I have an EI carb temp gauge, and use partial carb heat in cruise if conditions are right for icing.
Recently a couple of pilots have told me to only use full heat or none at all.
What do you guys think?

Terry wrote:I have an EI carb temp gauge, and use partial carb heat in cruise if conditions are right for icing.
Recently a couple of pilots have told me to only use full heat or none at all.
What do you guys think?

This is a good question. I have wondered the same thing, and heard the same about full or none. Full carb heat can really enrich the mixture. Seems like the temp difference between no carb heat, and full carb heat is significant. Depending on the how close the throat is to freezing temp, it seems like partial carb heat could be used while not losing nearly as much power?

I can't imagine why partial should be a problem. I think the training to use it all the way or none is to avoid forgetting about it.
With my o-470 on the 182, there is a significant mp drop with it all the way on due having to suck air through the tube to the carb and a further power loss from the warmer air. Overall, I use partial a lot to balance the combustion in the cranky induction system when I'm at cruise. I can get all my cylinders to reach peak very closely with it partially on and the throttle pulled back a little- impossible otherwise.

The carb heat is unfiltered of course, which simply shouldn't matter in the air anyway. I've had a few incidents with carb ice on my plane that I'd rather not wait for again. I like to have it around half on when the throat temp gets cold and there is visible moisture around for sure.

For what it's worth, Lycoming Service Instruction No. 1148C advises "all or nothing" unless you have a temperature gage.

I've heard two reasons for not running partial heat blind (without a gage). First, you run the risk of melting frost-like ice at the front of the carb, which then runs through it and re-freezes in solid form at the back. By the time you realize you need heat, it is less effective against the solid ice than it would have been against the frost-like ice at the front. Second, in really cold conditions where atmospheric moisture is in crystalline form that would not normally adhere to the carb or induction system, you run the risk of melting the crystals so they stick and form solid ice, as above.

The deHavilland Beaver with an R-985 Pratt and Whitney engine calls for carb heat to maintain +4 degrees C (or was that +7??) as noted on the installed carb inlet temperature gauge. Apply full carb heat in one of those things, and the engine almost dies....

I run some carb heat fairly regularly in cold temps to even out the EGTs and therefore allow me to lean more efficiently.

I would not run partial carb heat without a carb air inlet temperature gauge, however. The theory is that at certain temperatures, you COULD conceivably raise the carb inlet temperature INTO the carb icing range, thus actually causing carburetor icing.

MTV

Noe Flight Risk nailed it. Use all carb heat or nothing. Using a little can still cause icing

I run full carb heat as soon as I break ground till final. After I put that Pponk engine into service I experienced very high (100 over normal) EGT's, in discussion with Steve Knopp he commented that he used it all the time in cruise. His observations (and mine) is that it does enrichen mixture and after setting your RPM and MP you lean like you normally would. This saves me about .5-.8 GPH without any degradation of cruise speed. you just gotta remember to close it before touch down.

And for all that are going to ask how do you know, cause I'm using a JPI 450 for fuel management and a JPI 700 for temp trends and management.

akflyer2001 wrote:Noe Flight Risk nailed it. Use all carb heat or nothing. Using a little can still cause icing

Naw.
The Cessna 182 flight manual says partial is fine. I used to get ice anytime of the year if I was flying through misty weather. I use it all the time now, and the problems simply *went away*. I have a carb temp gauge.

If you have a functioning carb temp gauge, and use it properly, the chances of creating a problem from partial carb heat are pretty much nil. If you don't have a gauge, the chances are still pretty low, but vary from plane to plane.

I used to get carb ice a lot on takeoff on cool, humid mornings in a Cessna C-150. If you lose a hundred RPM in that plane with a passenger and a windshield with trees at the end of an otherwise adequate runway in Colorado, you are, in a word, unfortunate. If you decide to use the "all or nothing" approach, you end up detecting the ice after you've lost a good chunk of your already-thin section of reserve HP shortly after breaking ground. Then you ruin your shorts by pulling the carb heat all the way on and run a slalom course through those trees. If you run up and lean with a little on run-up and used it for takeoff, the problem ceased to exist. Other students/instructors who flew that plane never even knew they had been getting ice on a regular basis till they were told to try clearing the engine on climb-out, and shazam...they'd might lose 50 rpm, but then they would gain a mysteriously disappearing 100 rpm to put them at close to red-line again.

The all or nothing approach simply doesn't apply in every case, and doesn't apply at all if you have a carb temp gauge IMHO.

Regarding possible carb icing on takeoff... Carb ice is formed due to the vacuum that exists behind the throttle plate (or slide depending on your carb design.) Manifold pressure is always lowest at idle when the plate is closed. Full throttle, or wide open the differential is much less, so the risk of moisture condensing out of the air due to low pressure and low temperature is much less. I'm not trying to give a dissertation, just setting up my question.

I was taught (in an O-235 equipped 152) to pull full heat below 1800 rpm, my instructor's reasoning being that any greater rpm meant that the differential in air pressure behind the throttle plate above that rpm usually wasn't enough to cause icing. I've never been able to prove otherwise, but I'm sure the right conditions could disprove that.

So on takeoffs, like early morning at JC when it's dewey and I'm trying to warm the oil up, taxing along at 1000-1200 rpm, I figure that's the highest risk of carb ice. Once you mash it to the firewall, ice probably won't be forming. But you may have built some up during taxi. So I like to leave carb heat on until I run it up, and then back on. Then just before full throttle, I shut carb heat off.

This O-200 I've been flying is temperamental when it's cold. I keep carb heat on just to get it warm enough to run smoothly without it.

The Lyc O-235 has the induction passing through the oil pan, and the carb stays quite warm. The Cont o-200---very different animal. Got ice regularly- even at full throttle. My o-470...well, I found out it's famous for ice after having no shortage of nervous minutes along the way.

BTW- a lot of the temp drop through the venturi is due to the fuel vaporization- not just the ideal gas law. If you have a gauge, try going to full rich at a constant throttle/rpm setting. You'll see the temp take a noticeable dive if you are over 10k' or so.

I have a carb ice detector and have picked up carb ice on departure while climbing out at near full throttle.
Gary

I use partial Carb heat in cruise when conditions are cool and damp. Or at least I did with the 170. Continentals get Carb ice easier than Lycomings, so don't know as I,ve done it with the Cherokee. I was taught same as you, all or nothing. But experimenting showed that it was often useful to put some on, then lean to suit. Just pay attention, and give it full heat every few minutes to check if there's any buildup.

lesuther wrote:BTW- a lot of the temp drop through the venturi is due to the fuel vaporization- not just the ideal gas law. If you have a gauge, try going to full rich at a constant throttle/rpm setting. You'll see the temp take a noticeable dive if you are over 10k' or so.

I believe that most of the carb air inlet temperature probes are GENERALLY located UPstream of the fuel nozzle. I doubt you'll see much difference in temperature associated with fuel vaporization with a Carb temperature gauge.

Nevertheless, your point is well taken in that carb ice is caused generally by a combination of pressure changes PLUS fuel vaporization.

And, as noted, Lycomings are MUCH less prone to carb ice than are Continentals. Perhaps the worst carb ice machines are the small Continentals: C-65/85/90 and O-200, as well as the all time champ: the O470.

MTV

If I can't 'afford' an rpm loss (low, slow-for whatever reason [landing, scud running, sightseeing]) I pull and leave on carb heat if there are any clouds around at all.
If there are clouds around at cruise I pull carb heat on intermittently to ck for rpm loss that tells me that ice is forming.
Both of my current planes are Continental powered.
It has served me well, but I tend to be a fair weather flier as much as I can. I would suggest the REAL experts work/worked smallish planes in SE Alaska. Lets ask them to chime in here.....??
lc

Add: I was taught on/off for carb heat also. That is the way I have used it through the years. I am NOT saying it is the best way. Just that I have used it the way I was taught-and through the years haven't heard a discussion of other ways....... til now.
'ATTABOY for BCP
At least it gets us thinking about it.

When I was learning in 150s in Alaska, we often had to run carb heat on in really cold weather just to run smoothly--it was to help the fuel vaporize. But we didn't have carb heat temp gauges, so it was all or nothing.

Later, my 2nd airplane in partnership was a TR182 with a Lycoming O-540 which came with a carb temp gauge. There were numerous occasions over the 3 years of ownership in which I used partial carb heat, to bring the needle out of the yellow back to the green.

With my current airplane, a P172D with a Lycoming O-360, I had not experienced enroute carb ice until last summer returning from OSH. I was about halfway between Madison and Dubuque, when the engine started running just enough rough to be concerning. I pulled the carb heat, and after a short time, it smoothed out. It was a warm but very humid day. I also discovered that I had a spark plug problem, which seemed to make the engine more susceptible to carb icing, although I don't know why. With a full power mag check at Dubuque, the plugs cleared out completely, but half an hour or so after leaving Dubuque, I again experienced some carb icing at cruise. Once I had it cleared out, I checked the mags again, but both sides were fine.

Next month is annual time, and I'm having an Insight GEM installed, including a carb temp probe.

Cary

My 182 has iced up so bad I had a "precautionary" landing year or so back. Had departed in mid December from Southern Calif -climbed above wispy clouds.Carb heat would have helped if I would have added it early .O-470 has terrible air distribution system using carb heat even partial will richen up mixture and smooth out engine in cruise.Now I add carb heat anytime from Oct. -May or when OAT is less than 40 f.

182 STOL driver wrote:My 182 has iced up so bad I had a "precautionary" landing year or so back. Had departed in mid December from Southern Calif -climbed above wispy clouds.Carb heat would have helped if I would have added it early .O-470 has terrible air distribution system using carb heat even partial will richen up mixture and smooth out engine in cruise.Now I add carb heat anytime from Oct. -May or when OAT is less than 40 f.

I've gotten into a good habit of pulling the carb heat on periodically about every flight in all temps even during the summer (just out of habit). Been taught that it doesn't hurt a thing. I always push it back in (off) when field is made or over the numbers.

Great thread, BTW. Interesting info.

Pat
Just be sure to lean approprietly, and then when you turn it off, richen it back up or you will be too lean. I don't use mine unless I am building ice for fear of forgetting to turn it back off.

Seems Cessna recomends having carb heat on when landing, atleast on some engines(ours seem to be prone to carb icing)so if you go along with that line of thought and are flying with it on, I guess you just leave it on 'til you land.
Gary

I'm an all-or-nothing carb heat user in my Maule MX-7-180. I have a carb temp sensor connected to my JPI EDM800, but temperature alone doesn't mean much unless there's moisture in the air. I've had a drop in MP a couple of times on humid, cool days in the spring under VFR and had to use full carb heat to restore MP.

IFR in IMC is a different story. I stay away from flying in the clouds when it's cold enough for icing but I use full carb heat when descending through clouds at lower power settings even in the summer. One notch of flaps helps me keep the descent airspeed down to reduce the possibility of carb ice on an instrument approach. I can keep a higher power setting in this configuration, which further reduces the possibility of carb ice since the throttle plate in the carburetor is open wider and the engine is developing more heat.

At cruise I don't use carb heat unless I notice a drop in MP. Sometimes the drop can be hard to notice if it takes a while. If I suspect any kind of MP drop, then I use full carb heat to see if there's a rise in MP. If so, I keep it on full heat. The reason I don't use carb heat all the time or partial carb heat during cruise is that it robs me of power due to the reduced air density. If I use carb heat in cruise due to suspected icing, then I re-lean to avoid an over-rich situation. I do use carb heat on landing due to the lower power setting at a critical time when I may need full power quickly and can't afford to discover that ice has formed in the carburetor.