carb heat in cold weather

Peter Ha

Peter Ha

Pre-takeoff checklist
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Hey folks,
Is it best practice to turn on carb heat when weather is below 70s to prevent carb-ice?
Fear of carb ice on my C150 when I see colder then normal temps while flying below 2000ft.
How about turn on just a "little" carb heat to prevent carb-ice?
 
NEVER use partial carb heat, unless you have an installed carb temperature gauge.

Partial carb heat runs the danger of melting some of the ice, with the meltwater then running further down the intake manifold, where it refreezes. That ice may not be meltable even with full carb heat. In your 150, all or nothing! -Skip
 
Skip Miller said:
NEVER use partial carb heat, unless you have an installed carb temperature gauge.

Partial carb heat runs the danger of melting some of the ice, with the meltwater then running further down the intake manifold, where it refreezes. That ice may not be meltable even with full carb heat. In your 150, all or nothing! -Skip
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OIC, this must of been in my training lessons but I forgot the ALL-or-NOTHING. Thanks
I've been trained to turn on carb-heat during pattern to land. So I'll just turn on FULL below 2000ft when it's below 70s just to be safe.
 
Peter Ha said:
OIC, I've been trained to turn on carb-heat during pattern to land. So I'll just turn on FULL below 2000ft when it's below 70s just to be safe.
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Keep in mind that carb heat bypasses the air filtration for the engine, so other problems can develop if you use it that indiscriminately.

Read the PHAK section.
 
Peter Ha said:
I've been trained to turn on carb-heat during pattern to land.
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Yeah, that’s one thing I had to adjust to when flying the Archer. In the Cessna’s the carb heat is used to prevent carb ice and is part of the landing checklist, in the Archer, it’s only used on the suspicion that you have carb icing.
 
Another caution is to use it for long enough to see if it has any effect or not... the effect, if you DO have carb ice, is not instantaneous, but may take 15 to 30 seconds or more to see if you start to gain any RPM. If you apply it only for a few seconds and then turn it off, you can not only miss the fact that ice may already exist, but actually make things worse and increase the rate in which it develops. Applying carb heat is part of a pre-landing check in my Cherokee, but only as a check for ice... apply it, get the expected immediate reduction in RPM.... WAIT.... see if the rpms start to climb back on their own. If they don't, no ice, carb heat back out, and continue on...

IIRC, carb heat on was part of prelanding and landing in the Cessna 152 I initially trained on, but it's been a long time.
 
MuseChaser said:
IIRC, carb heat on was part of prelanding and landing in the Cessna 152
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It is, but realize that the O200 in the 150 is a bit different than the O235 in the 152.

In most Lycoming’s the carbs are updraft, located below the engine, with the intake manifolding routed through the oil sump, so they’re not as much of an ice maker as the carb’d Continentals are.
 
Ryanb said:
It is, but realize that the O200 in the 150 is a bit different than the O235 in the 152.

In most Lycoming’s the carbs are updraft, located below the engine, with the intake manifolding routed through the oil sump, so they’re not as much of an ice maker as the carb’d Continentals are.
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Okay, I’ve heard this a bunch of times, but I’m dying for someone to explain to me how heating the intake air that’s already gone through the carb helps to prevent carb ice.
 
Salty said:
Okay, I’ve heard this a bunch of times, but I’m dying for someone to explain to me how heating the intake air that’s already gone through the carb helps to prevent carb ice.
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Heat transfer for one, those pipes and manifold conduct heat. I had a 73 charger with a 318 that I bought for 600 bucks. That thing would run like crap when it rained. I finally pulled the air cleaner off and looked down the carb, you could see the throat of the carb coated with about an eighth of inch of rime ice. It would eventually stall out. So I went the library and got a factory manual and found out there was an exhaust channel under the carb, that was supposed to heat the carb. I pulled the manifold off and found that passage choked full of carbon. I chopped the carbon out with a screwdriver, lol, put it back together and never had the problem again.

Carb ice sucks. I experienced it a few times in the Skippers I did my primary training in here in New England. I was told that when the engine starts running rough, pull in the carb heat, on full and expect the engine to get worse as the ice melts. Also, as the engine is running rough, it generates less heat to melt the ice, so pulling it, then turning it off because the engine got rougher really handicaps your ability to rid the engine of ice when you need it most. Pull it in if you need it, and leave it in. If conditions were right, or if we suspected carb ice in cruise, we would add full carb heat, usually for about 30 seconds, if nothing happened then it was good, I'd shut it off, if the engine stumbled, then it would stay on for a while. We always used it in the pattern for landing.
 
Salty said:
Okay, I’ve heard this a bunch of times, but I’m dying for someone to explain to me how heating the intake air that’s already gone through the carb helps to prevent carb ice.
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Doesn’t have anything to do with that, it’s about carb placement between the two manufacturers. The spider duct that connects the carburetor to the intake runners going to the cylinders are cast right onto the oil sump and the carb is mounted directly to that. So the carb gets a lot of engine heat that naturally warms the carb body, not to mention that it's getting heat from the sump directly above...so it’s basically a two for one.

The carb on Continentals are much more isolated from the engine sump/case, so it gets colder much easier and is more susceptible to carb ice.
 
PaulS said:
So I went the library and got a factory manual and found out there was an exhaust channel under the carb, that was supposed to heat the carb.
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Ahh yes, the old Heat Riser. A thermostatically conrolled diverting valve in the exhaust system. The one I am (read: was) familiar with (318 ci 1955 Plymouth V-8) rattled like a 52 Chevy and did the job unless completely frozen by a combination of rust and carbon as you found out. I had forgotten about those!

-Skip
 
WannFly said:
read the graph that @Ryanb posted above, and dont make a rule - i will only use carb heat when its below 70 degrees
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For me in a 182 that’s the times I have suspected the most. Warm hazy evenings with wind down on approach. Thing is an icebox
 
PaulS said:
Heat transfer for one, those pipes and manifold conduct heat. I had a 73 charger with a 318 that I bought for 600 bucks. That thing would run like crap when it rained. I finally pulled the air cleaner off and looked down the carb, you could see the throat of the carb coated with about an eighth of inch of rime ice. It would eventually stall out. So I went the library and got a factory manual and found out there was an exhaust channel under the carb, that was supposed to heat the carb. I pulled the manifold off and found that passage choked full of carbon. I chopped the carbon out with a screwdriver, lol, put it back together and never had the problem again.

Carb ice sucks. I experienced it a few times in the Skippers I did my primary training in here in New England. I was told that when the engine starts running rough, pull in the carb heat, on full and expect the engine to get worse as the ice melts. Also, as the engine is running rough, it generates less heat to melt the ice, so pulling it, then turning it off because the engine got rougher really handicaps your ability to rid the engine of ice when you need it most. Pull it in if you need it, and leave it in. If conditions were right, or if we suspected carb ice in cruise, we would add full carb heat, usually for about 30 seconds, if nothing happened then it was good, I'd shut it off, if the engine stumbled, then it would stay on for a while. We always used it in the pattern for landing.
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Ryanb said:
Doesn’t have anything to do with that, it’s about carb placement between the two manufacturers. The spider duct that connects the carburetor to the intake runners going to the cylinders are cast right onto the oil sump and the carb is mounted directly to that. So the carb gets a lot of engine heat that naturally warms the carb body, not to mention that it's getting heat from the sump directly above...so it’s basically a two for one.

The carb on Continentals are much more isolated from the engine sump/case, so it gets colder much easier and is more susceptible to carb ice.
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The oil sump blanket I bought came with little insulating sleeves to snap over the intakes; they can ice up downstream of the carb.
 
I get carb ice all the time on my 150 even In Cruise configuration. Doesn’t hurt to leave it on in cruise as long as you properly lean. Better to prevent than to have to melt it away.
 
GeorgeC said:
The oil sump blanket I bought came with little insulating sleeves to snap over the intakes; they can ice up downstream of the carb.
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Perhaps, but that's not carb ice.

The argument starts with "the intakes warm the air going through the sump" and then changes to "heat transfer to the carb". I think it was just easier to put the carb on the bottom and they came up with some reasons to make it sound good. JMO. Maybe "intake ice" is also a problem, but it seems unlikely for ice to form there if it hadn't already formed in the carb.
 
Carb ice happens in the venturi. Proximity to the Lycoming sump has nothing to do with anything other than it robs power by warming the intake mixture. Lycoming powered Cubs make ice quite well. Meanwhile my 0-520 Skywagon rarely makes any. Both examples are contrary to popular myth.
 
Salty said:
Perhaps, but that's not carb ice.

The argument starts with "the intakes warm the air going through the sump" and then changes to "heat transfer to the carb". I think it was just easier to put the carb on the bottom and they came up with some reasons to make it sound good. JMO. Maybe "intake ice" is also a problem, but it seems unlikely for ice to form there if it hadn't already formed in the carb.
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I’m not really sure what argument you’re referring too. I’m not arguing which style is better, I’m just explaining why one variant tends to be more susceptible to making ice over the other.
 
It's not just ambient temperature that causes carb ice. It's the dewpoint, which is known by non-pilots as relative humidity. If the air is dry, carb ice is unlikely. It it's plenty wet, watch out. So guys in Arizona seldom see it, while guys in Oregon or New York might battle it all the time.

Check the METARs. They give the temperature and dewpoint for good reason. A small temp/dewpoint spread means ice is more likely. Big spread, not so much. But that also depends on the engine and installation; you have to figure out for yourself what it might do. Small Continentals are bad for it; Lycs, not so much, though some will make ice easily, as Stewartb noted. We had a Citabria 7GCBC with an O-320-A2B that would ice up in a heartbeat, while the O-320-E2Bs in the 172s wouldn't. Never did figure that one out.

That carb ice chart above gives an idea as to what the risk is depending on temp and dewpoint. Pay attention to the METAR and check the chart.
 
I know a guy with an identical year Skywagon. Same motor, same everything. He says his plane is an ice maker. Mine is anything but. I can say the same about my old Cub, which I sold to a friend after he crashed his due to carb ice. Mechanically it makes no sense. I believe ice is related to how a pilot operates the engine as much as anything mechanical.
 
Stewartb said:
I know a guy with an identical year Skywagon. Same motor, same everything. He says his plane is an ice maker. Mine is anything but. I can say the same about my old Cub, which I sold to a friend after he crashed his due to carb ice. Mechanically it makes no sense. I believe ice is related to how a pilot operates the engine as much as anything mechanical.
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Ice also forms at the throttle plate when it's nearly closed, not just at the venturi. The tight gap between the edge of the throttle plate and carb bore wall acts as a venturi, which is why the idle nozzles are right at that point in the wall. If a pilot is in the habit of making long, low, slow approaches at minimum power, carb ice could be more likely than if he comes in higher and closes the throttle altogether. Some folks, in an effort to reduce fuel flow ($$) or get the tach to record time more slowly, will run at reduced power where ice is more likely. And Mogas makes ice considerably more likely, since its volatility is higher. It evaporates faster and cools the carb more. Some 172s that might be otherwise identical, might have different ice propensities; there's a plastic baffle around the induction filter on those airplanes from about 1970 to 1986 that is supposed to prevent outside air coming around the filter housing, to improve the pressure differential above and below the engine for better cooling, and that baffle is often broken or missing and the air surging through there could cool the carb more. Worn and broken cooling baffles and seals can do funny stuff.

The variables are almost endless.
 
Peter Ha said:
OIC, this must of been in my training lessons but I forgot the ALL-or-NOTHING. Thanks
I've been trained to turn on carb-heat during pattern to land. So I'll just turn on FULL below 2000ft when it's below 70s just to be safe.
Click to expand...

Consult your POH regarding carb heat use. It varies from plane to plane. For Grummans you check carb heat, but do not use it continuously for landing unless conditions warrant. I've NEVER had carb ice in the pattern. When I trained in a C152, I think I recall that carb heat application was normal during all landing operations.

For any plane application of carb heat during extended partial throttle operations, especially in potential icing conditions, is warranted. The only time I have ever had a load of carb ice was during an IPC where we were doing a lot of extended holds and low power approaches in and out of the clouds. I check carb heat frequently in these conditions now, or just apply it prophylactically.

Full carb heat for sure. Partial application can exacerbate carb icing.
 
My O-320-E2D has never had a problem with carb ice. I have flown it in all types of weather. SE AK is humid to say the least, never had an issue with carb icing.

I always thought it was the way the carb was mounted and the induction system. Maybe it's the way I manage the engine, not sure but I'm happy.

Now the 150/152 I trained in, in northern IL would ice up on a summer day.
 
Peter Ha said:
OIC, this must of been in my training lessons but I forgot the ALL-or-NOTHING. Thanks
I've been trained to turn on carb-heat during pattern to land. So I'll just turn on FULL below 2000ft when it's below 70s just to be safe.
Click to expand...
I hope someone corrects me if I am wrong. I think there are other scenarios where you will want to pull carb heat sooner. For example, lets say you are arriving from 6500 and descending down to that pattern altitude of 2000. Sometimes you might be pulling the power way back for this descent. I would be pulling carb heat sooner (at a higher altitude) in this scenario.

Another one is if you are lined up behind 12 planes, idling and all waiting to take off on a humid day it could be making ice. In this case I will pull carb heat for a while prior to takeoff in case its starting to ice up. There are many takeoff power loss incidents where afterwards the engine runs just fine and its thought the evidence simply melted away.
 
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