Carb Heat: What's the deal?
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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?
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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 )
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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.
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.
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.
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.
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
and I selected full carb heat. After a few HARRUMPS from the engine it roared back to full power
.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.
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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"
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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.
This is completely unnecessary with a Lycoming engine
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.
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"
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.
Last edited by RTN11; 3rd May 2012 at 22:24.
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?
Presumably aircraft that continuously heat the carburettor, by whatever means, are effectively doing this?
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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.
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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?
The RAF used to lockwire the carb heat into hot on their chipmunks - not sure why actually, anyone?
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Originally Posted by custardpsc
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.
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 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.
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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
Simples :-p
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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 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!
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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.
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
Simples :-p
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
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.
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[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?
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?
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?
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.
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.
Last edited by Big Pistons Forever; 4th May 2012 at 01:44.
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.
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.