Carb Heat in rain

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What is your guys practice for use of carburetor heat while flying in rain? on/off? temps aloft dependent? humidity? only on when suspected carburetor ice or power setting outside of the normal operating range?
 
Just yesterday, moving the plane for annual the ceiling came down to MVFR and it started raining lightly. Air temps were in the low 40's and it was perfect wx for carb ice (182). It was only a 8 minute flight but I pulled carb heat every other minute in the rain to just to make sure ice wasn't getting a head start on me. If the air temps were in the 80's and raining I don't think I'd being pulling carb heat unless I saw the MP dropping. This for a carb'd aircraft of course. You probably want to check the POH as it may be in there and you missed it.
 
On the Cherokee 140 (1966 model) the POH (or whatever you call it in the older type Cherokees) actually state to only use carb heat during landing, if ice is suspected. I learned in Cessnas and they say always use carb heat when landing. I still follow that practice even with the Cherokee. Why does Piper not recommend using carb heat during the landing phase of flight?
 
Chrisgoesflying said:
On the Cherokee 140 (1966 model) the POH (or whatever you call it in the older type Cherokees) actually state to only use carb heat during landing, if ice is suspected. I learned in Cessnas and they say always use carb heat when landing. I still follow that practice even with the Cherokee. Why does Piper not recommend using carb heat during the landing phase of flight?
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It's because of how the intake piping is routed. Even in Cessna and pipers with the same engine, the pipers route the intake by the exhaust, which preheats the air in the intake

Sent from my SM-G996U using Tapatalk
 
Neal Wilkinson said:
It's because of how the intake piping is routed. Even in Cessna and pipers with the same engine, the pipers route the intake by the exhaust, which preheats the air in the intake

Sent from my SM-G996U using Tapatalk
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The Piper's exhaust runs around the front and back of the engine and radiates heat to all sides of the carb. A 172's is almost all up front.

Typical 172 exhaust system. The carb sits right about where the "1" is located.

upload_2022-10-26_18-23-5.png


Typical PA-18 exhaust system. The carb sits in the middle of all that.

upload_2022-10-26_18-25-21.png
 
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Chip Sylverne said:
Not sure that's the case. I was thinking more high humidity and air cooling as it passes through the venturi, not ingesting water.
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Yup. Rain falling through air will increase its dewpoint temperature. Virga is the result of rain falling through dry air, and the rain evaporates before reaching the ground. The dewpoint will rise in that case, too.
 
I fly a small continental- a model known to make ice better than a fridigidaire.

I’ve had this same wonder OP. Not conserned about ingestion but the idea of air going through a wet sponge out front is extra humid now….

I always pull carb heat periodically in cruise, leave it out 20 seconds and carry on. I always want to be ahead of it. In rainy weather I just try to be all the more vigilant.

looking for a minor power drop isn’t that helpful with a small engine as she’s always needing a throttle adjustment in or out, more than a big beast…

I see no reason to not periodically giving her a blast of heat- but if you do- just do it long enough to have effect not just melt it a bit n send it deeper in…
 
I was taught to use carb heat when flying in the clouds when there is a potential for carb ice.
 
When to use carb heat? When the temperature in the carb is conducive to ice formation. How do you know what that temperature is? Get a carb heat temperature probe/gauge. In the realm of aviation expenses, they are dirt cheap, and worth every penny.

There are a lot of OWT of how and when to use carb heat. I've had three very eye-opening experiences with carb ice, and all three were at about 300' AGL after takeoff. One in a PA28-160, the other two in a 182.
 
Jeff Oslick said:
When to use carb heat? When the temperature in the carb is conducive to ice formation. How do you know what that temperature is? Get a carb heat temperature probe/gauge. In the realm of aviation expenses, they are dirt cheap, and worth every penny.

There are a lot of OWT of how and when to use carb heat. I've had three very eye-opening experiences with carb ice, and all three were at about 300' AGL after takeoff. One in a PA28-160, the other two in a 182.
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If I did this in my 182, I would have to leave the carb heat on all the time in fall, winter and spring. Mine pretty much lives in the yellow other than hot days.
 
Jeff Oslick said:
When to use carb heat? When the temperature in the carb is conducive to ice formation. How do you know what that temperature is? Get a carb heat temperature probe/gauge. In the realm of aviation expenses, they are dirt cheap, and worth every penny.

There are a lot of OWT of how and when to use carb heat. I've had three very eye-opening experiences with carb ice, and all three were at about 300' AGL after takeoff. One in a PA28-160, the other two in a 182.
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Doesn't sound like fun. What did you do in each case, and what was the outcome?
 
Piperonca said:
Doesn't sound like fun. What did you do in each case, and what was the outcome?
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Moved my hand very, very quickly from the throttle to the carb heat control...

I experienced significant engine "stumble", and caught it before substantial loss of power.

Never let your guard down.
 
AlphaMike said:
If I did this in my 182, I would have to leave the carb heat on all the time in fall, winter and spring. Mine pretty much lives in the yellow other than hot days.
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I pull partial carb heat when the (digital) gauge gets below 40F unless the air is very dry. Carb heat isn't an all-or nothing, you can add just what you need. Many (carb'd) 182 operators find they have better fuel distribution when using partial carb heat, because the 182 induction system is about as primitive as it gets.
 
Jeff Oslick said:
Carb heat isn't an all-or nothing, you can add just what you need. .
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I agree with this but, you will hear from others that partial carb heat can cause ice to form deeper in the carb. I've heard "stories" of partial carb heat, causing ice deep in the carb that full heat would not melt.
 
AlphaMike said:
I agree with this but, you will hear from others that partial carb heat can cause ice to form deeper in the carb. I've heard "stories" of partial carb heat, causing ice deep in the carb that full heat would not melt.
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If you only pull heat to get a mid-thirties temperature, I could see that being a risk. When I pull heat, it is to get it at least into the low to mid 40s.
 
Checkout_my_Six said:
Cause he doesn’t understand how the carb heat box flows air. Rain will not enter the flow.
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It's not about the rain entering the carb, it's about the increased moisture in the air that generally happens when it rains and yes, some engines are definitely more susceptible to carb ice when it is raining.

Let's just say that I learned the hard way to use carb heat when flying near/around rain showers in the T6....
 
AlphaMike said:
If I did this in my 182, I would have to leave the carb heat on all the time in fall, winter and spring. Mine pretty much lives in the yellow other than hot days.
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Yup. Most carbs will. There's a temperature drop in there as a result of the venturi, and as the throttle is partly closed in cruise there's a drop there, too. Then we add the evaporation of the fuel on top of those and we can get a total drop of over 70°F. That will put the gauge into the yellow nearly all the time. The gauge is one of those false-security doodads. Knowledge and information is better.

The feds publish aviation weather info for a reason. They give us temperature and dewpoint for a couple of reasons, and carb ice is one of those. (The other is to give an idea of the likelihood of fog formation.) The pilot of a carbed airplane needs to check the METAR and pay attention to the temp/dewpoint stuff.

Current METAR for Vancouver, BC. Temp and dewpoint circled in red. Pretty close. Carb ice really likely. Look what's circled in green in the TAF: mist, due to the small temp/dewpoint spread.

upload_2022-10-27_16-5-29.png

On the chart (in °C), red dot:

upload_2022-10-27_16-11-5.png
 
Visible moisture? I'm using it, or at least checking for ice pretty regularly. If I need to pull power, it's going in, hopefully before power is pulled. Carb ice is no joke, great question. And sometimes it doesn't obey supposed rules.
 
Jeff Oslick said:
If you only pull heat to get a mid-thirties temperature, I could see that being a risk. When I pull heat, it is to get it at least into the low to mid 40s.
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low to mid 40s certainly isn’t going to melt ice if any formed Already. And your probe is in one spot, not necessarily always the coldest spot. I would not use carb heat in this way. For me, once it goes on, it’s staying on full for a minimum of 3 or 4 minutes. A little hot air doesn’t make ice disappear instantly. Pull out a hair dryer and melt an ice cube with it sometime. It’ll take awhile.
 
I have read the whole thread, and have not seen part two of flying in icing conditions, rain or other types.

Carb heat reduces HP for a number of reasons.
The longer path is more friction loss, and lower pressure at the intake valves.
The increase of air temperature reduces its density.
The lower air pressure at the venturi causes more fuel flow, making an over rich mixture.

The over rich mixture should be corrected with the mixture control, regaining some of the lost HP, reducing the fuel evaporation to reduce the evaporative cooling, and regaining most of the flight planned range.

I have encountered ice many times in high dew point days, and have learned that I had to leave the heat on for about a minute to be sire that any ice that had formed was gone. 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.

One of our planes had a carb temperature guage, and it gave me quite an education about temperature change from OAT to top of the carb.

Back to the OT's question, 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.
 
Interesting question.
According to the POH for a training aircraft I flew this week (N584SF), the POH for a 1968 Cessna 172 and Skyhawk states:
From page 2-11
"The use of full carburetor heat is recommended during flight in heavy
rain to avoid the possibility of engine stoppage due to excessiv,e water ingestion
or carburetor ice. The mixture setting should be readjusted for
smoothest operation.
In extremely heavy rain, the use of partial carburetor heat (control
approximately 2/3 out), and part throttle (closed_ at least one inch), may
be necessary to retain adequate power. Power changes should be made
cautiously followed by prompt adjustment of the mixture for smoothest
operation."

Being a student, I've not yet flown in any rain, so I was pondering that situation and specifically looked for the answer a few months ago.
 
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.
 
Salty said:
low to mid 40s certainly isn’t going to melt ice if any formed Already. And your probe is in one spot, not necessarily always the coldest spot. I would not use carb heat in this way. For me, once it goes on, it’s staying on full for a minimum of 3 or 4 minutes. A little hot air doesn’t make ice disappear instantly. Pull out a hair dryer and melt an ice cube with it sometime. It’ll take awhile.
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You're missing my point that I don't let the engine operate at carb temps that could form ice to being with. I'm religious about monitoring carb temp.
 
Jeff Oslick said:
You're missing my point that I don't let the engine operate at carb temps that could form ice to being with. I'm religious about monitoring carb temp.
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And you missed my point that a low 40s reading is no guarantee.
 
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