Carb Heat: What's the deal?
 

Under what conditions are light aircraft most susceptible to carb icing. I understand that in cold temperatures 10c and below and a close dew-point can lead to icing, however I recall reading something about icing occurring in far higher temperatures (20C etc.) due to the severe drop in the temperature of the carb venturi when the gasoline is converted from liquid to vapour . Is it a combination of outside temps, humidity and the venture temp drop? Is there a rule of thumb for “guestimating” when carb icing is most likely to occur?

Graph
 

Hopefully this link won't get blocked:
http://ibis.experimentals.de/images/...omcaassl14.gif

Engine and aircraft have a big effect. Pa28-161/Lycoming, C172/Continental? & Lycoming, and C152/Lycoming have given me far less icing than Jodel DR1050/Continental O200. Especially until the engine is hot. (after 10? minutes flying.) The O200 is an ice-maker. I've had the worst ice on warm, cloudy, summer conditions. (Warm for Scotland )

It's all down to the engine.

In Lycoming engines the carb is usually located close to, if not attached to, the oil sump. Therefore a lot of heat is automatically transferred reducing the overall risk of ice. Students are still generally taught to liberally apply carb heat, and in a lot of schools to use it for every approach. This is completely unnecessary with a Lycoming engine, but for the average PPL it's probably good to get into a habit which will work on every aircraft, and there are engines out there that will simply stop if you reduce power without carb heat. For more on use of carb heat see this thread http://www.pprune.org/flying-instruc...carb-heat.html

With older continental or rolls royce engines, the carb was positioned further infront of the engine leaving it much more exposed. I believe this was also the case on some older Gipsy engines found in Chipmunks and the like, but my experience on these types is fairly limited.

The main area of risk is from 5 - 20 degrees C, and the big danger is that it will happen in clear air if it is humid enough.

The only time I've had serious carb ice with a Lyco is taxiing on wet grass with a typical OAT of +8 to +15. Ultimately you have to know your engine and it's own dangers.

While it is generally true that Continentals are more prone to icing than Lycomings, I think one has to be careful to avoid making absolute statements.
The design of the cowling and air intake are also a factor. In my experience the worst 2 airplanes for carb icing were the BN Islander and the Piper Apache , and both are powered by Lycoming engines.

Carb ice is where you find it. I fly a little Grumman AA1B with a Lycoming O 235. The West Coast of Canada spends pretty much all of the winter sitting right in the max carb ice danger zone yet in over 10 years of flying I never once experienced carb ice...until last year. I was cruising at 6500 feet on a beautiful clear October afternoon with an OAT of about + 8. The engine starting loosing RPM and was sightly rough. I at first assumed it was plug fouling, a common problem with low compression engines run on 100 Octane AVGAS, but a mag check didn't make show any difference. However going to full rich made the engine run even rougher. It was at that point the penny dropped :O and I selected full carb heat. After a few HARRUMPS from the engine it roared back to full power:ok:.

Any engine will give you warning that it is icing first with a loss of power and then rough running so pay attention and icing will never be a problem.

RTN11, do you apply/cycle the carb heat as part of your pre-landing check-list regardless of whether you think you are in the "icing zone"

True, my statements shouldn't of been so absolute.

Should be "This is generally not required in a Lycoming"

I have mostly operated the O-320, but also the 235, pretty much every flight in either the "serious icing - descent power" or "serious icing - any power" bands, but only had carb icing twice in flight, and one of those was in cloud. On both occasions the engine gave plenty of warning with rough running, and a noticable drop in performance, and selection of carb heat solved it in seconds.

When flying for myself with in a PA28 with the O-320, before commencing the approach I check for carb ice. If none is found, and I am not in the serious icing any power band, I do not apply carb heat for the approach. This is in accordance with the POH.

Most schools I have taught at have the typical SOP to apply carb heat downwind, leave it hot for every approach and put to cold at 300'. This applies more to an older design of engine, or typically a continental and is not what the POH recommends for this aircraft type.

The reason for this SOP is that the schools think students are unable to think for themselves, and assess the real risk of carb ice, so they would rather have the carb heat hot for every approach. This does have some other affects, such as effectively enriching the mixture increasing the risk of spark plug fouling, something which I have experienced a lot more of on a Lycoming than I have ever encountered Carb Ice. As in the above post, the first thought was plug fouling, second was carb ice.

Is it true to say that the aircraft that are not prone to carb icing don't make full use of their engine's potential capacity? For example, you could fly a c152 around all day with carb heat on, but you wouldn't get the best out of it when it came to go-arounds and climb performance.

Presumably aircraft that continuously heat the carburettor, by whatever means, are effectively doing this?

Carb heating
 

I was also told that having carb heat on also burns through fuel at a helluva rate. So much do I was relayed a story about pilots who only got half way because they had carb heat on.

abgd - in short - yes. Performance suffers because warm air is less dense so less weight of air is inducted per cycle and it can expand less and exert less pressure on teh piston. Also the reduced mass flow rate affects the mixture although this could in theory be sorted out by leaning.

The RAF used to lockwire the carb heat into hot on their chipmunks - not sure why actually, anyone?

Not really - with mixture you can only set the fuel flow, which in terms affects fuel to air ratio, but you won't get 100% of rated power with carb heat to on, regardless of how you set mixture.

Rho = p/(R x T)

Carb heat reduces density of air just by increasing the temperature just as higher altitude reduces density of air by decreasing the pressure. Even with full throttle and mixture 50°F ROP for maximum power, the engine will not achieve its full rated power at altitudes higher than sea level (unless turbocharged), since the maximum engine output is more or less proportional to density of air entering the engine. The same goes for carb heat - once you set it to hot, the engine cannot achieve full rated power anymore.

I thought that carb heat only used waste heat from the engine, so that whilst that is correct, if you lean for cruise there shouldn't be any effect?

Yes it's true that if you apply full or partial carb heat, you neglect the full potential of engine output. A good example is on final approach - you descend at an aplied rate (say a 3 degree descent) with full carb heat set HOT, but before touch down (approx 100 feet) carb heat MUST be set to COLD. The reason for this is simples = if you are forced to do a go around, full engine output will be required to ascend, hence the reason for turning carb heat to cold - because carb heat set to HOT reduces output.

Simples :-p

Yes, if you lean properly after the application of carb heat, your engine is about as efficient as corb heat cold, for that power produced - you just cannot produce full power any more. Leaning after carb heat application also assures that the most carb heat is available. Other than aircraft equipped with a carb air temp indicator, if you need carb heat at all, you need as much as you can get.

I have more than 3500 hours in O-200, C-90, and C85 powered aircraft. Though I occasionally have had carb ice, and used carb heat, I have never felt that these engines were unusually prone to carb ice. Carb heat should be used as instructed in the aircraft flight manual. That said, in my C 150, I check carb heat for every flight, and otherwise do not use it at all, unless I detect symptoms of carb ice (loss of power). This has worked fine for me that way for a long time - but I do have a carb air temperature indicator too!

I have to be honest and say that one must NEVER guestimate the possibility of Carb ice. I beleive it good airmanship to routinely apply carb heat in the face of symptoms or not. I have been surprised in the past of carb ice prescence when no symptoms of such ever resided.

If it is so "Simples" how come No Grumman or Piper or Cessna POH requires you to select Carb heat off before touchdown ? What do you know that the flight test engineers at all the major light aircraft companies don't know ?

Since the optimum conditions for carb ice formation is high velocity air into the carb throat with a idle throttle setting, precisely the conditions on short final; what is stopping the formation of ice between the time you select carb heat off and then for what ever reason have to go around ?

And it should be noted that under the right conditions significant amounts of ice can develop in seconds:uhoh:

The fact is that the "requirement" to select carb heat off before touchdown at some arbitrary height is a unique to UK flying schools urban myth mindlessly passed down from Instructors who got it from their Instructors. Certainly it is not common practice in North America, where 70 % of the worlds light aircraft are operated.

The certification of light aircraft engines requires that the makers prove they can be run at full power with full carb heat on under any normal circumstances and so there is no reason to prevent you, in the event of a go around, from simply applying full throttle and then selecting carb heat off. If conditions warrant carb heat on final then I firmly believe it should stay on until you have landed and vacated the runway.

[quoteThe certification of light aircraft engines requires that the makers prove they can be run at full power with full carb heat on under any normal circumstances and so there is no reason to prevent you, in the event of a go around, from simply applying full throttle and then selecting carb heat off. If conditions warrant carb heat on final then I firmly believe it should stay on until you have landed and vacated the runway.][/quote]

PISTONS, do you do this?(keep the carb heat on till vacated) And how are you gauging "if conditions warrant carb heat" are you basing it on preflight weather reports and/or personal observations or pre landing OAT and visual cloud conditions or is there other factors you take into consideration when deciding to hold the heat on all the way down?

Well two of the aircraft I regularly fly have carb temp gauges which makes it pretty easy. If the carb temp is above 10 deg C then the carb heat stays off. I fit is below that I apply enough carb heat to get the temp up and out of the icing range.

If I am flying an airplane that does not have a carb temp gauge then environmental conditions dictate whether or not I consider carb heat. If, for example, it is dry and 25 C the carb heat isn't going on. :rolleyes:

If it is in the well in the icing range then I monitor the engine for signs of icing and always select carb heat on in the prelanding check and look for any signs of the heat clearing any ice. If I do think there was some icing present then the carb heat goes back on and stays on. In conditions particularly prone to icing like wet air with temps between zero and 10 deg I keep the heat full on during the approach as a prophylaxis against icing.

What are the feelings here about keeping carb-heat on prior to takeoff? My examiner, who clearly knew what he was about, encouraged me to use it whilst taxiing from the run-up area to take-off.

My concern:

1) You might forget it to reset it - not a problem in a light aircraft on a long runway, but a potentially major problem in a heavy aircraft on a short runway.

2) FOD - I just don't have the experience to know to what extent it's an issue on your average runway. Presumably a major consideration bush-flying in the Sahara, but is it worse on tarmac or grass? Is it likely to cause gradual erosion of the interior of the engine, or is ingesting a stone likely to cause a catastrophic failure e.g. by hitting a valve?

3) Presumably if you set full power against the brakes, prior to takeoff... if you have carb icing you'll detect it and can deal with the problem before take-off. If you don't have carb icing, you're not likely to get more after you let go of the brakes.

With both the C90 and O-200 engine aircraft I had taxiing on damp grass almost required carb heat - otherwise the engines would go all 'lumpy' and die. Carb ice at near idle.

I've had a similar 'conversation' with a friend whilst flying my Terrier (Gypsy powered) - when he commented I should have removed the carb heat on short finals. So I did as we taxied back to the hangar - and luckily(?) it then went all lumpy - until I re-applied carb heat.

Granted the humidity on the ground close to grass will always be higher but it does illustrate the point quite well. But my friends O-320 never suffered from these issues.....

Only times I've experienced carb ice was in a 172, taxiing out in cool or cold, and very humid conditions. (Dew on the ground, no wind.)

Carb heat remedied it in a few seconds.

Carb icing is more likely at a reduced throttle setting, because there is a greater resulting pressure difference between upstream and downstream of the butterfly.

Carb icing is slightly more likely if, in addition to the above, the mixture is left fully rich when it needn't be, because more gasoline is forced to evaporate, thus producing a greater cooling effect. It is not suggested that other than full rich is used on the ground when taxiing, nor during takeoff, when the enrichment also provides an important engine cooling function.

Dust/grit is the biggest enemy of piston engines. It can be too fine to see, but still get in and scratch things. Even melt on things, if silica based. avoid using carb heat on the ground any more than absolutely necessary when on a dusty surface.

Fortunately, around the parts I've usually flown, weather conditions conducive to dust blowing around are usually not those that lend themselves to carb ice.

Intake design !
 

Intake design also is a factor, I have had very little carb ice in a PA28 be it 140, 160 or 180 HP but. DR400's with the same engines fitted are very prone to carb ice.

Saying that having the carb attached to the engine sump will prevent most of the carb ice is tricking yourself into a false sense of security that is likely to end in tears.

I tried going to Cold on very short final - until I had severe carb ice on opening the throttle to turn after landing. I'm back to Hot until on the ground. In some conditions, I start the take-off run with Hot selected, and go to cold on the run. I've had carb ice stop the engine on a Konsin deiced runway on several occasions. (DR1050/O200 in Scotland.)
I seldom or never have touched carb heat in W. Colorado-Utah-Arizona.

Well, I have on occasion forgetten to set carb-heat to cold just prior to touchdown, so I'm pleased to hear it's not such a terrible sin.

http://www.lycoming.com/support/publ...fs/SI1148C.pdf

Lycoming, at least, are pretty clear on their use of carb heat. If you use it on the approach, it should only be selected cold after full power has been selected for a go around. Other than that, it can be selected cold once on the ground.

The whole putting it cold at 3-500' is a UK flying school nonesense and can simply act as a distraction at a rather critical phase of flight. The only exeption I would include is landing on rough grass when you wouldn't want unfiltered air going into the carb, so would select cold as soon as practical.

Big_Pistons_Forever

You have said some really important errors in BASIC aircraft operation and also in airmanship. Now you seem like a pretty opnionated person, so i will leave you to discover and note your shortfall.

ABGD

The best advice i can offer is to consult official sources. Dont listen to bods on here (including me)

If in doubt, there is NO doubt. And if in doubt, chicken out !

Ice is a problem - get rid of it! Carb heat induces a loss of power (recall carb heat check on run up) so lose it !..especially on final !

Inbox me dude, i'm from around your area :-p

Could you be more specific as to which "official sources" advocate turning off the carb heat on final ? I am asking because I am not aware of any aircraft manufacturer supporting this practice. Is the CAA the ones who are pushing this ?

I tend to listen to lots of people and also check with the official sources.

My view is that conditions and aircraft vary, so there can be no one true checklist to cover all situations. You get the basic protocol nailed, then occasionally adjust it on the basis of principles and experience. Hence my interest in what people here have to say. It would never have occurred to me that you could get carb icing as quickly as some here have experienced, for example, as the only carb icing I've ever encountered has been relatively insidious.

If the carb heat must be set to cold before landing, the aircraft flight manual will state this. The aircraft flight manual is the only "official source" of procedural information about the operation of the aircraft systems. It would not be an error to operate an aircraft in accordance with the procedures stated in the flight manual - in fact, it is expected with good airmanship! The selection of flight manuals I have here generally say "carb heat to cold after landing", though one flight manual I have says: "The approach technique is as follows....and carburettor heat off [cold] unless carburettor icing conditions prevail". I have never seen an instruction in a flight manual to reselect carb heat to cold on short final approach.

If, the pilot decides to overshoot, the carb heat can be moved to the cold position quickly - in many aircraft, without removing one's hand from the throttle. Certainly easily in any aircraft, and without detracting from the ability of the pilot to safely control the aircraft on climbout. A pilot who's workload is too high when having to select carb heat cold at the time of the decision to overshoot, is perhaps flying beyond their skill. It gets a lot more busy when you're flying a multi, and having to reconfigure flaps, cowl flaps, trim, and get the gear up after positive rate - carb heat is nothing compared to all of that!

The aircraft will safely climb with the carb heat hot, if all other things as they should be. Certainly at least enough climb to begin the overshoot, and get the aircraft reconfigured. If you think about it, the application of carb heat generally reduces the RPM (for fixed pitch prop engines) about 100 or so. Can you takeoff and climb away a C 150 at 2400RPM instead of 2500+? Of course you can. It's going to take more space, but is safely possible in most normal dimension runways. For the C 150, takeoff and climb away is possible at engine speeds as slow as 2100 RPM - though it's going to take a lot more runway and clearway. If you're going into a runway from which you cannot safely overshoot carb heat hot, you have already committed yourself to a landing from which a safe overshoot is not possible - and there are some, so it happens.

That would be because you are incapable of articulating where I have made any important errors. Go ahead and prove me wrong :rolleyes:

BTW I note you have listed your age as 28. I started my commercial flying career when I was 27, I am now 52 :E

Love/Hate
 

I always find these threads useful as they make me think about my own current knowledge/skill and what should I brush up on.... :)

What makes me despair however is the constant 'dick-measuring' which adds absolutely nothing to the debate... :(

As for the Trolls and armchair pilots on PPRuNe! :mad:

I'm delighted that I don't have to worry about selecting and deselecting carb heat.

I fly a Rotax 4 stroke engined aircraft. The carb bodies are permanently heated by the engine coolant but there is no power loss because the air entering the carbs is at ambient temperature.

I was always taught to select carb heat downwind and leave it on until you land. I can't see the problem on a go round, you just shove the carb heat and throttle forward at the same time. You might have to take another half second to do it on a 28. I suppose a minor advantage as well is that if your idle setting is slightly high you would have less residual thrust when on the landing roll.

As the OP of this thread I have found it extremely interesting and educational reading the different viewpoints on carb heat use, I have deduced the following:
1. Re-read and understand what the AFM says, and just as importantly what it DOES NOT say in regards to carb heat.
2. Carry with me the “carb icing danger zone chart” and understand how to relate its calculations to the combination of actual weather conditions before/ during flight and prior to the finals
3. When considering when to engage and disengaging car heat be cognizant of not just the elements (temperature, humidity etc.) but also field conditions such as thin grass with loose dirt, fleshly cut grass and wet grass both on TO and landing that can bypass air filtration when crab heat is on and cause additional problems. But remembering that if faced with a choice, unfiltered air is better than an iced carb
4. When in doubt whatsoever there is very little downside in any flight configuration to putting the carb heat on to verify there is no icing.
5. If landing in the conditions susceptible to carb icing keeping carb heat on all the way down till reaching ground effect, then only closing the carb heat due to either having to go around or conditions on the ground favouring filtered air such is the case on my airstrip which swings from wet to dusty with a thin cover of grass
6. Expect the unexpected and expect it happening quick and at times of high workload. And carb icing can happen pretty much at the entire normal temperature range for the area I fly in.

Thanks folks

Good summary, Piperboy.

Though you can probably select car heat according to the comfort level you require when driving to the airfield :D

And to add further... Crab heat is probably best left to the better restaurants.;)

Big_Pistons_Forever

Unfortunately i am very poor at articulating your errors, so i will leave you to measure how big your pistons are.........although i'm guessing there pretty big :-) Regarding my age....i'm actually a 75 year old ballet dancer..which confirms ???

Good try at a reasonable thread Piperboy. I'm happy for you that you got some useful responses, before the thread drifted into silliness. I would take Big Piston's advice verbatim any day. You can see that there are varied ways in which carb heat is used by pilots. and some passion about it too.

Keep your eyes open for an opportunity to fly an aircraft equipped with a carb air temperature indicator - watching that while selecting carb heat, and then adjusting the power and mixture to optimize the heat, is an education. What you will see is that the techniques which are taught, are often only a part of the story. The flight manual techniques err to the simple and conservative, as one would expect. Pilots who feel confident that they have assured their security from carb ice simply by selecting carb heat, and then ignoring the other engine controls could be in for disappointment - I have been. Like any control of the aircraft, carb heat should be applied as a part of good, informed pilot decision making.

I applaud your effort to become more informed. Good attitude! Perhaps another will begin to act their age - well, at least one of them.....