you use carb heat on the ground? For takeoff and climb out? Does little Continental or Lycoming matter? My previous employer required engine heat on any time there was visible moisture and temperature was below 10C (granted that was for turbine operations).
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In conditions like that I checked for ice immediately before takeoff. Run the RPM up, pull the heat and wait a bit. Often see an RPM rise, indicating accumulated ice.

That AOPA article also mentioned some accidents right after takeoff due to accumulated ice, limiting the power and resulting in mushing to an accident off the end of the runway somewhere.

Most Continentals way worse than Lycomings, but a Lyc will quite willingly ice up when the conditions are right, especially soon after startup when the oil temp is low and isn't warming the carb body yet. Never believe the baloney about Lycs not icing up. Mr. Murphy is never far away with an exception to that.
 
Cap'n Jack said:
What is the humidity when it is raining?
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Depends a lot on the ambient temperature. Cold air is more easily saturated than warm air, as it can't hold as much moisture. You could see a temp-dewpoint spread of zero, or several degrees like this one:

upload_2022-10-28_11-43-36.png
 
Cap'n Jack said:
And the humidity?
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Dewpoint vs. temperature is just another indicator of relative humidity. Dewpoint is an absolute measure of water vapor in the air. RH moves around as air temperature changes.

upload_2022-10-28_16-51-30.png
 
Dan Thomas said:
Dewpoint vs. temperature is just another indicator of relative humidity. Dewpoint is an absolute measure of water vapor in the air. RH moves around as air temperature changes.

View attachment 111842
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Both statements are true, but people tend to think in terms of relative humidity rather than dew point. Note that in an evaporative situation such as the example in your post 51, the ambient temperature goes down too.
 
Cap'n Jack said:
Both statements are true, but people tend to think in terms of relative humidity rather than dew point. Note that in an evaporative situation such as the example in your post 51, the ambient temperature goes down too.
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Yup, it does. You can also find strong downdrafts under virga as the air chills and gets denser as the heat goes to evaporate the water.

But for aviation, where we need to know carb ice risk and potential for fog formation, the dewpoint is a better reference. It's why the MERTARs post it rather than RH. If we look up the relative humidity and find that it's only 60%, say, we might not be concerned. But once the sun goes down, the air starts cooling and the RH goes way up and we might run into trouble. Knowing what the dewpoint of the air mass is, we can watch the OAT and know when to expect cloud formation. Altitude also affects temperature, but unless we have frontal passage or an inversion, the dewpoint will be about the same as it is on the ground, and we know what to expect.

50 years ago, on my first dual cross-country trip, we were travelling through a valley. The sun was headed toward the horizon, and clouds started forming "out of nothing" all around us. We turned around and got out of there, headed for more open country and home. A stark lesson on temperature and dewpoint.
 
Dan Thomas said:
Yup, it does. You can also find strong downdrafts under virga as the air chills and gets denser as the heat goes to evaporate the water.

But for aviation, where we need to know carb ice risk and potential for fog formation, the dewpoint is a better reference. It's why the MERTARs post it rather than RH. If we look up the relative humidity and find that it's only 60%, say, we might not be concerned. But once the sun goes down, the air starts cooling and the RH goes way up and we might run into trouble. Knowing what the dewpoint of the air mass is, we can watch the OAT and know when to expect cloud formation. Altitude also affects temperature, but unless we have frontal passage or an inversion, the dewpoint will be about the same as it is on the ground, and we know what to expect.

50 years ago, on my first dual cross-country trip, we were travelling through a valley. The sun was headed toward the horizon, and clouds started forming "out of nothing" all around us. We turned around and got out of there, headed for more open country and home. A stark lesson on temperature and dewpoint.
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Going back to the original question- we know the relative humidity will be 100 % because it is raining. The dew point will suggest how quickly ice will form by how much water the air can hold as per your chart in post 32.
 
Cap'n Jack said:
we know the relative humidity will be 100 % because it is raining.
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I question that presumption. It certainly was 100% where the rain formed, but probably not where you are below that, if rain is falling on you.
 
Salty said:
I question that presumption. It certainly was 100% where the rain formed, but probably not where you are below that, if rain is falling on you.
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Why do you question it? What happens to rain when it falls into dry air?
 
Cap'n Jack said:
Why do you question it? What happens to rain when it falls into dry air?
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Rain doesn’t evaporate instantly when it hits air at 99% humidity. Unless you are in a cloud or fog, it’s not necessarily 100% humidity when it’s raining, and I posit most of the time it probably isn’t even close.
 
Salty said:
Rain doesn’t evaporate instantly when it hits air at 99% humidity. Unless you are in a cloud or fog, it’s not necessarily 100% humidity when it’s raining, and I posit most of the time it probably isn’t even close.
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Enough rain, eventually the air will be saturated. A small thunderstorm over dry air, you'll get a few drops of rain, the humidity will increase, and the temperature will drop.
Fly into the virga in the picture below, you'll find 100% humidity

In the context of the question that started this thread, I'll assume 100% humidity while flying in the rain and act accordingly to keep the engine running.
JAK_1786 by Jack Silver, on Flickr
 
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Once again:

upload_2022-10-29_11-31-11.png

At 100% RH you start seeing fog or cloud. If it's raining but you're not in fog or cloud, it's not 100% RH.

RELATIONSHIP OF DEWPOINT AND RELATIVE HUMIDITY TO CLOUDS AND PRECIPITATION:
If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog (at the surface) and clouds aloft (which consist of tiny water droplets suspended in the air).

From https://www.weather.gov/lmk/humidity
 
This is one of those cases where 99 or 100 percent humidity doesn't matter. My job is to err on the side of caution. The poh usually has a directive, if not, I will at least check for ice. Low power, carb heat coming on.

The thing about carb heat is that once the engine is in trouble, you have a very small window where it will produce heat to fix the problem. Once it stops running the muff cools very quickly. You need to be quick and proactive. Really no reason to be sparing with it.
 
Dan Thomas said:
Once again:

View attachment 111852

At 100% RH you start seeing fog or cloud. If it's raining but you're not in fog or cloud, it's not 100% RH.

RELATIONSHIP OF DEWPOINT AND RELATIVE HUMIDITY TO CLOUDS AND PRECIPITATION:
If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog (at the surface) and clouds aloft (which consist of tiny water droplets suspended in the air).

From https://www.weather.gov/lmk/humidity
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Re-read your own post. I highlighted a key word. "May" != "always"
Once again, get enough rain, it will eventually saturate the air with water vapor. I'll consider flying in the rain to be flying at 100% RH and act accordingly. Is there a reason not to do so?
 
Cap'n Jack said:
Re-read your own post. I highlighted a key word. "May" != "always"
Once again, get enough rain, it will eventually saturate the air with water vapor. I'll consider flying in the rain to be flying at 100% RH and act accordingly. Is there a reason not to do so?
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Lower performance when it’s almost always not necessary.
 
Salty said:
Lower performance when it’s almost always not necessary.
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Then I guess we'll just disagree.
Put water into dry air, it will evaporate. The water has to go someplace, and that's into the air. A decent amount of rain will make it close enough to 100% humidity that I'll deem it to be 100%.

You do what you want.
 
Cap'n Jack said:
Then I guess we'll just disagree.
Put water into dry air, it will evaporate. The water has to go someplace, and that's into the air. A decent amount of rain will make it close enough to 100% humidity that I'll deem it to be 100%.

You do what you want.
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Virga is not a default, it's actually relatively rare. Rain far more often stays rain until it hits the ground. Like, nearly always. I'm mystified as to your position on this. Have you never gotten wet in the rain? Hint: That means the water's not in the air.
 
Salty said:
Virga is not a default, it's actually relatively rare. Rain far more often stays rain until it hits the ground. Like, nearly always. I'm mystified as to your position on this. Have you never gotten wet in the rain? Hint: That means the water's not in the air.
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I'm just as mystified by your position.

If rain is falling through dry air, the water in that drop has to evaporate to some degree. If the air is very dry, the drop evaporates quickly. If the air is humid, less of the drop evaporates before hitting the ground. That doesn't mean the drop has to completely change into water vapor before hitting the ground. The drops just get smaller in size. You get enough rain, it saturates the air to 100% humidity.
 
If you have carb heat use it periodically in the cruise and put it on in the descent.

If your engine starts playing up use it as part of emergency drills

You get carb ice due to temp drop in the carb Venturi.. if the humidity and temp intersect at the relevant point you get carb ice. We get this in humid climate like UK a lot. Flying in the rain will increase humidity too..
 
I only have about 120 hours on my plane but I have never used carb heat. I am in Michigan so I am prime for it but my plane has a carb temp sensor. Even on the coldest day I flew last winter it was never below 100 once the engine was warm enough for take off. I have an O-320 with the carb essentially mounted to the bottom of the oil tank so if the oil is warm the carb is also. That and how the air box is set up, keeps the carb warm. I do check to make sure it works on run up and on downwind just to make sure it works.
 
Dana said:
The manual for my Lycoming basically says, use carb heat when needed, don't use it on landing approach unless icing conditions are known or suspected, and don't use partial heat unless you have a carb temp gauge.
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Lycs are a completely different animal. I've flown many years behind a Lyc, and I had to use carb heat only once. Flying behind a Conti, you are going to use it almost every flight. In a two hour flight today, the engine monitor was flashing red about carb temp almost continuously. I only used CH as a precaution on landing because there was no visible moisture. But any more moisture and it would have been on almost all of the time.
 
rockwoodrv9a said:
I only have about 120 hours on my plane but I have never used carb heat. I am in Michigan so I am prime for it but my plane has a carb temp sensor. Even on the coldest day I flew last winter it was never below 100 once the engine was warm enough for take off. I have an O-320 with the carb essentially mounted to the bottom of the oil tank so if the oil is warm the carb is also. That and how the air box is set up, keeps the carb warm. I do check to make sure it works on run up and on downwind just to make sure it works.
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If your carb temp is over 100 all the time on a cold day, there's something wrong with the carb heat rigging or the temp sensor or gauge are lying to you. Carburetors cool the air a whole lot, and if yours isn't, you not only have a mechanical issue, you are sacrificing power due to the lowered air density.

I have had carb ice many times in O-320s.
 
Another thing to remember about carb heat is that when you have ice and turn it on, there is a good chance the engine will run worse than with it off. Leave the carb heat in until the engine at least smooths out, increased roughness means you have ice. Apparently this is a thing, or so I was told when I flew airplanes that have carb heat. People get wigged out pull the carb heat out and the engine stops running due to the ice.
 
PaulS said:
...increased roughness means you have ice.
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Well, more likely it means you had carb ice and now that it's melting you have carb water. There's only one way for carb ice to exit and that's through the combustion chambers and out the exhaust, so yes, you can expect increased roughness for a short while.
 
I will have to recheck this winter the temps. My engine runs about 385 max CHT and 1400 EGT with oil on the low side and difficult to get hot in the winter. I have to use aluminum tape over my oil cooler and block part of my air intake to get it up to 130. I have all my flights uploaded to Savvy and so I will go check the carb temps. One thing about the G3X is it records everything. I can not believe the amount of data each flight produces.

Edit - I just checked.
Nov 2021
OAT - 24 F
Oil 128 MAX
CARB 68-101F

JAN 2021
OAT 9 F
OIL - 134 MAX
CARB 74-110

It looks like I was remembering incorrectly. But, even on those clod days, the lowest carb temp was 68. I know the fuel venturi can lower the temp and cause carb ice in some planes more than others. The temp sensor works and when I pull the carb heat cable, the temp rises pretty quickly. I check it at each run up and begore I cut power, I just have never needed to use it.
 
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Virga happens quite often where I live. I see it a couple dozen times year.

I’m my O520, I see carb temp in the low teens in the winter, low 40’s on a hot day. Haven’t had ice in a long time, but have had it bad on a couple of occasions that in the past. I run carb heat in heavy rain if the carb temp is less than 35. But, if carb heat isn’t on and it’s not a dry day, I check for ice regularly.
 
rockwoodrv9a said:
It looks like I was remembering incorrectly. But, even on those clod days, the lowest carb temp was 68. I know the fuel venturi can lower the temp and cause carb ice in some planes more than others. The temp sensor works and when I pull the carb heat cable, the temp rises pretty quickly. I check it at each run up and begore I cut power, I just have never needed to use it.
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I would still suspect that the carb heat is misrigged, leaving it partially on. The sump's heat can't fight the cold that much. Cold air coming in can suck so much heat out of the carb body it's not funny, and adding the pressure drop and evaporation of the fuel really refrigerate it.
 
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