What is the purpose of the Carb heat

Allison Riccardi

Allison Riccardi

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Angelwings
How detrimental to different phases of flight is it? What exactly does it do? How can I be sure that it is working effectively? What is the purpose of it? Any other information I should know about this thing? Thanks for the input.
 
The purpose of carb heat is to divert heated air into the carburetor to eliminate or avoid ice buildup in the carb. You can tell if it is working if you turn it on and the RPM's drop a little.

That's the short answer. Go read the material cited by Mike
 
It's there for you to use to eliminate ice in a carburetor venturi. It usually isn't used in normal flight but is prescribed for landings, where many engines tend to ice up with reduced power descents. You want all available power if you need to go-around. If you pull carb heat in normal ops you'll enrich the mixture. There may be times when that's helpful, like in super cold conditions where your mixture may be too lean as a result of temperature. Some guys use partial carb heat to improve furl distribution, too, like in 0-470 Continentals. But not all engines need it.
 
It is important to know that carb icing can happen in temperatures well above freezing.

carbur1.gif
 
Allison Riccardi said:
How can I be sure that it is working effectively? Thanks for the input.
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During your run up pull carburetor heat and the engine should slow down if the carburetor heat is working. There should be something specific about checking to see if your carburetor heat is working in your Pilot Operating Handbook (POH) for your particular aircraft. There will also be instructions on when and how to use carburetor heat in your POH.

Not all aircraft have carburetor heat.
 
Vance Breese said:
During your run up pull carburetor heat and the engine should slow down if the carburetor heat is working. There should be something specific about checking to see if your carburetor heat is working in your Pilot Operating Handbook (POH) for your particular aircraft. There will also be instructions on when and how to use carburetor heat in your POH.

Not all aircraft have carburetor heat.
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Okay but what if it works on the ground, then decides it doesnt want to in the air?
 
It'll work better in the air since your exhaust system's temps will be higher in flight ops than on the ground. The ground check is to verify operation. Don't be afraid to pull it during normal flight. You'll know whether it's working or not very quickly. (Reduced power with carb heat on)
 
Allison Riccardi said:
Okay but what if it works on the ground, then decides it doesnt want to in the air?
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Carb heat is actuated by a cable acting on a butterfly valve that allows hot air to flow from the aircraft heater (muffler surround) into the carb. The throttle and mixture operate by similar cable mechanisms, the likelihood of carb heat working on the ground and then failing in the air is very small.

Carb-Heat-diagram.jpg
 
Also your procedure for when to or not to use it may vary with aircraft type. For example, I understand the carbed 172 models are prone to it and have carb heat on as part of their landing checklist. However, those of us flying Piper Cherokees generally only use it on an as-needed basis since our carbs are not prone to icing. Read the POH for your aircraft/checklist for that aircraft.
 
The airplane won't crash if carb heat fails... Conditions are not "always" right for it to form... And odds are in your favor that it won't... (at normal operating rpms)... In the pattern, if you're worried about it, keep RPMs up until you have runway made either way...

EDIT: Spoken as the VFR-only pilot that I am...
 
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cowman said:
Also your procedure for when to or not to use it may vary with aircraft type. For example, I understand the carbed 172 models are prone to it and have carb heat on as part of their landing checklist. However, those of us flying Piper Cherokees generally only use it on an as-needed basis since our carbs are not prone to icing. Read the POH for your aircraft/checklist for that aircraft.
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The tach in the aircraft can give some indication as well. PA-28's say to use carb heat only if carb icing is suspected. Note the first picture is a Piper tach, it has a green arc from 500rpm up to redline indicating no carb heat needed. Cessna 172's require carb heat at lower power settings, the second photo shows a Cessna tach. It shows a green arc from 1900rpm up to redline. When operating below 1900rpm, one should be using carb heats.

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Engine_instruments_Cessna_152_RPM_gauge_tachometer.jpg
 
From personal experience, I've never had carb ice during descent, but I've had it 3 times on initial climb out, and once in cruise.

The first time was in a PA-28-160, at about 300' AGL. Airplane had a carb ice detector, but not a temp gauge. Power loss happened at the same time the ice detector light came on - no prior warning.

The other times were in my O-470 182Q. Twice on initial climb at around 300-400' agl, the other time in cruise between cloud layers.

If environmental conditions are within the range of carb ice formation, and you're flying a carburated plane, you should always be alert for carb ice.

Jeff
 
It is there to keep the pilots adrenaline at a manageable level.
 
One last thing on carb ice, your instructor should be teaching you the practice of "clearing the engine" on long descents. From " Flying Wisdom, The Proficient Pilot, Volume 3" by Barry Schiff:

"During prolonged glides, be certain to "clear" the engine by applying power every 30 to 45 seconds. Many pilots believe that this prevents the spark plugs from fouling, but "clearing" usually does not do that because the power application is too short. The most important reason for applying occasional power is simply to confirm that the engine is still running. After all, an idling engine and a windmilling propeller are almost indistinguishable.

If an engine does fail during a prolonged glide, it is best to determine this at a relatively high altitude. A pilot cannot afford to wait until power is genuinely needed to discover that it is unavailable.

Applying power periodically during power-off descents also helps to keep the engine warm and provides a modicum of carburetor heat. (Carburetor ice can form during prolonged idling even when carburetor heat is applied because there often is not enough heat being generated by an idling engine to prevent ice from forming.)

During prolonged idling, fuel can condense and form small puddles in the induction lines of some carbureted engines, especially those using autogas. Adding power periodically also clears out this fuel before it can accrue sufficiently to cause the engine to falter.

When "clearing" the engine during a glide, do not jab the throttle or apply a large amount of power. Instead, move it gently, especially when operating engines with counterweighted crankshafts."
 
Applying power during the glide simply keeps the mufflers hot, in case you do need to clear ice.
 
Carb icing is caused by evaporative cooling, fuel is transferred to the Venturi by suction caused by the Venturi, as the fuel atomizes it cools, and drops the temp in the Venturi to below freezing. when water of any kind enters the Venturi it freezes and destroys the Venturi and throttle plate's shape, until no fuel is transferred, then the engine quits and no heat is made for the mufflers to melt the ice in the Venturi. when the mufflers are too cold to heat the carb, you are going to land.
It requires but a few seconds to cool the mufflers, they are made of thin Stainless steel sheet, not much mass to hold heat.
So when carb heat does no good, try adding fuel by the accelerator pump, get that engine making heat, any way we can.
 
Keep in mind, that if you have carb ice built up and your engine is not running well because of it, your engine may run even WORSE when you apply it as the heat begins to melt the ice and it is ingested by the engine. Resist the urge to pull the carb heat back out, give it a chance to work, it won't hurt anything. As mentioned, when the engine stops firing the mufflers cool quickly when flying, so it should be one of the first things you try if you have an airplane that has carb heat.
 
Because I obtained my PPL flying the fuel injected DA20, I had to amend GUMPS to be GUMPS-C when flying the Skylane. Took me many hours to remember add the habit of adding Carb Heat after flying aircraft that didn't have that function.
 
mscard88 said:
:rofl: Last?! Shirley you jest Bill. This is POA, it ain't over til it's, I dunno, resurrected.
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My aircraft doesn’t have carb heat. Can I get a field approval to add it?
 
mscard88 said:
Go by Tom's, he'll fix ya right up. Expensive though.
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Will I need to get a carb before I get carb heat? Should I get a Stromberg or a Marvel? Can I substitute a cheap import?
 
Clark1961 said:
Will I need to get a carb before I get carb heat? Should I get a Stromberg or a Marvel? Can I substitute a cheap import?
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One of those old SUs from an MG will be good enough. Doesn't have to hooked up either, just look like it is.
 
Allison Riccardi said:
Okay but what if it works on the ground, then decides it doesnt want to in the air?
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Do what you would do if any piece of required equipment failed...land and have it inspected. It is not an emergency unless the atmospheric conditions fall into the danger zones in the chart Bill Jennings posted....note that a clear July sky with temps in the 70s can be icy if the air contains enough moisture in invisible form.

Interesting experiment: Go to a dispenser of canned or bottled soft drinks and buy one. Wait a few seconds to see if water droplets form on the sides of the container. That water is the product of condensation and is a rough idea of how much moisture the air contains on a given day.

Bob
 
Jeff Oslick said:
...The first time was in a PA-28-160, at about 300' AGL. Airplane had a carb ice detector, but not a temp gauge. ...
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Do you recall the conditions on that day? Full power carb ice in a PA-28 is by all accounts a very rare event.
 
Tom-D said:
...........................................So when carb heat does no good, try adding fuel by the accelerator pump, get that engine making heat, any way we can.
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Never heard of that before. Would I have thought of it? Maybe, most likely not. Now I will. That's going into my bag of tricks
 
Tom-D said:
Carb icing is caused by evaporative cooling, fuel is transferred to the Venturi by suction caused by the Venturi, as the fuel atomizes it cools, and drops the temp in the Venturi to below freezing. when water of any kind enters the Venturi it freezes and destroys the Venturi and throttle plate's shape, until no fuel is transferred, then the engine quits and no heat is made for the mufflers to melt the ice in the Venturi. when the mufflers are too cold to heat the carb, you are going to land.
It requires but a few seconds to cool the mufflers, they are made of thin Stainless steel sheet, not much mass to hold heat.
So when carb heat does no good, try adding fuel by the accelerator pump, get that engine making heat, any way we can.
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I’m willing to bet that you quadruple spell checked this.
 
mscard88 said:
One of those old SUs from an MG will be good enough. Doesn't have to hooked up either, just look like it is.
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I want individual carbs. Who can draw up an induction diagram to provide carb heat to the air from the turbocharger after it passes through the intercooler?
 
Tom-D said:
Carb icing is caused by evaporative cooling, fuel is transferred to the Venturi by suction caused by the Venturi, as the fuel atomizes it cools, and drops the temp in the Venturi to below freezing. when water of any kind enters the Venturi it freezes and destroys the Venturi and throttle plate's shape, until no fuel is transferred, then the engine quits and no heat is made for the mufflers to melt the ice in the Venturi. when the mufflers are too cold to heat the carb, you are going to land.
It requires but a few seconds to cool the mufflers, they are made of thin Stainless steel sheet, not much mass to hold heat.
So when carb heat does no good, try adding fuel by the accelerator pump, get that engine making heat, any way we can.
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Evaporative cooling is the major source of cooling, but there's also the pressure drop in the venturi that's responsible for some of it as well. IIRC, the evaporative cooling can be as much as 40 degrees F, and the pressure drop can cause as much as another 30 degree F drop. Add them up and we see that we can get as much as a 70 degree drop, explaining why we can get ice on a 100-degree day.

Pressure drop is the primary cause of carb ice when the throttle is closed. Air being pulled around the tiny gap at the edge of the throttle plate accelerates tremendously, which also means that its pressure temp also fall considerably.

You are right about that muffler getting cold in a hurry. Carb ice is suspected to be the chief factor in many practice forced landings that became the real thing when the pilot tried to overshoot and the engine was dead. I say "suspected" because the ice is long gone by the time investigators start looking, but nothing else can be found wrong with the engine or any of its systems. "Clearing" the engine is a really poor term and implies that we're just blowing out any fuel that might be puddling in the intake; the real risk is carb ice and no heat to drive it off. Many syllabi call for a ten-second clearing every 1000 feet during the glide; ten seconds is a long time, but we need a bunch of hot air and time for it to melt any accumulated ice.
 
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