Carb Heat

Carb Heat

I am interested in carb icing and am curious as to under what conditions GA pilots apply carb heat.  The issue of carb heat when gliding to land is fairly straight forward.  But what about carb heat when in cruise.  Is it strictly the dew point dry bulb temperature spread?

I look at the icing probability chart and it looks like it gives equal probability between low dry bulb and dew point and high dry bulb and dew point.  From an air conditioning perspective, after all this is a thermodynamic air conditioning process, that is not true.

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Re: Carb Heat

I have a 150 with the O-200 engine, which is notorious for carb ice. I think Continental put carb ice into the design! I apply carb heat anytime I notice the RPM's decline while cruising, or getting engine roughness. Other than landing, I only apply heat when the engine needs it.

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Re: Carb Heat

Anytime I'm enroute and there is visible moisture (haze etc.) I apply carb heat (after moving the mixture to rich) at least occasionally for a while to either detect ice or remove it - I only apply heat when I'm within gliding distance of an airfield (it is possible to kill an engine by applying carb heat for a number of reasons including faster ice buildup than I may have expected - because carb heat also leans an engine)  then return to cruise settings and proceed on my way. Of course all the other normal times - before reducing power, etc.,  just expect that someday when you put carb heat on you may kill the engine - so be prepared for that in advance - if you suspect real ice, then apply carb heat slowly or in stages so you don't send all that water thru the carb all at once.

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Ken Wanagas

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Re: Carb Heat

Before I overhauled the engine in the ole 150K,I used to get carb ice quite often.I have yet to have any carb ice problems on the new engine.Im not sure exactly why the new engine configuration makes any difference,but it appartently has.Almost 10 hours on it,and not a stumble

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Re: Carb Heat

Using Carb Heat richens the mixture; not leans it.  If the engine is choking with ice (which will richen it) and you richen the mixture then put in carb heat you are more likely to have it die than leaving the mixture lean and pulling heat on.

Plus, a leaner mixture will create more heat to battle the ice than a rich mixture.

In cruise I just pull the carb heat for 10 seconds every 15 minutes or so if I feel conditions are good for carb ice (leaving mixture where its at).

My philosophy is to use carb heat to prevent carb ice vs waiting till I have it.

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Re: Carb Heat

Sorry Ken but I have to disagree with you on the carb heat leaning the mixture. Hot air is less dense than cold air so the mixture becomes richer with the application of carb heat. (I learned this the hard way at the race track while carb tuning during the summer.)

I have a C150H that is IFR equipped and often flown in IMC. I check for carb ice at regular intervals (determined by atmospheric conditions) by applying carb heat. If ice is detected by a larger than usual rpm drop or rough operation carb heat is left on until smooth operation returns. When the engine returns to normal operation carb heat is turned off and normal cruise is resumed.
Since I spend quite a bit of quality time with my bird I can usually hear the engine change sound before I pick up the rpm loss or rough operation on the tach. The objective is to be proactive and make carb ice a non event. My past instructors have called this staying ahead of the plane. I have used this method in 2 and 4 seat Pipers as well as 150s and 172s it has worked for me for 23 years.

If you pay attention to all the warning signs and take early action I would be hard pressed to believe that one could shut down an engine with carb heat due to water ingestion. I have never let carb ice ever go that far unnoticed.

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Re: Carb Heat

Carb heat makes the mixture richer.  To test this, the next time you do a run up, pull the carb heat on, and note the rpm drop, and then with the carb heat on, lean the mixture and you will see the rpm increase back to the starting point.  The reason carb heat richenes the mixture is warm air is less dense than cold air, changing the fuel/air mixture to one of less air, which creates a richer fuel mixture.

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Re: Carb Heat

That's my question.  Do pilots use it carb heat as a reactive measure or a proactive measure.  So far, it seems to be a split decision.  Have a look at the ice probability chart and

http://www.fishcreekflyingclub.com/safety/carbice/

see if there are thoughts as to when heat should be applied.

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Re: Carb Heat

We have a 182R with the TCM O-470-U also know affectionately as the icemaker. Carb ice is well known here in southern California even tho' technically it's a desert. I was going crazy with the carb ice problem and finally spent the few bucks (relatively) and had Iceman installed. This uses an optical probe to actually see carb ice. Sensitivity can be set depending on one's preference. This has really resolved the issue for me. Unless the alarm goes off after being set and tested I don't worry about carb ice. Not to say I don't monitor OAT, humidity, etc. but it's worked great for us.

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Re: Carb Heat

One of the nice things about the 0-470 engines, is that you can use partial carb heat to keep the carb heat gauge in the green.
Glenn

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Re: Carb Heat

<HTML>I have an 0-470R and was intrigued by your mention of a carb heat guage.  Someone gave me a carb temp guage, but I haven't installed it.  Is it worth doing?  What kind do you have?

Also any thoughts about running with carb heat partially on for extended periods of time? (Is it sort of like turning on deicing boots too early?) I think it's ok and do it occasionally, but just looking for thoughts.</HTML>

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Re: Carb Heat

<HTML>I have an 0-470R and was intrigued by your mention of a carb heat guage.  Someone gave me a carb temp guage, but I haven't installed it.  Is it worth doing?  What kind do you have?

Also any thoughts about running with carb heat partially on for extended periods of time? (Is it sort of like turning on deicing boots too early?) I think it's ok and do it occasionally, but just looking for thoughts.</HTML>

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Re: Carb Heat

Very useful thread.

I have a 172D with an O-300-D and am a low time VRF'r.  The general guidelines about carb heat is to apply 'when necessary'.  Perhaps I'm getting too wrapped up on the details, but any insight into these questions would be greatly appreciated!

- How should carb heat be applied when the mixture is leaned?  Should the mixture be richened, lean or left as is?
- Can carb heat be 'partially' applied or is it either 'on' or 'off'?  Is there anything to be gained with a partial setting or would this stress the carb heat box or air intake system?
- During run-up, what is normal RPM drop?
- During landing, what is the general practice on applying carb heat (for example, carb heat on when RPM is outside the green arc)

Thanks!

Jim

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Re: Carb Heat

JoeT,

I like your analogy with the de-icing boot.

I would think the problem with partial leaning is two fold. How do you know you have leaned it enough, and 2) weather and atmosphic conditions change as you fly, so even if you are able to set it correctly, how would you know when it won't become somewhat inadequate later. 

I see the problem as an energy balance.  The carb heat has to apply more energy than the cooling process that is forming the ice.  As the ice builds up less air is taken into the engine and with it less energy available to melt the ice.  Change in the amount of energy added to the air stream will be less by going from a partially open to a full open than would be available if you go from full off to full on.  Therefore, as you suggest, it may become ineffective, like the de-icing boot..

I was taught to think of carb heat as an off and on switch, and not to do it partially.  If needed, all the time then open it fully, set the mixture, and as Vinny would say "forget-about-it". 

I haven't gotten to the point yet where I can detect carb ice by the sound of an engine.  I usually pick it up when I perceive a performance change.  I decide my rpms have decreased slightly and have been increasing the throttle progressively.  I've detected carb ice from the time of starting my engine until the time I did my run-up. Instead of an RPM drop when I pulled carb heat I got an RPM increase. - That was spooky.

My 2 cents.

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Re: Carb Heat

It is an energy balance.  To vaporize a specific amount of fuel, you need a specific amount of energy.  Think about your propane barbeque.  It's a liquid in the tank, but the grill needs a propane vapor.  In order to vaporize the propane, it needs energy.  Where does the energy come from?  The propane itself.  Latent heat is needed to vaporize the propane.  Put your hand on the tank when your grill is on and the tank is cold.  The enrgy removed from the liquid propane causes a temperature drop in the liquid.  Under certain conditions, frost will form on the tank. 

The same is true to for avgas.  The heat required to vaporize the fuel comes from the combustion air.  The temperature of the air drops to the dew point and moisture starts to condense out.  Under certain conditions, the amount of energy taken out of the moist air stream will push the temperature of the combsution air down below 32 F and the condesnation will freeze.  Carb heat adds energy to the combustion air which is then used to vaporize the fuel.

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Re: Carb Heat

I incorrectly stated that carb heat leaned the mixture - I should remember that even without detailed info that if you increase airflow (or density) in a air you also increase fluid flow, just the fluid flow increase lags behind just because of its resistance, and of course the reverse - decrease airflow (or density as in applying carb heat ) and decrease fuel flow just at a lesser rate - resulting in a "richer mixture".

I guess that I still don't use carb heat till the mixture is rich - after reviewing my habits and the application of carb heat, probably has more to do with that I usually do this in preparation for descending, where I rich the mixture first.

I don't like to use "partial heat" because it seems more unpredictable first of all, you are killing off a few HP right off the top, and it seems like that could be easy to forget if "partial" is a habit, and the worst offender is that with carb heat you are introducind unfiltered air any time carb heat is applied partial or full.

I don't like to use carb heat for only a few seconds (or partial except in special circumstances) as I believe that I have read from credible sources that that could warm just enough in marginal situations to cause more problems than not - therefore when I apply carb heat its long enough to provide substantial warming then I return to "normal" settings.

On the issue of detecting ice first - it works out great if you can -ALL the time - but I have read so many times where carb ice was a factor -and sometimes an unpleasant factor - that I suspect it all the time and believe that there could be a time where I could overlook it till its a problem.

I have even had my automobile carb - ice over twice (that I know of) in a few decades, and I never suspected that - Once was a fault of the muffler, but most people would never know even if their automobile carb did ice over and quit the engine, and by the time they had it towed and "repaired, no one would ever know- just while I was stuck on the road I pulled the air cleaner and looked down thru the carb bore amazed to see the thing iced almost shut....   thats another story.

Alan -  I'm happy your plane is back in the air and that it is now producing enough heat to keep the ice at a distance.


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Ken Wanagas

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Re: Carb Heat

Dave...

More specific ans.

Proactive and Reactive both will help keep you in the air and not using the "E" word

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Ken Wanagas

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Re: Carb Heat

Short answer to your question is procative.

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Re: Carb Heat

I put the carb heat on while the engine is still in the green before power reduction.  This gives a better shot of warm air than doing it after power reduction.

My goal is to never have carb ice; not get rid of carb ice when I get it.

As mentioned C150's have a habit of forming it.

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Re: Carb Heat

more thoughts -

I had an instructor tell me once -and have always tried to follow his advice since - In a real emergency if the engine has quit ( and in all your practice simulated emergencies ) immediately pull carb heat.

If the engine has quit due to ice you may have enough residual heat left (maybe only seconds)  if you apply carb heat right away to have a chance at restart - slim but anything is a help at that point, and if you're worried about introducing water from the thawed ice the engine is allready dead so give it a try.

advice forwarded - take it or leave it.

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Ken Wanagas

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Re: Carb Heat

Obviously proactive is preferred, but it seems that there is a bit of a split here.  In order to be proactive, you need to recognize when conditions exist for the problem of carb icing.  What criteria do GA pilots use to determine that the risk of icing is there?

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Re: Carb Heat

I'm no expert but I look at the overall humidity and the temperature/dew point.  If the air is warm and humid it can hold more water vapor than the same humidity at a lower temperature.

If the temperature and dew point are close that can also indicate a higher chance.

Either way I tend to occassionaly pull the carb heat when at a low power descent etc every so often for 10 seconds or so.

I may be overzelous but so far I haven't had ice cause the engine any grief (and its a C150H).

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Re: Carb Heat

I'm an HVAC engineer and student pilot.  I started this thread after looking at the carb ice probability chart that I listed in an earlier comment.  I look at the issue of carb ice as strictly a thermodynamics air conditioning problem, which it is.  In looking and talking to a few other pilots, I found that everybody had a different answer as to when they should apply heat and under what conditions icing will occur.  The books really don't tell you what is happening.

Let me ask this question then.  Which condition is more likely to freeze:  70 deg F db (dry bulb) and 90% rh and a dew point of 66.9 deg F or 85 deg F db, 42.1 % rh and 59.4 dew point?

You can either post your response or e-mail me for a discussion at This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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Re: Carb Heat

The condition I use for probable carb ice is "visible moisture"

If its high enough humidity you can usually see it - haze or whatever - if its low ceilings or heavy haze then its real humid - without having to know the temp-dewpoint spread, If its very very clear then there is probably not enough moisture to be a substantial threat -

there is always a possibility of carb ice - just much higher risk when the available moisture is high.

There are some temps more succeptable to carb ice, and those are good to keep in mind, and thats supporting info, but if you can see moisture then ice is possible or probable.


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Ken Wanagas

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Re: Carb Heat

Having a 150K,and prior to the overhaul I experienced carb ice quite frequently,I purchased a carb temp kit from Wag-Aero.Installation was a breeze,and while it will NOT indicate carb ice,it will give you the carb throat temp in a Yellow arc(which means most likely probability of Ice formation)...When I suspect that carb ice is possible I add just enough carb heat to keep the needle out of the yellow arc....Never encountered it again using this method.The PMAd version was about 125.00 and was well worth it.

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