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