Why isn't the carb heated all the time?

[rich_g85]

This question popped up yesterday but I forgot to ask my instructor, so here you go..

I just wondered why couldn't the carb heat either A) stay on constantly or B) the carb be modified so that there was always a flow of warm air to/through it, removing the need to 'remember' (not that I have an issue with remembering!) to switch it on when reducing power, during the FREDA check, etc.

Perhaps the answer to this will become obvious when I've done the work for the Aircraft Technical exam, but in the meantime could anyone explain this for me?

Thanks,
Rich

[flybymike]

Cold air is denser than warm air and gives more power and better performance with less likelihood of engine damage due to knocking/pinking/pre detonation etc

Having said that I believe some aircraft engines do have a permanent warm air supply to the carb

[Flight Inspector]

Carb heated air is thinner (less density) and that's what you need to have a better (optimized) combustion. The thicker, colder or denser the air the better your combustion. (If the mixture is right.) See it as flying always 2 or 3000 feet higher than actual altitude. The air is les dens, so you need to lean, to have an optimum mixture. Performance will be less with Carb Heat on. You’ll notice when you switch it on while checking for Carb Ice. Why it is still on modern light airplane’s, its cheap. Other solution is the injected engine. This will have the Carburetor remove and there is no change of Carb Ice.

Hope this helps. Gool luck with the training. :ok:

[tomtom_91]

Economy - I remember thinking this a few weeks ago!

Tom

[Iwasoneonce]

Applying carburetor heat sends a warm air through the carburetor, which has the effect of giving a rich mixture, hence the RPM drop. Also when carburetor heat is applied the air is unfiltered. IIRC.

[Deeday]

I believe some aircraft engines do have a permanent warm air supply to the carbI've heard of a mod for Rotax engines that takes warm fluid from the cooling system and makes it circulate around the carburettor body, so that the body gets the heat (and so preventing any ice accretion), instead of the air/fuel mixture.
Now that's a smart solution, always-on and costing no power.

[IanPZ]

Yeah, the rotax engines in microlights (912UL) have the water jacket extended to the carb bodies. This stops ice forming on the air intakes, but at the same time does not sufficiently heat the air going in to impact density that much.

It's still possible for them to ice up, but much more unlikely, I am told.

[Nearly There]

Ive often wondered why aviation still has these uneconomical and very outdated engines, and there not cheap, I heard someone talking £10k+ for a reconditioned lycoming recently!

[Rod1]

The Rotax 912 is very common in Group A / SEP aircraft as well as micros. There is an electrical system which heats the body of the carb which does a similar job to the water jacket. It is available on Rotax 91X and Jab engines.

Rod1

[FlyingStone]

As explained, use of carburetor heat increases the temperature of air in the carburetor venturi, which means that density of the air before the venturi (where mixing of fuel and air takes place) drops (if I remember correctly the mathematical equation is Rho = p / (R * T) ). Don't take me for granted, but I believe the rise in carb temperature with carb heat is usually around 10-15 degrees Celsius, which basically means that density (and therefore engine power) drops for around 5% - provided the air-to-fuel ratio remains the same. Not much of importance on low-level cruise, but could be lethal at high-elevation takeoff/go-around.

I think at this point it should be noted that ANY aircraft engine can suffer from icing, it's just the type of icing engine is prone to that differs from engine to engine. Most people immediately think of carb ice when mentioning engine icing, but that is only one reason. It's important to know that ice can form on the air filter (cold and humid air), which prevents air from going into intake system and this type of icing doesn't affect only carbureted engines, but injected too. This is also one of the reasons why carb heat isn't ON all the time, since the design was made to be easy to use and carburetor heat and alternate air switch were incorporated into a single lever/knob now called carb heat so by applying carb "heat" the engines also "breathes" air without a filter blocking the way.

As said, the technology is here for a few decades (classic gasoline injected engines) and is improving (direct injection diesel engines with accurate mass air flow sensors which makes possible to set the optimum mixture), many manufacturers even put injected engines into training aircraft (C172SP, DA20-C1, ...), but the price and "reliability" seem to be on side of carbureted engines. Ah, new technology in aviation - seems to take forever to get it certified and almost impossible to convince aircraft owners to buy/use it :ugh:

[BackPacker]

Obviously the Rotax 912 is (partly) water cooled so there's coolant available to heat the carb bodies. On an air-cooled Lycosaurus you would need a different solution.

This stops ice forming on the air intakes

Actually, it doesn't. Ice can still form due to the venturi effect and the evaporation of the fuel. But ice can't stick to the carb walls anymore. Either it contacts the carb walls and melts, or is sucked into the engine itself. Both of which are harmless.

Other solution is the injected engine. This will have the Carburetor remove and there is no change of Carb Ice.
Another issue is that the carb heat mechanism also ensures that the intake air bypasses the air filter, thus enabling an alternative air intake path when the air filter is clogged up (for instance by impact ice). That's why you try to minimize carb heat application on the ground - the unfiltered air might have sand particles or other foreign matter in it that's been thrown up by the prop blast. You could of course decouple the carb heat and alternate air intake functionality, but that would mean you'd get another lever in the cockpit.

[POBJOY]

A "oil heated carb" (jacketed) was not uncommom on pre war engines, however they are expensive to produce and the carb would need to be "heat soaked" to work.Their purpose is to prevent a build up when the throttle is closed under normal conditions.However in severe conditions if a build up occurs it would still need another source of hot air to clear it.
The same principal can use the coolant as the heat source on a liquid cooled engine.

[englishal]

Ive often wondered why aviation still has these uneconomical and very outdated engines, and there not cheap, I heard someone talking £10k+ for a reconditioned lycoming recently!
Don't be ridiculous. Who's ever heard of a Lycoming costing 10k to rebuild !!!

Ours cost 28k to zero time.... :eek:

[lotusexige]

He did say £10k+, a big + though:)

[Genghis the Engineer]

Don't be ridiculous. Who's ever heard of a Lycoming costing 10k to rebuild !!!

Ours cost 28k to zero time.... :eek:

Or about the price of two brand new Rotax 912s!

(This is one reason why I try to avoid owning more than 1/10th of any one light aeroplane!)

G


N.B. Standard practice on most Gypsy Major installations is also to leave carb heat on all the time.

[Big Pistons Forever]

Lycomings run the intake tubes through the oil sump and for that reason they are not particularly susceptible to carb ice. Small Continentals do not, and carb heat should be used liberally in icing conditions.
.

While in general your statement is correct the engine installation has a significant effect on the propesnsity to suffer from carb icing. The two worst aircraft for carb icing I have ever flown were the BN2 Islander and the PA 23 Apache, and both have Lycoming engines.

The smaller carburated Continentals are getting rare in the training fleet as almost all training aircraft in use today are fitted with the lycoming O 235 or O 320. The C 172 and Pa28 series in particular seldom develop ice so it is common that students complete their training without ever actually expereincing carb ice, thus I think it is doubly imprortant that proper procedures for detecting and dealing with carb ice are taught.

[funflier44]

When I did my flying training on RAF Chipmunks back in 1965 and when subsequently flying them on holding postings the Carb Air was always "wired HOT".

[bingofuel]

If I recall, on the chipmunk it was common to wire the selector in the 'warm' position. I don't think it had a 'hot' setting and warm had minimal effect on power

[T21]

Carb icing occurs when the ambient air is moist and between plus 5 and plus 15 degrees. If you have carb heat on all the time you can raise the temperature of colder air up to the above range. If you get icing then you do not now have extra carb heat to clear it.

[SNS3Guppy]

Carb icing occurs when the ambient air is moist and between plus 5 and plus 15 degrees.

Actually, no. That's structural icing. Not carburetor icing.

Carburetor icing can occur at much higher temperatures, because the carburetor experiences a significant temperature drop at the venturi throat. Carburetor ice can happen on hot summer days, and often does.


I just wondered why couldn't the carb heat either A) stay on constantly or B) the carb be modified so that there was always a flow of warm air to/through it, removing the need to 'remember' (not that I have an issue with remembering!) to switch it on when reducing power, during the FREDA check, etc.

The purpose of carburetor heat is to move the carburetor air temperature out of the icing range. This is properly done using partial carburetor heat, and a carburetor air temperature gauge. Given that most light airplanes don't have a carburetor air temperature gauge, manufacturers take a ballpark shotgun approach and have the user apply full carburetor heat, or no heat. This is surgery using a meat cleaver.

Carb heat, when not needed, is detrimental. It reduces engine power.

[moggiee]

All this stuff about "heated air being less dense" is a bit of a red herring.

If the appropriately sized carb is fitted and the mixture is set correctly then there would be no appreciable loss of power but there would be a significant reduction in the risk of suffering a loss of thrust due to icing. Fitting a slightly wider bore carb to permit greater airflow and matching mixture to density would give as much power from the engine as you presently get with no loss of economy.

Unfortunately, very few people properly understand the physics involved and FIs trot out the same old inaccuracies to each new generation of pilots. It's just an internal combustion engine, after all.

[aviate1138]

My SkyArrow650 Rotax 912UL was fitted with a device that heated the carb body. Similar to below......

Carb heat, carburator heater for 912 Rotax, carb heat for Bing carburetors. (http://www.ultralightnews.ca/bing/carbheater.htm)

From the moment the device was fitted [and the original carb heat system via lever retained] was the day any carb icing conditions seemed to vanish.

Using the original system [with the Nigel Beale device attached] on really bad carb ice days engine output was smooth/no misfiring etc whereas before there would be a hiccup as the slivers of ice were consumed.

It was the best Rotax engine add-on device I ever bought.

[SNS3Guppy]

All this stuff about "heated air being less dense" is a bit of a red herring.

If the appropriately sized carb is fitted and the mixture is set correctly then there would be no appreciable loss of power but there would be a significant reduction in the risk of suffering a loss of thrust due to icing. Fitting a slightly wider bore carb to permit greater airflow and matching mixture to density would give as much power from the engine as you presently get with no loss of economy.

Unfortunately, very few people properly understand the physics involved and FIs trot out the same old inaccuracies to each new generation of pilots. It's just an internal combustion engine, after all.

The R3350's I used to operate, or the R2600's, were also internal combustion, carburetted engines, but were very different in operation than say, an O-320. All of them use carburetor heat, but are different animals.

Yes, one does lose significant power with the application of carburetor heat, depending on the specific installation and the phase of flight in which is it applied. This is not a red herring, and can be a crucial issue, especially when one is flying a heavily loaded light airplane at high density altitudes. Limited performance demands all the power one can muster under some circumstances, and the application of carburetor heat may be enough to tip the scales such that insufficient power is available.

One doesn't have the option of rejetting the carburetors, or selecting a bigger carburetor, or one with more barrels.

You can rest assured that the carburetors I used on large radial engines were substantially larger than what you see on small airplane piston engines. Never the less, carburetor icing was still a consideration.

Put a bigger carburetor on the engine and apply carburetor heat, and one still has a decrease in air density. Given the low RPM's and low relative power output from aircraft piston engines (due to limitations by the propeller RPM in fixed-drive installations), a significantly bigger carburetor isn't going to benefit the engine much, but increases in induction air density (temperature and altitude) will always have an impact.

There is no inaccuracy in explaining, nor in demonstrating that the use of carburetor heat causes a loss of power, because it's indeed the case.

Do you intend to swap carburetors during the approach to get the different results you describe? Do you intend that the student install a different carburetor? Have you successfully managed to STC a new carburetor and thus eliminate the need for carburetor heat in certified, piston engine airplanes? Do you in fact, then, have a basis for what you're attempting to suggest here?

Large volume or small, induction air density as a function of both temperature and altitude impacts engine performance. It's just an internal combustion engine, after all.

[IanPZ]

Gotta agree with sns3guppy. from my days as an engineer...

Power is related to the amount of fuel burned, and that has an ideal ratio of fuel to air. Therefore, to burn more fuel (and so to increase power, you need more oxygen. To get more oxygen into the cylinder, there are only two options (three, if you include nitrous oxide injection!) either increase the density of the air by cooling it, or increase the density by pressurising it. (denser air has more oxygen per unit volume)

Hence, subaru impreza turbo intercooler! Cools and pressurising, so getting both benefits.

By the same logic, heating air reduces the density, which makes less oxygen available to burn fuel, which means less power.

Now, where did I leave my propeller hat! (sorry, bit of a geek :-)

[FlyingStone]

If the appropriately sized carb is fitted and the mixture is set correctly then there would be no appreciable loss of power but there would be a significant reduction in the risk of suffering a loss of thrust due to icing. Fitting a slightly wider bore carb to permit greater airflow and matching mixture to density would give as much power from the engine as you presently get with no loss of economy.

Carburetor heat DOES, believe it or not, reduce maximum available engine power. It's - as you say - simple physics. By heating the air you reduce the air's density (the equation is written in my previous post) and by not changing any other parameter, such as dynamic (velocity) and static pressure, you effectively decrease mass flow of the air through the carburetor, the effect being the previous set mixture too rich, so one needs to adjust it to a new setting. But even with correctly (let's say stochiometric) set mixture without and with carburetor heat there will be approx. 5% drop in available power, since with reducing mass air flow due to increased temperature, you also reduce mass flow of the fuel through the jet into the carburetor. Again - simple physics: less fuel, thus less power (with maintaining stochiometric ratio of course).

As you say fitting the wider bore carburetor would reduce chance of carb ice, but the real way to go are injected engines, which eliminate many problems (except ram/intake ice).

[FL575]

Thinking about Funfliers post, the RAF Chipmunks did have the carbheat 'wire-locked' in hot in 1969.

That is when I learned to use 'wire-locking' pliers whilst taxying in. Should any other 'dual' Chipmunk be spotted during a dual sortie, a quick 'I have control' from the rear, followed by 'Carbheat to cold'. Then followed the usual 5 to 10 mins of 'combat'. As I say, it was the students job to 'tidy up' whilst taxying in. Is that correct D H-H?

[Miroku]

I used to fly an AA5 which had a device to monitor whether carb ice was forming (it didn't work mind). I'm surprised that nobody has come up with a device along these lines which could be easily (?) retrofitted due to the number of problems caused by carb icing.

[Genghis the Engineer]

I used to fly an AA5 which had a device to monitor whether carb ice was forming (it didn't work mind). I'm surprised that nobody has come up with a device along these lines which could be easily (?) retrofitted due to the number of problems caused by carb icing.

Generally a pain to retrofit such things to old engines, and if going for a new engine - have something fuel injected, then you don't have a carb ice problem anyhow.

G

[englishal]

We have a carb temp meter....but then we're very unlikely to get carb ice as we have a turbo charger.

[Mark1234]

Chippies are perhaps a little bit of a red herring. They have a warm air system, not a hot air system (hence less impact to performance if warm is continuously selected), and according to my handling notes are 'highly likely' (in red) to quit at low throttle settings w/o the warm air applied. I presume this means they make ice rather readily, I've never felt the need to test that. Certainly application of heat has a much less dramatic effect than in other a/c I've flown.

If the only difference carb heat made was a richer mixture, you could recover the power by using that little red lever that so many appear afraid of moving - no need to re-jet. However, as others have said, hot air is less dense, and cylinder volume is not variable, so hot air = less power. That's one of the reasons why a high ambient cripples your takeoff performance.

[IanPZ]

So, I know I said stuff about air temp and density, cos I've had to deal with that from car mechanics, but I wonder if someone can clarify the whole carb-icing thing.

I understand why it happens, but surely whether you have a carb or injection, ice will still form around the trumpet of the air intake. Is the problem ice there, or actually on the metering needle of a carb.

If its not the metering needle, then why don't injection systems suffer from carb icing? Presumably if ice forms around the venturi (which are still there in an injection system) then the cross sectional area gets smaller, the air flows faster, pressure drops, more ice, etc etc.

Or am I missing something (which I am sure is true)!

[BackPacker]

We have a carb temp meter....but then we're very unlikely to get carb ice as we have a turbo charger.

I've had carb ice twice now in the R2160 (O-360 or something like that; not injected) when taxiing with about 1000 RPM in high humidity conditions (low T/D spread, in both cases just ahead of a front passage).

At those RPMs I would assume that the turbo would not be spinning at all, or only give a negligable rise in MAP/temperature, so you would be able to get carb ice anyway.

(But how many non-injected turbo installations are there anyway?)

[Jan Olieslagers]

(But how many non-injected turbo installations are there anyway?)

Rotax 914 - not a very common engine, indeed.

[englishal]

(But how many non-injected turbo installations are there anyway?)
Quite a few (Rockwell Commander). It is funny because although a carb, we don't have carb heat, just "alternate air".

[Mark1234]

So, I know I said stuff about air temp and density, cos I've had to deal with that from car mechanics, but I wonder if someone can clarify the whole carb-icing thing.

I understand why it happens, but surely whether you have a carb or injection, ice will still form around the trumpet of the air intake. Is the problem ice there, or actually on the metering needle of a carb.

If its not the metering needle, then why don't injection systems suffer from carb icing? Presumably if ice forms around the venturi (which are still there in an injection system) then the cross sectional area gets smaller, the air flows faster, pressure drops, more ice, etc etc.

Or am I missing something (which I am sure is true)! There are (broadly) two places where ice can form:

External impact icing on the intake trumpet/air filter stopping the supply of air. That's why you have alternate air, and is equally likely for injected and non-injected, but less common than 'carb icing'

Typically what we refer to as 'carb ice', which forms in the venturi/throttle plate etc. There are two contributing factors to the temperature drop here - the pressure drop in the venturi, and also the cooling effect of evaporating fuel (pour fuel or another volatile liquid on your hand, you'll feel a significant chill as it evaporates, so long as you're not close to a flame!)

In an injected engine, the fuel is typically added further downstream, nearer the inlet valves, thereby removing at least one of those.

As for venturis, I'm not familiar enough to comment on the specifics of aviation application, but typically fuel injected automotive engines do away venturi, using other mechanisms such as a heated wire for air mass sensing - I doubt aero engines are that advanced (they'd not need a mixture lever if they were); I rather suspect fuel metering is a simple mechanical linkage to how far open the throttle plate is. Long and short, I wouldn't expect to find a venturi in an injected aero engine - but I may be wrong.

[edit to add]
Turbocharging usually involves a significant rise in inlet air temperature - both through compression, and heat transfer of the turbine being rather warm, having several hundred degrees of exhaust gas flying through one end. Obviously the extra air more than compensates for this, but a turbo will raise inlet temp far more than the normal carb heat control anyways - I'd rather suspect that is why the rockwell has alternate air. I'm also assuming the carby is pressurised, and downstream of the turbo - I'm not sure if it's possible to do it the other way round and draw the fuel through the turbo.

[IanPZ]

Thanks, Mark1234

So, is carb icing really about the fuel cooling (or at least, thickening), and thus less vapourised fuel reaching the engine, rather than ice causing blockage of metering valves etc? I think that's what you're saying.

Certainly, most old-style fuel injection systems on cars (like old Mercedes) just had the equivalent of a distributor for fuel, and then an injector straight into the chamber, rather than into the neck of a throttle body (like modern cars).

I can see that doing that, the fuel mixture wouldn't be much affected by the air temp, especially as the cylinder itself would be hot from exiting exhaust gas....

Hey, why don't they just use 2-stroke engines instead? (Or do they?)

I'm slowly understanding the differences, and the light is starting to dawn. I suppose design has to be based around the premise of "nothing must cause the engine to stop, if possible" rather than improving fuel efficiency and trusting in the AA!

Thanks for the help.

[Mark1234]

Nope, that's not what I'm saying :)

Carb icing is ice, and occurs when moisture (humidity) in the air is turned into water ice in the carb - most likely the venturi, and/or throttle plate, which restricts the breathing.

That happens because the temperature in the throat is a lot lower than ambient, THAT is driven by the venturi pressure drop, and also evaporative cooling of the fuel - it requires energy to transform the liquid into a vapour (think water spray in high performance automotive applications).

Aero engines are generally pretty crude, and use large capacities to achieve low power outputs. They have to operate over a far wider range of conditions, and are they're deliberately very low stressed; but probably the main reason is it costs a lot to certify, and there are liability concerns doing anything radical. Therefore we're mostly stuck with technology from the 60's.

[IanPZ]

Ooooohhhhh

So carb icing IS about restricting airflow, but its the combination of the pressure drop AND the evaporative cooling that pushes the humidity over the edge and causes freezing. And by taking the fuel away, or squirting it in further down the intake side, you only get one effect (pressure drop) and so icing is less likely.

Is that right?

[n5296s]

I've heard of a mod for Rotax engines that takes warm fluid from the cooling system and makes it circulate around the carburettor body, so that the body gets the heat (and so preventing any ice accretion), instead of the air/fuel mixture.
Now that's a smart solution, always-on and costing no power.

In the Cessna TR182 (I think all the R182s) the carb is at the back of the engine. Mine has a carb temp gauge and it is never below 20 degrees. So in effect this is what happens. Don't know why other designs don't do it, it seems so common-sensical to me.

[Mark1234]

Ian, that's kinda it. As I said, I don't think there's a venturi in an injected (aero) engine either, but I don't take them apart, only sit behind them..

[flyme273]

"I believe some aircraft engines do have a permanent warm air supply to the carb"

The flight manual for the Morane Rallye recomends using partial carb heat to operate the engine outside the amber range on the carb temp gauge.

[ShyTorque]

IanPZ, Yes, I think you have now "got it".

I liken the effect of fuel evaporation in the venturi of a carburettor to the principle of a refrigerator. Inside a fridge, a liquid is deliberately evaporated to provide the required cooling effect. But in this case, the vapour/gas is subsequently collected, compressed and the ensuing heat produced is allowed to disperse elsewhere.

Just as water ice forms on the exposed metal parts inside of a fridge, it can also do so inside a carburettor.

The carburettor is a precision instrument and is carefully calibrated (jetted) to provide the correct air/fuel mix. It relies on a steady flow and an increase in airspeed in the venturi to cause a pressure drop, which draws the correct amount of fuel from the jet/s.

If ice upsets the airflow (volume or flow pattern) it will upset the mixture. It is also possible for ice to partly block a fuel jet.

By injecting the fuel downstream of the intake, any liquid water or ice formed by evaporation will go straight into the inlet ports, which is harmless to the engine. Having said that, closer to the engine also means warmer, so ice forming from the fuel evaporation is less likely, in any case.

[24Carrot]

Granted carb heat reduces max power (the throttle is already full open), is there any problem with leaving it on all the while you are not at full power, and opening the throttle a little to compensate?

I know there is a issue where extremely low air temperatures (-25 deg C IIRC) can be heated into an icing range, and in the aircraft I trained in, carb-heated air was unfiltered, but is there any other issue, e.g. with the mixture?

No burning urge to do it, just wondered.

[Jim59]

Some of Mr Lycoming's thoughts on carb ice - but what does he know about it!

Lycoming Publication SA25-10/05
To prevent or melt the ice, use the carburetor heat. Since warm air is less dense than cold air, effectively enriching the mixture, you will see an rpm drop when the carburetor heat is turned On. The rpm drop will continue until the carburetor heat is turned Off.

Lycoming Flyer- Operations
CARBURETOR HEAT OR ALTERNATE AIR HEAT
AS POWER THIEVES
In the opening paragraph, carburetor heat was used as an example of a cause of power loss, but many pilots aren’t sure they understand the reason for it. Flight tests conducted with a precision torque meter installed have measured fairly accurately a loss of as much as 15% of engine power when full alternate air or carburetor heat have been applied. As a specific explanation, there is a small power loss when we use heat because the pilot has switched from the direct, colder ram air to an indirect carburetor heat muff, or a similar indirect source of warm air with an alternate warm air source from inside the cowling. This accounts for an average 3% power drop because of the loss of ram air. The major portion of the
engine power loss is caused by the carburetor heat or alternate air heat. Aircraft engines are checked for their horsepower output at a corrected standard temperature of 59˚ F. Engineering has provided a simple rule of thumb for the effect of heat on power,
i.e., for every 10˚ F of heat above the standard 59˚ F, there is a 1% power loss. Since the average heat source on an engine provides at least 100˚ F of heat above standard, this heat condition causes an average power loss of 10%. Our measurable total power loss at sea level with standard conditions is already up to 13%.
When warm air is used by the pilot, the mixture becomes richer, and the engine may roughen with another slight power loss as a result. In addition, the higher the altitude with its less-dense air, the greater the enriching effect because the fuel-metering
device will become richer at altitude and the engine less efficient. Thus, there will be another small, difficult to measure, power loss to be added to the 13% loss already accumulated.
With full carburetor heat applied, most float-type carburetors react very sluggishly or inefficiently on a straight-arm throttle technique during a touch-and-go landing or an aborted landing. In some cases, the float-type carburetor may refuse to accept the throttle when it is applied in this manner. A gradual, steady application of the throttle is always the best approach.
We should also remind the pilot that when using carburetor heat or alternate air heat at cruise power, to adjust the mixture lean, otherwise the mixture will be rich. If the heat causes an undesirable power loss at cruise, and there is throttle available, the pilot may bring the manifold pressure up at least to the power reading before application of heat; and if additional power is needed and available, add a maximum of two inches of MP, or 100 RPM (fixed-pitch prop) above the previous power, and then adjust the mixture. It is possible to compensate for the horsepower loss due to heat by means of the latter technique if throttle or RPM are available.

[rich_g85]

Wow! I didn't expect this sort of reponse to my question, thought it would all be over in a couple of posts. :)

Thanks to everyone who has replied, for all the useful information and varied points of view. As a student pilot I'm keen to gather as much knowledge as I can, who knows when it may come in handy. :ok:

[IanPZ]

Mark, ShyTorque,

Thanks for that. I'm just starting out with this flying stuff, and I've always found the better I understand something, the better it goes in, and the more intuitive it seems. I appreciate the time to clarify. Always easiest when comparing it to something I'm already familiar with.

Rich, bet you wished you'd never asked now!

IPZ

[Maoraigh1]

"I know there is a issue where extremely low air temperatures (-25 deg C IIRC) can be heated into an icing range,..."
At that temperature, the air would hold very little water. Why would raising the temperature have an icing risk, with no available water source to increase the humidity?

[24Carrot]

I think there has to be some visible moisture, ie some water has condensed out, though it doesn't have to be as bad as cloud. I was re-taught this recently, but I can't find any notes or reference now. I also remember thinking I wouldn't be flying on days like that.

I believe Backpacker may have posted something on this a while back.

[ShyTorque]

24Carrot, You may well be flying on days where carb icing is possible.

Check this out:

http://www.caa.co.uk/docs/33/ga_srg_09webSSL14October.pdf

[24Carrot]

I meant specifically carb ice that forms because carb heat was applied.

I take the normal carb icing risk very seriously, because yes, I could be flying in those conditions.

It's the -25 degs OAT I don't plan to fly in.

[ShyTorque]

24Carrot, OK, understood.

Depending on the installation, the carb heat will raise the intake temperature by a specific amount, as per manufacturer's design and certification. The trick is to ensure the icing range is kept out out of the carb's interior. Yes, although there does need to be some water present for ice to form, fuel icing (water in fuel) can occur in very low temperatures. Hopefully that will be taken care of by correct fuel quality checks.

[BackPacker]

I believe Backpacker may have posted something on this a while back.

I believe not...:confused:

However, my limited knowledge of physics leads me to believe that Moraigh is right:

At that temperature, the air would hold very little water.

If you have air with a temperature of, say -15, and this air is almost fully saturated, and then you raise the temperature to, say +15 (30C difference - does sound about right for carb heat application), and then you cool it down again to -15 (30C difference sounds about right for venturi and evaporative cooling), the air is still *almost* fully saturated. So no, or virtually no ice would form because the air can still hold all of the moisture.

So getting carb ice by *applying* carb heat and thus moving the temperature into the icing range sounds like a very unlikely scenario to me. Unless the temperature rise due to carb heat application is rather low compared to the temperature drop due to venturi and evaporative cooling.

Nevertheless, why would you apply carb heat if there's no risk of carb ice? There's a nice graph somewhere from the CAA, replicated in all PPL books (my Pratt book shows it on page Gen54), that show you that the chances of carb ice are highest at +15 OAT and fully saturated air, and virtually nonexistent below -15C or above +35C, irrespective of the humidity of the air.

I work for a large IT firm and one of the problems IT faces is datacenter cooling. If you put a (small) air conditioning unit somewhere in a server room, and just draw in datacenter air from an arbitrary location, it cools the air to a level that's below the ambient air temperature, and in a lot of cases below the dew point. This leads to condensation (which needs to be flushed away, may lead to corrosion and may actually lead to a datacenter that's so dry that it's very bad for humans to stay in for longer periods of time, and may increase the chances of static electricity to form - some datacenters actually have humidifiers precisely for this). But if you move the cooling location to directly behind the heat source (in the form of a rack door which doubles as a radiator), and make sure that you cool the air to the ambient temperature but not lower than that, you virtually eliminate the problem of condensation. Same problem, same physics. Condensation occurs when the temperature is forced below the dew point. (And when the temperature is below zero, condensation will become ice.)

[24Carrot]

It seems this topic has a history! I can't work out how to link to a thread this late at night, but there is a pprune thread titled:

Carb heat - can it cause ice?

[flybymike]

Not sure if this has been previously mentioned, but there is a school of thought which says that carb heat should either be fully applied or not applied at all. This is because partial application may cause previously formed ice pellets which would otherwise pass harmlessly through the engine , to become "sticky" and clog up the venturi. Thus if heat is applied it should be sufficient to completely melt all of the ice.

[flybymike]

It seems this topic has a history!


Everything on here has a history and has all been discussed before....;)

[24Carrot]

I finally found where I saw it: Wagtendonk's 'Principles Of Helicopter Flight', p185.

By intelligent use of carburetor heat, the risk assciated with
carburetor icing can be avoided.
At temperatures below approximately -15C (+5F), the ice formed
is dry and unlikely to adhere to carburetor parts such as walls
or the butterfly. Applying carburetor heat when the carburetor
temperature gauge shows values colder than -15C may encourage the
formation of carburetor ice.And it has been discussed or mentioned on pprune before, eg:

Pilot DAR post #15 in:
http://www.pprune.org/private-flying/344053-carb-heat-warrior-lycoming-o-320-d3g.html

BackPacker post #2 in:
http://www.pprune.org/private-flying/348687-winter-flying.html

Concerning the water content of cold, clear air:

According to the chart in this wikipedia entry:
Relative humidity - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Relative_humidity)
The saturated water content of air at -20 to -10 degs C
is 2-3g/kg, vs 5g/kg at 0 deg C. So it is lower,
but not spectacularly so.

Also from personal observation of cold frosty days,
the visibility away from the sun can be excellent,
(I was certainly not flying in cloud), but the visibility toward
the sun can be terrible. That might be dust, but I suspect it is
fine water or ice. Something similar may happen at extremely
low temperatures.

Edited to include a proper link to the earlier thread, which concluded carb heat could not cause icing.
http://www.pprune.org/tech-log/75295-carb-heat-can-cause-ice.html

[Maoraigh1]

Thinks: At very cold air temperatures, how much warmer than the ambient air temp might the carburetor be, due to heat conduction from the engine? (No use of carb heat.)

[ShyTorque]

I think it will still be colder than ambient, due to the latent heat of evaporation of the fuel.

[SNS3Guppy]

Not sure if this has been previously mentioned, but there is a school of thought which says that carb heat should either be fully applied or not applied at all.

It has been discussed, at length, in this thread.