Carb Heat
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Carb Heat
I was once told by a fairly experienced Pilot / Engineer that Carb Heat should only ever be used to melt accumulated ice and never as a preventative measure to stop it's formation in the first place.
The theory being that under certain operating conditions, it is actually possible for the application of CH to riase the air temp into the range optimal for ice formation. At which point you're left with no possible way to clear it.
Have never flown anything with a Carb Heat Temp guage before - anyone out there who has and could confirm / refute this one?Sounded like bull**** to me but you never know.
PS Please no replies along the lines of "The correct answer is in you flight manual"......
The theory being that under certain operating conditions, it is actually possible for the application of CH to riase the air temp into the range optimal for ice formation. At which point you're left with no possible way to clear it.
Have never flown anything with a Carb Heat Temp guage before - anyone out there who has and could confirm / refute this one?Sounded like bull**** to me but you never know.
PS Please no replies along the lines of "The correct answer is in you flight manual"......
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Well if you think about it, the only way to prevent the formation of ice, is to leave it on all the time so in theory, if you turn it on say, after 15 mins of flight, then you may be getting rid of tiny ice particles 
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Carb Heat should only ever be used to melt accumulated ice and never as a preventative measure to stop it's formation in the first place
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I'm a new boy, but the instructors at our club really empasise how important carb heat is.
My feelin is that carb icing is the number one killer for light aircraft.
We are taught to put on the carb heat whenever the engine is running below cruise power and to always check from time to time in flight.
I suppose you could run with it on all the time but it would reduce engine power?
Also I think it should either be on or off,never in between.
Last weekend I did a touch and go and forgot to put the carb heat off before going round,but my instructor said OK so you've lost a bit of power but the engines still running!
Recently a locally owned Stearman was diving with idle engine and the engine stopped due to ice.He was lucky and managed to land on an old airstrip nearby without harm.
I wonder why a carb ice warning system isn't fitted to all light aircraft?
Lister
My feelin is that carb icing is the number one killer for light aircraft.
We are taught to put on the carb heat whenever the engine is running below cruise power and to always check from time to time in flight.
I suppose you could run with it on all the time but it would reduce engine power?
Also I think it should either be on or off,never in between.
Last weekend I did a touch and go and forgot to put the carb heat off before going round,but my instructor said OK so you've lost a bit of power but the engines still running!
Recently a locally owned Stearman was diving with idle engine and the engine stopped due to ice.He was lucky and managed to land on an old airstrip nearby without harm.
I wonder why a carb ice warning system isn't fitted to all light aircraft?
Lister
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The theory being that under certain operating conditions, it is actually possible for the application of CH to riase the air temp into the range optimal for ice formation. At which point you're left with no possible way to clear it.
This argument will go on and on, and has, on numerous threads. When all engines are either fuel injected or fitted with carb ice detectors, they can finally stop. Till then, I would suggest applying carb heat frequently, for long enough to melt ice if you have it OR if you've inadvertantly caused it to form by applying the heat in the first place. In misty, damp conditions, leave it on all/most of the time, especially if over mountains or water. Get to know your engine, and if the RPM is dropping, don't keep increasing it; ask why, as carb ice is the most likely answer. And don't listen to people who don't really know what they're talking about, how ever many hours or qualifications they've got; check to see if they're right first.
At least, all of the above is what I do.
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And if you do turn it on and the engine starts burping and farting don't turn it off again. Which is what your instinct will tell you to do.
Pick a field and stay close to it and try and maintain as much height as possible without going IMC. Cover your bum and tell ATC if you like and run your normal engine drills as if it was a PFL. Then wait. After a while it will stop burping and farting and you can continue. If you have told ATC remember to tell them that you are OK now. Then continue on your way being a bit more careful how often you use it. If it doesn't sort itself out put it in the field and wait for the police to turn up.
MJ
Pick a field and stay close to it and try and maintain as much height as possible without going IMC. Cover your bum and tell ATC if you like and run your normal engine drills as if it was a PFL. Then wait. After a while it will stop burping and farting and you can continue. If you have told ATC remember to tell them that you are OK now. Then continue on your way being a bit more careful how often you use it. If it doesn't sort itself out put it in the field and wait for the police to turn up.
MJ
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Don't Chipmunks have the carb heat permanently on?
I understand one of the reasons for not having it on all the time is the power reduction caused by the air being less dense when it's hot. Then the other reason is that the carb heat intake isn't filtered, so on or near the ground there's the chance of airborne dirt getting into the engine.
I understand one of the reasons for not having it on all the time is the power reduction caused by the air being less dense when it's hot. Then the other reason is that the carb heat intake isn't filtered, so on or near the ground there's the chance of airborne dirt getting into the engine.
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I vaguely recall reading an explanation (based on physics of water vapour/liquid/ice; the old "steam tables") that merely warming the airflow (e.g. with partial carb heat) is highly unlikely to create ice, in a temp/DP scenario where ice would not have been created in the absence of carb heat.
I don't have the reference for this however. Bookworm, where are you?
If correct, this would explain why planes which have their air intake warmed up slightly by the exhaust (various PA28s, I think) don't get carb icing.
Anyway, I think carburettors should be banned
I don't have the reference for this however. Bookworm, where are you?
If correct, this would explain why planes which have their air intake warmed up slightly by the exhaust (various PA28s, I think) don't get carb icing.
Anyway, I think carburettors should be banned
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Chipmunks (well ex military Chipmunks) do have their warm air arrangements wire locked on.
You'll note warm air - not hot air - the Gypsy draws air off the cylinders and so it is nothin glike as warm as coming from an exhaust muff. Warm air is recommended on all Gypsys in dusty conditions as the incoming air takes a 90 degree bend and so a lot of dust is removed from the incoming air when in 'warm air'.
I cannot remember the exact specification but hot air systems are supposed to be capable of at least a 30 deg C temperature increase. So to cause icing to occur when hot air is selected it is going to have to be bl**dy cold for icing to occur hot air is selected. So the next time you're flying on a crystal clear -20 Deg C day you might need to be careful with carb heat. All the rest of the time it is a theoretical blind alley.
You'll note warm air - not hot air - the Gypsy draws air off the cylinders and so it is nothin glike as warm as coming from an exhaust muff. Warm air is recommended on all Gypsys in dusty conditions as the incoming air takes a 90 degree bend and so a lot of dust is removed from the incoming air when in 'warm air'.
I cannot remember the exact specification but hot air systems are supposed to be capable of at least a 30 deg C temperature increase. So to cause icing to occur when hot air is selected it is going to have to be bl**dy cold for icing to occur hot air is selected. So the next time you're flying on a crystal clear -20 Deg C day you might need to be careful with carb heat. All the rest of the time it is a theoretical blind alley.
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IO540 - I can assure you that PA28s can get carb icing, I've had it with an OAT of 25C! I think as a general spamcan rule, Lycomings are less prone to ice than Continentals, but as a useful rule of thumb, if you've got a carb heat lever in the plane, your carb can ice up.
Lister Noble - carb icing is not the number one pilot killer, that's pilots doing stupid things - like not knowing exactly how and when the plane they are flying can generate carb ice. Also being 'fairly experienced' at anything does not give you an immunity from spouting bullsh1t. Look at Sven Goran.
Lister Noble - carb icing is not the number one pilot killer, that's pilots doing stupid things - like not knowing exactly how and when the plane they are flying can generate carb ice. Also being 'fairly experienced' at anything does not give you an immunity from spouting bullsh1t. Look at Sven Goran.
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The carb heat isnt wired on in all chipmunks (like ours).We select the carb heat on the climb-out and deselect when vacated the runway (and a check during the 1800 power check pre takeoff)
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I've heard this one before too. And likewise it was from an engineer. But I don't believe it.
Lets say you're nice and high on a cold day, and the outside air temp is -20C. You use Carb heat and this brings the air temp up 30C to +10.
Then surely there is such a large gap between the +10C and the dew point of the air, that there is no possibility of Carb Ice forming?
At -20C I can't imagine the air holding very much water. Just because you raise it's temperature to 10C very quickly, isn't going to make much difference. Where is the water going to come from?
Of course this is just my amateur physics, and but I can't see the logic.
dp
Lets say you're nice and high on a cold day, and the outside air temp is -20C. You use Carb heat and this brings the air temp up 30C to +10.
Then surely there is such a large gap between the +10C and the dew point of the air, that there is no possibility of Carb Ice forming?
At -20C I can't imagine the air holding very much water. Just because you raise it's temperature to 10C very quickly, isn't going to make much difference. Where is the water going to come from?
Of course this is just my amateur physics, and but I can't see the logic.
dp
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If you wait for ice to form on a VW or Continental engine there is a possibility that by the time you have recognised it, the engine won't be able to generate enough heat to melt the ice - then you are really stuffed.
I've had the engine stop on the ground when taxying. As a result I tend to use it in a precautionary way before it is needed, but always to leave it on for some time, not a quick blast
I've had the engine stop on the ground when taxying. As a result I tend to use it in a precautionary way before it is needed, but always to leave it on for some time, not a quick blast
Tabs please !
Manual carb heat control always amazes me. Car manufacturers solved this problem in the 1970s before almost universally ditching this technology in favour of fuel injection. I don't recall ever having a fault develop on a car where the induction heating system failed and the engine developed a problem as a result.
In some aspects, aviation is still in the dark ages and it's a wonder that we have managed to progress to electric starters.
Perhaps that's part of the charm
In some aspects, aviation is still in the dark ages and it's a wonder that we have managed to progress to electric starters.
Perhaps that's part of the charm
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shedding ice !
The ice formation in a carb is not only down to the atmospheric conditions I have noticed that the Robin DR400 aircraft are much more prone to carb ice than Pipers with the same engine/carb the only differance is the intake/filter.
When shedding ice it may take some time to shift and if you are at altitude with the mixture lean and the engine starts to loose power or run rough then persist with the carb heat and dont what ever you do put the mixture to rich This will only send the mixture over rich reducing not only the power from the engine but also reducing the heat avalable to the carb heat heat exchanger.
If you can moniter the EGT and set the mixture to the hottest setting that you can get if the carb has a lot of ice then the EGT will be well below the normal cruise setting but keeping the heat in the engine is the only way to melt the ice. If you have no EGT indication then set the mixture for max RPM (the throttle should already be fully open).
When shedding ice it may take some time to shift and if you are at altitude with the mixture lean and the engine starts to loose power or run rough then persist with the carb heat and dont what ever you do put the mixture to rich This will only send the mixture over rich reducing not only the power from the engine but also reducing the heat avalable to the carb heat heat exchanger.
If you can moniter the EGT and set the mixture to the hottest setting that you can get if the carb has a lot of ice then the EGT will be well below the normal cruise setting but keeping the heat in the engine is the only way to melt the ice. If you have no EGT indication then set the mixture for max RPM (the throttle should already be fully open).
Then surely there is such a large gap between the +10C and the dew point of the air, that there is no possibility of Carb Ice forming?
At -20C I can't imagine the air holding very much water. Just because you raise it's temperature to 10C very quickly, isn't going to make much difference. Where is the water going to come from?
At -20C I can't imagine the air holding very much water. Just because you raise it's temperature to 10C very quickly, isn't going to make much difference. Where is the water going to come from?
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Don't forget that the temperature in the carb venturi will be significantly lower than the OAT; this is due to a combination of adiabatic cooling as the pressure drops in the venturi and to the latent heat taken from the air and the carburettor body by the evaporation of the fuel. This is why you can get icing in the carb with the OAT above zero.
So, in dublinpilot's scenario with an OAT of -20 and a further drop in the carb temperature of 10deg due to these two effects, to -30 deg, any water will be ice crystals which will go straight through the carb. Now apply heat and raise the temperature 30 degrees to...... around zero. Bingo, ice forming in the carburettor because you applied heat!
3 Point
So, in dublinpilot's scenario with an OAT of -20 and a further drop in the carb temperature of 10deg due to these two effects, to -30 deg, any water will be ice crystals which will go straight through the carb. Now apply heat and raise the temperature 30 degrees to...... around zero. Bingo, ice forming in the carburettor because you applied heat!
3 Point