Alright,
I'm in the middle of getting my instructors rating and before I actually get into teaching students I thought I'd get some more opinions on something that has been nagging me for two years now. And so far, all opinions I've heard seem to be spread 50/50. Everyone here knows the advantages and the safety reasons to having/using carb heat, but the following has always bothered me. In extreme cold conditions where one would be brave enough to still climb into their 172 piston banger, and you are well below the temperature where carb ice will form. Standard practice requires us to pull the carb heat every fifteen minutes or whenever we are in low power settings to prevent carb icing. HOWEVER, pulling the carb heat would actually raise the temperature back into the icing range. So here's the question, when in extreme cold temperatures, do you or do you not pull the carb heat?
Carb Heat...To use or not to use?
Moderators: North Shore, sky's the limit, sepia, Sulako, Right Seat Captain, lilfssister
Refer to the POH/AFM:
It depends on what type of piston aircraft we are talking about here. You mentioned the C172, which of course, is stated and taught to use carb heat below certain power settings but that is not the case for all aircraft. An example would be the PA28 (Piper Warrior). When I used to fly them, we never touched the carb heat unless it was required (meaning you see indications of carb ice) no matter what the OAT and power setting. Thats how piper instructed in the POH to deal with carb heat in this aircraft.
As for extremely low temperatures in the C172, I'd say turn it on as there are probably more factors than we are aware of that must be considered (such as residual heat from the engine itself around the carburator) which of course we would assume that Lycoming & Continental have thought of and wieghed in when designing their engines.
LJD
It depends on what type of piston aircraft we are talking about here. You mentioned the C172, which of course, is stated and taught to use carb heat below certain power settings but that is not the case for all aircraft. An example would be the PA28 (Piper Warrior). When I used to fly them, we never touched the carb heat unless it was required (meaning you see indications of carb ice) no matter what the OAT and power setting. Thats how piper instructed in the POH to deal with carb heat in this aircraft.
As for extremely low temperatures in the C172, I'd say turn it on as there are probably more factors than we are aware of that must be considered (such as residual heat from the engine itself around the carburator) which of course we would assume that Lycoming & Continental have thought of and wieghed in when designing their engines.
LJD
Follow the POH. For the 172 it says to use carb heat, except if you have a carb air temp gauge. In that case it says to "keep the needle out of the yellow arc during possible carb icing conditions." As LJ said, there are many other aircraft that only call for carb heat if carb icing is indicated.
Now, if the OAT is so low that applying carb heat will only raise the carb air temp into the yellow arc, what are the chances that there's enough water vapour present in the atmosphere to create carb ice?
Now, if the OAT is so low that applying carb heat will only raise the carb air temp into the yellow arc, what are the chances that there's enough water vapour present in the atmosphere to create carb ice?
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Right Seat Captain
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The short answer that I'd give is: do what the POH says to do.
I'm not sure if you've flown a 172 with a Carb Temp gauge, but I think you'd find that with normal power (in the green arc range) you're still warm enough to be above the yellow arc, which represents the range of temperatures that ice is deemed to be possible. When you turn on your Carb Heat, if the temperature is still not very high, and this is during cruise where as you said you're checking every 15 minutes and you have relatively higher RPMs, you have a problem, or it's really too cold to be flying.
Think about it, if your Carb Heat is not hot at all, what kind of temperature is your engine at?
I'm not sure if you've flown a 172 with a Carb Temp gauge, but I think you'd find that with normal power (in the green arc range) you're still warm enough to be above the yellow arc, which represents the range of temperatures that ice is deemed to be possible. When you turn on your Carb Heat, if the temperature is still not very high, and this is during cruise where as you said you're checking every 15 minutes and you have relatively higher RPMs, you have a problem, or it's really too cold to be flying.
Think about it, if your Carb Heat is not hot at all, what kind of temperature is your engine at?
I'll put my 2 cents in. If your extreme low temperature is like mine, we are both talking about -20 and colder temps. In those cold temps flying with a piston engine may not be ideal to try in the first place, but if the plane can do it, the performance you get is unbelievable.
Another question to concider. Is there enough moisture in that cold air to even cause carb ice to form, even if the engine gives off enough heat to bring the carb temperature above zero?
Another question to concider. Is there enough moisture in that cold air to even cause carb ice to form, even if the engine gives off enough heat to bring the carb temperature above zero?
Only a few times did I ever fly in an 172 that had a carb temp gauge and it was up in Thunder Bay when I was still a student at Confed. And we were up doing some practice in Kakabeka (or however you spell it) falls. And my instructor pointed out that the temperature coming into the carb was below the freezing arc, then he had me pull the carb heat, putting it into the arc. True, at those temperatures, the chances of carb ice forming is slim, but none the less....it's always a question that gets people talking. And yeah, I remember my PPC in the PA-28, and my training in the seminol....but with those aircraft it's because of the poor cooling of the engines, prolonged use of carb heat can cause those engines to overheat. Poor Cooling + Really Effective Carb Heat = Deep shit! But I can definitely see the point about following the POH, when all else fails, refer to the POH.
The other factor to consider is that carb heat is not just for carb ice. It is also to keep the engine warm.
For instance, if you are descending at -20C, if you don't turn on carb heat to keep the engine warm, the engine will cool extremely quickly (of course, you would plan ahead and do a power on descent). If you overshoot it is very easy to crack a cylinder. So, yes, in general, continue to use the carb heat in cold temperatures unless you notice that the use of carb heat is causing carb ice to form.
For instance, if you are descending at -20C, if you don't turn on carb heat to keep the engine warm, the engine will cool extremely quickly (of course, you would plan ahead and do a power on descent). If you overshoot it is very easy to crack a cylinder. So, yes, in general, continue to use the carb heat in cold temperatures unless you notice that the use of carb heat is causing carb ice to form.
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Cat Driver
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Oh jeeses is this actually the instructors forum?
Some of the stuff I'm reading here makes me wonder who is failing to teach these subjects.
Does anyone here know where carb heat comes from and the temperature curve in relation to power settings and the time factor?
Does anyone understand the very small area that carb heat affects, and the relation to engine shock cooling?
Cat
Some of the stuff I'm reading here makes me wonder who is failing to teach these subjects.
Does anyone here know where carb heat comes from and the temperature curve in relation to power settings and the time factor?
Does anyone understand the very small area that carb heat affects, and the relation to engine shock cooling?
Cat
The hardest thing about flying is knowing when to say no
After over a half a century of flying no one ever died because of my decision not to fly.
After over a half a century of flying no one ever died because of my decision not to fly.
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Right Seat Captain
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Last time I checked, richening the mixture (ie turning on the Carb Heat) cools the engine.chewsta wrote:The other factor to consider is that carb heat is not just for carb ice. It is also to keep the engine warm.
For instance, if you are descending at -20C, if you don't turn on carb heat to keep the engine warm, the engine will cool extremely quickly (of course, you would plan ahead and do a power on descent). If you overshoot it is very easy to crack a cylinder. So, yes, in general, continue to use the carb heat in cold temperatures unless you notice that the use of carb heat is causing carb ice to form.
when operating in cold o a t, provided the use of carb heat would not cause carb iciing, i would use carb heat. in the case of an overshoot when adding go around power the fuel will mix and vaporize better with the warmer carb heat. if you check the 172 p o h it more less says, add full throttle. then carb heat cold, flaps etc.
RSC,
Was under the impression that the heat from carb heat more than offset the cooling effect of enritchening the mixture. Also, you would be cooling it gently compaired to the shock cooling that would occuring in the event of an overshoot. Let me know if you disagree.
Cat, I thought carb heat came from the same place that babies do
In all seriousness I doubt any of us has seen a chart that depicts a temperature curve in relation to power settings and the time factor. Most of us have just seen the TC carb ice probability curves (temperature vs. humidity). If you have the charts please post them (or email to me, I know you're computer challenged 
.)
Was under the impression that the heat from carb heat more than offset the cooling effect of enritchening the mixture. Also, you would be cooling it gently compaired to the shock cooling that would occuring in the event of an overshoot. Let me know if you disagree.
Cat, I thought carb heat came from the same place that babies do
In all seriousness I doubt any of us has seen a chart that depicts a temperature curve in relation to power settings and the time factor. Most of us have just seen the TC carb ice probability curves (temperature vs. humidity). If you have the charts please post them (or email to me, I know you're computer challenged .)-
Cat Driver
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Chewsta :
What I was driving at is in the bug smashers like Cessnas used in flight training the carb heat is a muff around an exhaust pipe, when you throttle back the exhaust pipe cools very rapidly and by default the temperature of the air being induced into the carb gets colder in direct relation to time and power setting.
Shock cooling is a different animal alltogether and is related to the air flow past the cylinders if you throttle back quickly the heat produced by the fuel burning is rapidly dissipated as the fuel burn amount decreases. Shock cooling results in cylinder choking as the cylinger shrinks and the piston does not. Some engines are more suseptible to cylinder choking than others.
Carb heat will not have any effect on shock cooling for the simple reason that the hot air only affects the carb throat.
Cat
What I was driving at is in the bug smashers like Cessnas used in flight training the carb heat is a muff around an exhaust pipe, when you throttle back the exhaust pipe cools very rapidly and by default the temperature of the air being induced into the carb gets colder in direct relation to time and power setting.
Shock cooling is a different animal alltogether and is related to the air flow past the cylinders if you throttle back quickly the heat produced by the fuel burning is rapidly dissipated as the fuel burn amount decreases. Shock cooling results in cylinder choking as the cylinger shrinks and the piston does not. Some engines are more suseptible to cylinder choking than others.
Carb heat will not have any effect on shock cooling for the simple reason that the hot air only affects the carb throat.
Cat
The hardest thing about flying is knowing when to say no
After over a half a century of flying no one ever died because of my decision not to fly.
After over a half a century of flying no one ever died because of my decision not to fly.
Cat:
I was always under the impression that the carb heat carried through to the cylinders. Thinking about it more, I see that the heat wouldn't get far past the throat due to the temperature drop in the venturi. As always, appreciate your wisdom.
Now, why can't we just fuel inject everything
I was always under the impression that the carb heat carried through to the cylinders. Thinking about it more, I see that the heat wouldn't get far past the throat due to the temperature drop in the venturi. As always, appreciate your wisdom.
Now, why can't we just fuel inject everything
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Cat Driver
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Chewsta :
I really enjoy giving suggestions to flight instructors, especially when they show apppreciation.
Like every young flight instructor here I also started with basic knowledge taught to me by my instructors, and like everyone else I just kept on learning more as the years passed.
There are several things that new flight instructors should do and one of the most important is to hang out with the mechanics and offer to help.
Nothing beats hands on learning.
The second thing that will really improve your understanding of flight is to go to a school that teaches aerobatics on tail wheel airplanes and learn basic aerobatics.
Cat
I really enjoy giving suggestions to flight instructors, especially when they show apppreciation.
Like every young flight instructor here I also started with basic knowledge taught to me by my instructors, and like everyone else I just kept on learning more as the years passed.
There are several things that new flight instructors should do and one of the most important is to hang out with the mechanics and offer to help.
Nothing beats hands on learning.
The second thing that will really improve your understanding of flight is to go to a school that teaches aerobatics on tail wheel airplanes and learn basic aerobatics.
Cat
The hardest thing about flying is knowing when to say no
After over a half a century of flying no one ever died because of my decision not to fly.
After over a half a century of flying no one ever died because of my decision not to fly.
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Right Seat Captain
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ahramin,
At least the way I understand it, by turning on the carb heat, as you said, it decreases the air density. If the mixture control is left unchanged, you now have an excess of fuel for the amount of air going in, and more fuel is left unspent. Unspent fuel in the cylinders has the capability of cooling the cylinder further, as it carries the heat away from the cylinders as it is exhausted. Thus it still has the effect of enrichening the fuel to air ratio, and causes cooling.
This is what I was taught, and it makes sense to me, however I may be wrong.
At least the way I understand it, by turning on the carb heat, as you said, it decreases the air density. If the mixture control is left unchanged, you now have an excess of fuel for the amount of air going in, and more fuel is left unspent. Unspent fuel in the cylinders has the capability of cooling the cylinder further, as it carries the heat away from the cylinders as it is exhausted. Thus it still has the effect of enrichening the fuel to air ratio, and causes cooling.
This is what I was taught, and it makes sense to me, however I may be wrong.
Last edited by Right Seat Captain on Sun Feb 13, 2005 4:46 am, edited 1 time in total.
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I don't think you need to go to a school. Booting around with an experienced pilot would probably do the trick. Maybe in a plane like this?The second thing that will really improve your understanding of flight is to go to a school that teaches aerobatics on tail wheel airplanes and learn basic aerobatics. -Cat Driver
-istp
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shimmydampner
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Cat Driver
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istp :
It's to late, I am selling it, in fact I am on my way to the airport to day to finish the annual as I have a possible buyer comming to look at it in a few days.
I finally gave up trying to teach in Canada because every time I try TC finds a way to stop me.
What do you think of that? I guess I just don't have the right stuff to meet the requirements of TC's flight training experts.
Cat
It's to late, I am selling it, in fact I am on my way to the airport to day to finish the annual as I have a possible buyer comming to look at it in a few days.
I finally gave up trying to teach in Canada because every time I try TC finds a way to stop me.
What do you think of that? I guess I just don't have the right stuff to meet the requirements of TC's flight training experts.
Cat
The hardest thing about flying is knowing when to say no
After over a half a century of flying no one ever died because of my decision not to fly.
After over a half a century of flying no one ever died because of my decision not to fly.
Regarding pressure carbs and fuel injection, he states, "There is no potential for carb ice on either of these types". This is not correct. Pressure carbs do ice up, albeit far less readily than float carbs. Carb heat is normally only used with a pressure carb when icing is detected, not as a routine preventive measure (ie, not automatically for every landing or when at reduced power) as with a float carb.
His caution to not use partial carb heat is correct, but his rationale is incomplete. Partial carb heat can not be used on some aircraft because of the mechanical function of the cold/hot air valve. In some types, partial carb heat will cause a restriction of cold air flow, but not open the heated air valve. The restricted air flow causes a power reduction.
Like so many issues that come up for discussion here, universal procedures can not be safely used in many circumstances. One must follow those from the POH specific to their aircraft type.
His caution to not use partial carb heat is correct, but his rationale is incomplete. Partial carb heat can not be used on some aircraft because of the mechanical function of the cold/hot air valve. In some types, partial carb heat will cause a restriction of cold air flow, but not open the heated air valve. The restricted air flow causes a power reduction.
Like so many issues that come up for discussion here, universal procedures can not be safely used in many circumstances. One must follow those from the POH specific to their aircraft type.
