How long should it take for Carb heat to clear ice?

brcase

brcase

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Brian
Been my month for Carb Heat issues.

After previously having some issues with throttle cables I have developed the habit of checking the cable connections to the carburetor as best I can during the preflight. Earlier this month found a Cherokee where the Carb Heat cable that had become disconnected (probably would have found it on the run up also).

Then 2 hours into a 3 hour flight with a Cessna 140, had the carb heat cable fail and the carb heat defaulted into the On position, causing about a 500 RPM Drop after leaning a bit more and I assume melting a bit of ice it turned into a 150 RPM drop until I could land to get it fixed. It did take a surprising amount of time for the RPMs to come back.

This morning I was flying a Cessna 150 35F and 93% Humidity. Runup was normal but perhaps a hint of ice during the carb heat check (250 rpm drop then came up to 150 drop). I took off with what seemed like normal TO power at about 400 feet RPM's started dropping, I pulled the carb heat and power dropped to about 2100 RPM which pretty much stopped my climb at about 500 feet. I started a close in turn back toward the runway but proceeded down wind keeping the runway withing gliding range, about midfield power started coming back up to normal. Probably took at least a full minute (seemed longer) to get back to full power.

Brian
 
Its been my experience that Carb Heat clears out whatever ice is there in a few seconds. Unless, of course, there is enough ice to completely choke the carb.
 
The small Continentals (like those in the Cessna 140 and 150) are famous ice machines. In conditions like those posted, you are guaranteed carb ice at some point before you even take off. It's smart to check for ice again immediately before takeoff if you've taxied a while or been idling after the runup, as it will be making ice in that time. In mine, I will taxi to the takeoff point with the carb heat on just to get it warmed up good for the takeoff, and after the throttle is wide open the ice is much less likely.

Don't think about percent humidity. Aviation weather uses dewpoint instead, and at high relative humidity the spread between the ambient temperature and dewpoint temperature is small, raising the risk of carb ice. And it can happen at much warmer temperatures, too. Carb ice is one of those things not taught nearly well enough to students, and some pilots, sooner or later, eventually lose an engine to it. It's one of the major causes of engine failure.

There are numerous examples of the carb ice chart on the 'net. This is one:

FAA-Carb-Ice-Chart-0609a.jpg


Dan
 
brcase said:
Been my month for Carb Heat issues.

After previously having some issues with throttle cables I have developed the habit of checking the cable connections to the carburetor as best I can during the preflight. Earlier this month found a Cherokee where the Carb Heat cable that had become disconnected (probably would have found it on the run up also).

Then 2 hours into a 3 hour flight with a Cessna 140, had the carb heat cable fail and the carb heat defaulted into the On position, causing about a 500 RPM Drop after leaning a bit more and I assume melting a bit of ice it turned into a 150 RPM drop until I could land to get it fixed. It did take a surprising amount of time for the RPMs to come back.

This morning I was flying a Cessna 150 35F and 93% Humidity. Runup was normal but perhaps a hint of ice during the carb heat check (250 rpm drop then came up to 150 drop). I took off with what seemed like normal TO power at about 400 feet RPM's started dropping, I pulled the carb heat and power dropped to about 2100 RPM which pretty much stopped my climb at about 500 feet. I started a close in turn back toward the runway but proceeded down wind keeping the runway withing gliding range, about midfield power started coming back up to normal. Probably took at least a full minute (seemed longer) to get back to full power.

Brian
Click to expand...

Sounds to me your issue is poorly maintained airplanes more than carb ice.
You should have enough power for climb out at takeoff with carb heat on and the ice should clear in a few seconds at full power.
 
Last edited:
DanThomas- Just realize that taxiing with carb heat also allows bypass of the induction filter, which is where you suck fine particulates into the engine.
 
Clip4 said:
Sounds to me your issue is poorly maintained airplanes more than carb ice.
You should have enough power for climb out at takeoff with carb heat on and the ice should clear in a few seconds at full power.
Click to expand...

That is my concern that there might be something wrong, but don't have any idea what would cause the carb heat to work as expected. 150 to 200 drop during runup and full power, but when it seems to have ice then it drops 400 RPM and can take up to a minute or more for it to clear.

Brian
 
Jaybird180 said:
DanThomas- Just realize that taxiing with carb heat also allows bypass of the induction filter, which is where you suck fine particulates into the engine.
Click to expand...

Not on pavement unless there's a dust storm, unlikely at high humidity.

You learn to think and take your pick. Tolerate ice buildup to eliminate any possibility of ingesting a bit of dust, or carefully use the heat to prevent ice and be alive to finish the flight? Sometimes students learn too much by rote: "We simply cannot use carb heat on the ground!"

You'd be surprised at how many cracks and crevices there are to let dust into an engine, right past that filter even when carb heat is off. The carb heat flapper valve is a good example.

Dan
 
usually long enough to scare and tempt you to put the carb heat back in as it ingests melted ice and the RPMS drop, but don't :).

In any case, it should stabilize in seconds.

And unless you're on a dirt field, I always test it on the ground. If not, I test it before going too far.
 
I am beginning to think the answer is, it depends. In most cases carb ice seem to clear within a few seconds. However looking at the chart posted above I suppose it might be possible that the carb heat just moves the temperature from one end of the orange area to the other end. I know have had a few pilots tell me that in very cold temps turning on the carb heat can just warm it up enough to induce carb ice, don't know if that is true or not.

I have flown a few planes with a carb temp guage, next time I will have to observe how much the carb heat increases the carb temperature. I don't know if I have ever seen a gauge in a 150, so it probably won't be and apples to apples comparison.

Brian
 
brcase said:
I am beginning to think the answer is, it depends. In most cases carb ice seem to clear within a few seconds. However looking at the chart posted above I suppose it might be possible that the carb heat just moves the temperature from one end of the orange area to the other end. I know have had a few pilots tell me that in very cold temps turning on the carb heat can just warm it up enough to induce carb ice, don't know if that is true or not.

I have flown a few planes with a carb temp guage, next time I will have to observe how much the carb heat increases the carb temperature. I don't know if I have ever seen a gauge in a 150, so it probably won't be and apples to apples comparison.

Brian
Click to expand...

If the weather is VERY cold and IF there are ice crystals in the air, you might induce ice by melting those crystals (or sublimating them into water vapor) and that then ices in the carb. A rather rare event. Most of us don't care to fly at temps below -20C.

Continentals have a much more powerful carb heat system than the Lycomings. A 150's system will heat the air significantly. An older 182 or 180 will run pretty rough with full carb heat since it gets so hot and the mixture goes way rich. The Lycomings have the carb bolted to the hot oil sump, heating the carb body and reducing the need for lots of heat. You'll notice a really minimal RPM drop on a Lyc when pulling the heat, compared to a Continental.

If you'll remember that the dewpoint stays the same when the carb heat is on, you'll see on the chart that raising the ambient temperature with the heat moves the icing risk out of the worst areas. As the throttle closes and less air is taken in, that air gets hotter as it takes longer to flow through the heat muff, correlating to the need for hotter air at low power settings where the risk is the worst.

Dan
 
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