Use of Carb Heat

[SammyG]

I've been having a discussion with a pilot friend of mine regardingwhen its apporpriate to use the carb heat. I advocate using it duringanytime you're using a reduced power setting and/or in a descent. Hesays its only necessary when the RPM is below the green arc on thetach. I say that that's dumb because reduced power to me means anythingless than cruise power, regardless of the green arc. I sometimes evenuse the heat during cruise every once in a while, just for a fewmoments, to make sure all is ok.

So what's the reasoning behind the green arc theory? Is there any, or am I just being overly-careful?

Thanks.

 

[minitour]

I've been told below the green arc, but sometimes I'll find myselfusing it on a cross country just to make sure I'm not picking upanything along the way. I do always put it full on with whateverpower setting I have (usually full throttle or high RPM anyway) to letthe air stay nice and hot flowing into the intake.

It drives me crazy when an instructor or check airman is going to"demonstrate" something and they'll pull the throttle all the way toidle without first hitting the carb heat. Then they sit there andb*tch at you when you do something that you've never even heard ofbefore, but they want you to do it. Principle of Effectanyone? Even if you're nice about "oh let me get the carb heatfor ya" they still end up turning it around on you...

Anyway, the only thing I've been told is to turn it full on while at ahigh power setting to get the hottest air possible going into theintake, THEN reduce your power below the green arc. Remember thatgreen arc is the "normal operating range" for the engine, so anythingless would be "reduced power".

That's all I got. I'm sure someone can offer a more technical explanation...

-mini

 

[Kream926]

why

I do always put it full on with whateverpower setting I have (usually full throttle or high RPM anyway) to letthe air stay nice and hot flowing into the intake.

\


why?

cold air gives you better performance. if theres no icing conditions present why use it? if you encounter carb ice you should notice it rather quickly. then you should use it, clear the carb then decide on what action you should take. thats what ive been tought and thats what would probably do. i would never run it full on with cruise power, unless i picked up some induction icing, then i would find a place to land. especially in a 52


my 2 cents

 

[PropsForward]

The recommendation (base of the green arc on the tach) is per manufacture. I think it has to do with a combination of factors. The butterfly being closed, how the duct work to the carb is layed out, etc.

I have heard that the reason Piper has the green arc all the way down is because of the cowl design and how/where the air flows to get to the carb is somewhat preheated where as Cessna is not (same engine).

Another issue with carb heat is the fact that it causes a mixture-rich condition that can over cool the cylinder heads. So you may want to take caution if you just got done working the engine real hard.

I had a carb ice experience one time when I took off, through the climb I started losing power, I did the memorized items which included carb heat and noticed a rough engine, I suspected ice so I leaned the mixture and sure enough things got better. I elected to keep the heat on and just fix the rich mixture by leaning a bit more.

 

[minitour]

True, cold air does give you better performance and if there is noicing conditions present you don't "have" to use it. I use itdownwind abeam the numbers every time (as well as other places). Why? Habit.

Now, why leave the RPMs up when you turn on the heat? Let's thinkabout it. Reduce the RPM, what happens? The engine startsputting out less power, and cools down. The air in the enginecompartment is what the carburetor uses when you turn on the carbheat. Its basically just a "valve" that switches from the airfilter to an alternate source which draws heated air from theengine area. If you have any ice in there and haven'tnoticed it (which a student pilot may not notice it the first time -hell, even an experienced pilot may not notice it the first time) andthe air is too cool to melt the ice, what have you done foryourself? Nothing. Plus, now run the RPM back up and youmay not be able to get it hot enough to melt the ice in the firstplace.

Kinda like "clearing the engine" while simulating an engine failure, Ilike to "clear the carburetor" in cruise flight. Not often, maybeonce an hour or if I'm not getting the performance I was expecting, butyeah I want to be sure. Then if I see the RPM come up when I putit on, I know I'm picking it up and I need to look for a place to setit down. If not, then I know I'm going to be slow, can calculatemy new range and write up the plane when I get back to be looked at.(After I've checked things like mixture, Density Altitude, etc.)

Strictly habit and making sure the hottest air possible goes into the carburetor.

-mini

\


why?

cold air givesyou better performance. if theres no icing conditions present why useit? if you encounter carb ice you should notice it rather quickly. thenyou should use it, clear the carb then decide on what action you shouldtake. thats what ive been tought and thats what would probably do. iwould never run it full on with cruise power, unless i picked up someinduction icing, then i would find a place to land. especially in a 52


my 2 cents

 

[Ralgha]

Its basically just a "valve" that switches from the airfilter to an alternate source which draws heated air from the engine area.

Nope, you're thinking of alternate air for an injected engine. Carb heat comes from a shroud around the exhaust manifold, which is MUCH hotter than what the typical alternate air valve nabs.

 

[Ralgha]

Then if I see the RPM come up when I put it on, I know I'm picking it up and I need to look for a place to set it down. If not, then I know I'm going to be slow, can calculate my new range and write up the plane when I get back to be looked at.(After I've checked things like mixture, Density Altitude, etc.)

Picking up a bit of carb ice in cruise is not necessarly reason to divert, nor is it a reason to write it up for maintenance. Carb ice is expected in some situations, this time of year, it's unusual for our 152s and 172s to NOT get carb ice, especially after startup on the ground. The design of a carburetor makes it prone to ice under certain conditions, it's just a fact of life. If you have the opprotunity, fly a plane that has a carb air temp gauge in it a bit so you can see how various conditions affect it, as well as what the "probable icing" temperatures are.

Good job on the range considerations when using carb heat though.

 

[minitour]

Nope, you're thinking of alternate air for an injectedengine. Carb heat comes from a shroud around the exhaust manifold,which is MUCH hotter than what the typical alternate air valvenabs.

uh...the exhaust manifold is where? In the engine compartmentunder the cowling... Yeah, I didn't quite explain that as well asI could have...my bad. But since we're on the subject, next timeyou get a chance, put on the cabin heat at full power, notice it getshot real quick (shroud around the exhaust manifold heats air, bam cabinair, same principle as carb heat) now run the rpm down to...hell I dontknow 1700, 1500 rpm...it won't get as hot. Still warm, but not ashot. I want that air going in there to be REALLY hot..melt thatice off, not just kinda melt it a little bit maybe...but really frythat sh*t...

Picking up a bit of carb ice in cruise is not necessarly reason todivert, nor is it a reason to write it up for maintenance. Carb ice isexpected in some situations, this time of year, it's unusual for our152s and 172s to NOT get carb ice, especially after startup on theground. The design of a carburetor makes it prone to ice under certainconditions, it's just a fact of life. If you have the opprotunity, flya plane that has a carb air temp gauge in it a bit so you can see howvarious conditions affect it, as well as what the "probable icing"temperatures are.

Good job on the range considerations when using carb heat though.

I think you misunderstood me. What I meant to say (and probablydidn't type it up this way) was that if I don't have the performance Iwas expecting (say my TAS is slow) and I check for carb ice and thatisn't it, then I'll go through and check the mixture (too rich? toolean?) as well as maybe my calculations are off (maybe the PressureAltitude is a bit different - sorry, I said Density...which wouldaffect performance, but the numbers in the POH listing TAS and fuelburn are based on P.A., not D.A. my bad). If it isn't any ofthose things, then I'll talk to maintenence on the ground.

Would I divert? It depends on where I'm going. If I have torun the carb heat on to kill the ice, then I'm losing performance(which is why I'd re-check range n stuff), so yeah, I'd start thinkingabout places to set it down. Maybe I don't have enough fuel nowto get to my destination, so I have to stop for fuel.

If not, then I know I'm going to be slow, cancalculatemy new range and write up the plane when I get back to belooked at.

This is where I was getting at if the plane isn't performing, and itsnot carb ice and its not P.A. or mixture or anything else I can thinkof (Pitch + Power = Performance) then I'm having a chat with mx forsure, if not writing it up for service.

As a side note on the "alternate" source of air. The carb heatactually can work like an "alternate air" source on an injectedengine. Get one of those foam air filters up front and soak it insome cumulus clouds or even get some ice on it (oops - instrument ridetoo, ouch!) and air won't go through it as well. No problem, justhit the carb heat and you're letting all kinds of air into thecarburetor/induction area now. Watch those RPMs go WAY up...Ithink we finally decided the other day that we picked up about 300RPMs...we're still talking about it...

but anyway...

-mini

 

[Kream926]

lets not forget that its an unfilitered air source

 

[Lead Sled]

I've been having a discussion with a pilot friend of mine regardingwhen its apporpriate to use the carb heat. I advocate using it during anytime you're using a reduced power setting and/or in a descent.

A wise man once told me, "When in doubt, read the manual."

I've can't believe that not one person asked SammyG what type of airplane is he flying? Guys, that has to be the number one question. The proper way to use carb heat is found in the aircraft POH. Don't assume that you will operate the carb heat in a Continental powered Cessna the way way that you would do it in a Lycoming powered Piper.

Sammy, the ONLY way to properly operate the carburator heat in the aircraft you're flying is found in the POH or aircraft flight manual. Period. Reread the manual then you can answer your own question.

'Sled

 

[keith]

Lead Sled you are correct Sir !!

Bravo, Lead Sled, bravo. I have been watching this thread and waiting for someone (a flight instructor perhaps?) to point out the obvious source for the correct answer.

As Lead Sled quoted, "When in doubt, read the manual".

 

[Ralgha]

uh...the exhaust manifold is where? In the engine compartmentunder the cowling... Yeah, I didn't quite explain that as well asI could have...my bad. But since we're on the subject, next timeyou get a chance, put on the cabin heat at full power, notice it getshot real quick (shroud around the exhaust manifold heats air, bam cabinair, same principle as carb heat) now run the rpm down to...hell I dontknow 1700, 1500 rpm...it won't get as hot. Still warm, but not ashot. I want that air going in there to be REALLY hot..melt thatice off, not just kinda melt it a little bit maybe...but really frythat sh*t...

Yes I've noticed that, many a time, and I've also noticed that the cabin air intake (in Cessnas) is just a hole that happens to have an exhaust pipe passing by right in front of it. Not exactly the best way to get really hot air, however the carb heat shroud is wrapped and sealed all the way around a sizable section of the exhaust manifold that will heat the air much more effectivly than the cabin heat. I'm not saying you're wrong, but I think the air is hotter than you think it is.

I think you misunderstood me. What I meant to say (and probablydidn't type it up this way) was that if I don't have the performance Iwas expecting (say my TAS is slow) and I check for carb ice and thatisn't it, then I'll go through and check the mixture (too rich? toolean?) as well as maybe my calculations are off (maybe the PressureAltitude is a bit different - sorry, I said Density...which wouldaffect performance, but the numbers in the POH listing TAS and fuelburn are based on P.A., not D.A. my bad). If it isn't any ofthose things, then I'll talk to maintenence on the ground.

Would I divert? It depends on where I'm going. If I have torun the carb heat on to kill the ice, then I'm losing performance(which is why I'd re-check range n stuff), so yeah, I'd start thinkingabout places to set it down. Maybe I don't have enough fuel nowto get to my destination, so I have to stop for fuel.



This is where I was getting at if the plane isn't performing, and itsnot carb ice and its not P.A. or mixture or anything else I can thinkof (Pitch + Power = Performance) then I'm having a chat with mx forsure, if not writing it up for service.

Yeah I did misunderstand you. That's not at all the idea I got from what you wrote previously.

As a side note on the "alternate" source of air. The carb heatactually can work like an "alternate air" source on an injectedengine. Get one of those foam air filters up front and soak it insome cumulus clouds or even get some ice on it (oops - instrument ridetoo, ouch!) and air won't go through it as well. No problem, justhit the carb heat and you're letting all kinds of air into thecarburetor/induction area now. Watch those RPMs go WAY up...Ithink we finally decided the other day that we picked up about 300RPMs...we're still talking about it...

but anyway...

-mini

When you're using carb heat, the air is not coming from the main carb inlet, therefore, it is by definition, "alternate air". I've never said otherwise.

 

[Lead Sled]

Thanks Keith. (It just so happens that I am a current CFI.) As pilots we have to be on constant watch that "irregularities" don't creep into our flying style. (This is one of the main reasons for the BFR and other periodic checks that all pilots are responsible for.) We are all suseptable to this, it makes no difference the amount of flying time we have or the ratings that we hold. CFI's are just as prone to this as anyone else if they're not careful.

"Creative thinking" has got a lot of pilots into trouble. It's usually best to stick with POH recommendations - they were put in there for a reason. Personally, I'd see what your POH says and follow it to the letter. If your CFI says something different you may have other problems to deal with. As a student, now is the time to develop proper habits and procedures. As I mentioned earlier, using checklists and following the AFM/POH is perhaps one of the best habits that you can form.

'Sled

 

[minitour]

...I'm not saying you're wrong, but I think the air is hotter than you think it is.

No doubt its hot hot hot air...hmm...I guess that would be kinda neatto see exactly what the temp of that air is...cool idea, thanks!

Yeah I did misunderstand you. That's not at all the idea I got from what you wrote previously.

Yeah, that's my fault...I seem to be able to explain things better by actually talking and/or showing...it's all good

When you're using carb heat, the air is not coming from themain carb inlet, therefore, it is by definition, "alternate air". I'venever said otherwise.

A'ight...I misunderstood ya there...my bad....

It's all good G-diggity-dawg...word up

-mini

 

[JonJohn82]

In the Piper, I generally do not use carb heat unless I suspect it. On a couple of X-C flights, I have turned it on for a bit since I suspected the beginnings of it.

 

[bobbysamd]

Carb heat

Pull it on during any time you operate at reduced power. Such asbeginning your decent from downwind to base. Typically, in a 172 youreduce power from 2000 rpm for your downwind to 1500 rpm; at that time,pull on the carb heat. Pull on carb heat without fail when practicingsimulated emergency landings, steep spirals, etc., anytime you are inflight and you reduce power to idle.

You really don't want to use carb heat for the hell of it during normalcruising because the hot air enrichens the mixture unnecessarily.

Of course, one can get carb ice at high power settings. I'll neverremember the time I was flying a 172 on a nice spring afternoon inOklahoma. The throttle was forward with power set at 2400 rpm but I waslosing power. Couldn't figure out why - but I pulled on the carb heatand shortly thereafter, all was well. Classic carb ice conditions thatday - temperature between 32 and 70 and humidity over 50%.

Finally, I found this FAA article on carb ice. Enjoy!

 

[Tinstaafl]

Carby ice in the Islanders I used to fly over the North Sea was a common occurrence. After a while it became automatic to apply carby heat every 10 or 15 mins or so, and on for all reduced power settings.

Sometimes I'd have to have carby heat hot for most of the flight.

 

[avbug]

Application of carburetor heat prior to power reduction is important, both to ensure that no ice exists, and to prepare the carburetor for that initial reduction when landing. Ice is most likely to form, and to form agressively at greater throttle settings, for reasons already mentioned. At idle, the throttle plate is closed, and the only place for ice to form is directly adjacent or after the throttle plate where the airflow is serviced by the idle jet.

If this is the case and idle icing as formed adjacent to the throttle plate, the remedy is to open the throttle; the little bit occluded by icing is now a big bit and not readily affected by the icing. Stumbling at this point indicates formation of ice (or a rich or lean mixture), and the engine should be cleared as required using ice.

If the engine is stumbling, I would strongly discourage trying to lean out the mixture. Especially on departure when close to the ground (such as on a go-around). This isn't the time to troubleshoot, but to take the fewest and most effective steps to handling all possible problems at once. This includes going rich on the mixture, rather than leaning it out. Unless the carburetor is grossly misadjusted, going rich isn't going to put your engine out...and unlike a turbine engine, you have spark plugs to keep it firing anyway. Go rich in a rouch engine situation prior to applying carburetor heat. After you ensure you do have a continuing power source, you can lean as required.

Another very common misapplication of carburetor heat occurs during ground operations. Often pilots are taught to apply it for only a moment during the runup. They are told this is to ensure an RPM drop (in fixed pitch normally aspirated installations)...then to push it back in and forget it. Carburetor heat should be left on for fifteen seconds to ensure that it has indeed removed any built up ice. One should also ensure upon shutting off the carburetor heat that the engine does return to the same level of operation that it had before the carb heat was applied. Application of carb heat is best done immediately prior to entering the runway for departure, or on the runway prior to brake release (if doing a braked departure).

 

[Peanut gallery]

Now we know the Continental anwer

POH POH POH and did I mention the POH

 

[SiuDude]

The carburetor on a Lycoming engine is attached to the oil pan, whereitabsorbs a lot of heat from the engine. I have never used carb heatinthe air in a Piper or the aircraft that I fly now (Lycomingengine).Cessnas are a different story. The carb is mounted farther fromtheengine, and thus absorbs less heat. You will not be able to getawaywith not using carb heat for very long.

The green arc is there for a reason.