Carb Heat in IMC?

1

121Dispatch

Pre-takeoff checklist
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I was flying a Warrior on a trip to SC this weekend. We filed IFR in anticipation of getting on top of a layer of broken to overcast clouds. Unfortunately, we spent the entire trip in the soup at 5,000. I would have gone up to 7,000, but we were showing +3C, and noticable water around the OAT sensor, so I didn't feel like seeing if we would pick up ice on the climb.

Anyways, something that I've asked a few CFI's, and never really gotten a solid answer is: Do I need to use Carb Heat the whole time I'm in IMC? I'm pretty sure the answer is 'no, turn it on if the RPM/MP begins to drop once it is set at cruise power.' Well, we were at max gross, which included a newly minted PPL, his girlfriend, and my wife, so I didn't really want to deal with having the engine start running rough, then add carb heat and have it run even rougher, so I elected to keep the carb heat on the whole time we were in the soup.

As a commercial pilot, I should really know this stuff, but what is the rule of thumb on this...and are there any side-effects to keeping the carb heat on for so long...other than the obvious loss of RPM?
 
increased chance of detonation, if i remember the books right. running at lower power output and richer mixture. of courrse you're putting unfiltered air into the engine too.
 
tonycondon said:
increased chance of detonation, if i remember the books right. running at lower power output and richer mixture. of courrse you're putting unfiltered air into the engine too.
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Detonation is a lean phenomenon--not rich. IOW, carb heat will not induce detonation.

To the OP--the hazard in running carb heat constantly is that you potentially move induction system icing (if it occurs) deeper into the intake system--and with carb heat applied when/if the ice formed you now have no way to remove it.
 
121Dispatch said:
Do I need to use Carb Heat the whole time I'm in IMC? I'm pretty sure the answer is 'no, turn it on if the RPM/MP begins to drop once it is set at cruise power.'
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Close. Drop off the "once it is set at cruise power" part and you'd be correct. If you see a drop in RPM with roughness during climb, it may well be carb ice, even at full throttle.

Well, we were at max gross, which included a newly minted PPL, his girlfriend, and my wife, so I didn't really want to deal with having the engine start running rough, then add carb heat and have it run even rougher, so I elected to keep the carb heat on the whole time we were in the soup.
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The problem with doing that is you lose power in the climb. In addition, in really cold conditions (like well below zero C), the application of carb heat can actually cause carb ice to form -- and you can't get rid of it. What happens is the carb heat warms the incoming air above freezing, so the frozen water melts, and then refreezes in the carb throat. Since the outside air is so cold, carb heat can't warm it enough that it will stay above freezing as it goes through the carb throat, and that means you can't get rid of what's formed.

All in all, unless you have a carb air temp gauge and can modulate the carb heat to keep the carb throat temp just above freezing, don't use carb heat in climb or cruise unless you have carb ice symptoms, and then only long enough to melt out the ice that's formed.
 
Thanks guys. That ties together some loose ends I have received from other folks. Had I known we would have been spending so much time in IMC, I would've asked the question here on the board before the trip.

BTW...I've spent quite a bit of time doing airline travel lately, and have listened to several of the pilotcast's...great stuff...I really enjoy them. Keep it up!!!
 
tonycondon said:
of courrse you're putting unfiltered air into the engine too.
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That really is not that big of a risk at altitude. There is not much in the air that will really be caught by the filters in our airplanes. Those filters seem to have problem catching anything that is smaller than a grain of sand anyways. Up high in the sky the air is pretty clear of that stuff unless you are flying through volcanic ash or something like that.

But I would only use the carb heat if the engine is showing signs of carb ice. Leaving it on all the time will cost you a lose in power and could end up causing ice in the carb in very cold conditions.
 
Ron Levy said:
In addition, in really cold conditions (like well below zero C), the application of carb heat can actually cause carb ice to form -- and you can't get rid of it. What happens is the carb heat warms the incoming air above freezing, so the frozen water melts, and then refreezes in the carb throat. Since the outside air is so cold, carb heat can't warm it enough that it will stay above freezing as it goes through the carb throat, and that means you can't get rid of what's formed.
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Hey Ron,

How is that any different than applying carb heat when the temps drop? By that I mean if this is true:

Ron Levy said:
Since the outside air is so cold, carb heat can't warm it enough that it will stay above freezing as it goes through the carb throat, and that means you can't get rid of what's formed.
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then how does carb heat do you any good at all when it's applied after ice has formed?

I guess what I'm asking is, what difference does having it on before the symptoms appear make? If it can't melt ice in the carb, it can't melt ice in the carb.

What am I missing?
 
N50909 said:
Hey Ron,

How is that any different than applying carb heat when the temps drop? By that I mean if this is true:


then how does carb heat do you any good at all when it's applied after ice has formed?

I guess what I'm asking is, what difference does having it on before the symptoms appear make? If it can't melt ice in the carb, it can't melt ice in the carb.

What am I missing?
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The carb heat is only so hot. It's entirely possible that the air might still be below freezing after full carb heat. For example If the OAT is about 5 degrees and you apply carb heat you could be raising the temperature of the air to just barely below freezing which is where it's going to cause a big problem. You also must remember that even if the air going into the carb is above freezing it may cool in the carb enough to freeze.

I made the engine in a C-150 quit by applying carb heat when it was about 10 degrees outside.

The colder the air gets, the less moisture it can hold, and the less likely you are to have carb ice problems.
 
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jesse said:
The carb heat is only so hot. It's entirely possible that the air might still be below freezing after full carb heat. For example If the OAT is about 5 degrees and you apply carb heat you could be raising the temperature of the air to just barely below freezing which is where it's going to cause a big problem. You also must remember that even if the air going into the carb is above freezing it may cool in the carb enough to freeze.

I made the engine in a C-150 quit by applying carb heat when it was about 10 degrees outside.

The colder the air gets, the less moisture it can hold, and the less likely you are to have carb ice problems.
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Yeah, I get that carb heat only raises the temps so much. What I don't get is what difference it makes if carb heat's on the whole time or just used if there's already ice. Either way it seems it won't melt the ice based on what you both have posted.
 
N50909 said:
Yeah, I get that carb heat only raises the temps so much. What I don't get is what difference it makes if carb heat's on the whole time or just used if there's already ice. Either way it seems it won't melt the ice based on what you both have posted.
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If the air is really cold, moisture in the air will be frozen and the temperature drop in the carb will not do anything to change that. It'll bounce off and go through the engine as if it was dry.

If you turn on carb heat in that situation, it will raise the temp of the air until it's liquid and then it'll get into the carb and re-freeze to the carb. You now have carb ice and no way to get rid of it.

That's my understanding, anyway. I used to have a really good link with all kinds of graphs and such that talked about carb ice and made it much more understandable, but I can't find it. :dunno:
 
flyingcheesehead said:
If the air is really cold, moisture in the air will be frozen and the temperature drop in the carb will not do anything to change that. It'll bounce off and go through the engine as if it was dry.
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Got it Kent, thanks!

I guess I never really thought about ice going THROUGH the carb.
 
Actually, if the already-frozen ice goes through the carb, you increase engine power -- poor man's water injection. Just don't expect that tiny mass of water to have a significant or even measurable effect.
 
Ron Levy said:
Actually, if the already-frozen ice goes through the carb, you increase engine power -- poor man's water injection. Just don't expect that tiny mass of water to have a significant or even measurable effect.
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Sheesh!

How does that increase power?
 
N50909 said:
Sheesh!

How does that increase power?
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As Ron said, the effect is marginal. Turning solid water into steam increases cylinder pressure, increasing power.

Water injection was a built-in system on many of the large radials. On these engines, the primary benefit of water injection was to reduce cylinder temperature making detonation less likely. Therefore you could advance the spark further without damaging the engine, and also extend the time you could run flat out without damage.

-Skip
 
Resurrecting an old thread here as I specifically remembered the advice to only using carb heat long enough to take care of the ice and then to shut it off when the ice cleared.

I was reading through an AOPA ASF report on icing and their instructions are to

1) apply carb heat long enough to clear the ice
1b) it will take longer than you think for your engine to return to full power...wait until it does
2) shut off the carb heat
3) if carb ice returns, turn carb heat back on and leave it on (remembering to lean so as not to drastically increase fuel usage)

I fully agree that you should not use carb heat unless you are experiencing symptoms, but I thought that this was an important point of clarification.

Has anybody here ever had carb ice form in clear air in the spring/summer?

Edit: Link to the report... (PDF FILE)
 
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FlyNE said:
Has anybody here ever had carb ice form in clear air in the spring/summer?
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I have not, but looking at the chart is is quite possible.
 

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Bill Jennings said:
I have not, but looking at the chart is is quite possible.
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Yeah, that's what they were saying in the report. They said that in high humidity, it was possible to form carb ice with the OAT at as high as 90 degrees. I was just wondering how often "possible" became a reality.
 
FlyNE said:
Yeah, that's what they were saying in the report. They said that in high humidity, it was possible to form carb ice with the OAT at as high as 90 degrees. I was just wondering how often "possible" became a reality.
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I've seen it at about 65°F in a helicopter. It was hazy VFR, slight but noticable power loss, went away with carb heat.

Joe
 
Ed Guthrie said:
To the OP--the hazard in running carb heat constantly is that you potentially move induction system icing (if it occurs) deeper into the intake system--and with carb heat applied when/if the ice formed you now have no way to remove it.
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Thats what I understood the danger to be.
 
FlyNE said:
Yeah, that's what they were saying in the report. They said that in high humidity, it was possible to form carb ice with the OAT at as high as 90 degrees. I was just wondering how often "possible" became a reality.
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The only time I have ever experienced major carb ice was on a sweltering day around St. Louis.
 
AdamZ said:
Any boy you must be light people and zippo luggage and partial fuel
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yep. calculated right at max gross with fuel at the tabs. it sucked having to plan the fuel stop, but the girls needed a bathroom break anyways, so it worked out okay.
 
I've only experienced carb ice one time with an OAT of about 60 F. Fortunately I was alert enough to recognize it and cutting the carb heat on for a few minutes (if that long) resolved the problem.
 
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FlyNE said:
I fully agree that you should not use carb heat unless you are experiencing symptoms, but I thought that this was an important point of clarification.

Has anybody here ever had carb ice form in clear air in the spring/summer?
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Yes, but it was high humidity not really hazy but close. Can't remember the OAT but it was pretty warm. The chart posted pretty much nails it.
 
smigaldi said:
80 degree OATs in Florida with very high humidity, the engine started running rough. I tried a carb heat and within 3 minutes the engine was running smooth.
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Good grief! Carb heat can take that long to take effect? My limited experience with carb ice differs, but then again I fly in southern CA mostly. Pardon my ignorance, but why is that (the time-to-clearance, not my flight in CA;))? I mean, that much ice can build up in there ? ..... I suppose at more open throttle settings maybe. But then if so, what if all that stuff gets dumped into the engine in one bolus?

Or should I just stay out of Floridian airspace in the summer ... ?:yes: To put it another way, if I applied carb heat and the engine didn't perk up in maybe 30 seconds (after a short period of losing more power sometimes of course), I'd be starting to sweat just a tad. But then again, in FL my passengers probably wouldn't notice .....
 
I believe if you read the poh for the warrior it will tell ou the only time you should apply carb heat is when cab ice is thought to be present.
 
It resolved fairly quickly. I am sorry I did not get out my stop watch when it happened so I could give you the exact time. Next time I'll do so so that you can sleep better.

SIGH!
 
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