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Carb Heat in IMC
- Thread starter NYCPilot
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That's the outside air temperature range in celcius for structural icing, not for carburetor icing. Carburetor icing has a much, much wider outside air temp range for icing potential, and the internal temps vary with the carburetor. The reason for this is that the pressure drops and subsequently temperature differentials vary from carburetor to carburetor, power setting, throttle setting, etc.
In order to determine carburetor air temperature, you must have a calibrated carb air temp gauge. Without it, you are performing guesswork. Some systems exist which illuminate a carb ice light, a form of idiot light, which tells you that ice is forming. This may be better than nothing, but still leaves you in the position of guessing at the carb air temperatures.
Carburetors are updraft and down draft for almost all aircraft installations. Placed beneat the engine where it's cooler, carburetors ice. Placed above the engien where it's hotter, carburetors ice. Placed above and behind the engine where it's hottest, carburetors ice. Carburetors ice. It's inherent to the nature of the functioning of the carburetor.
Carburetors ice when the engine is at full power. This is when the temperature drop is the greatest. (In or just down stream of the venturi) Carburetors ice at idle. This is when the pressure differential is the greatest across the throttle plate, where idle carb ice forms. (Adjacent to the idle jet)
Sounds good on paper, wonderful over flat terrain...a very, very bad idea in mountainous terrain...which makes up most of the country. Most of the time you'll never been in any position to glide to any airport. Knowing the terrain beneath you if you can't see it is guesswork. Flying single engine IMC over terrain isn't okay because you think you know it or might be able to glide to an airport. It's a massive gamble, and it's not just about losing a powerplant. It's about losing your one source of electrical (the battery doesn't count as a second), or your instrument power (vacum), etc.
That was the plan?? Is that anything like skydiving and pulling a ripcord to see if anybody bothered to pack the parachute? Or drinking things in a pharmacy to see which one is good or bad for you?
Your mixture was probably incorrectly set to begin with. When you applied carburetor heat, you enrichened the mixture, and you saw an increase in power. You were probably set too lean. It could have been you, or it could have been a maintenance function. If you didn't see structural ice on the airplane and on the windscreen, on the OAT probe or other small protrusions around the aircraft, then you weren't getting external induction icing, either.
My reply here was too long...thus endeth part one...
And thus beginneth part two...
In a fixed pitch airplane, that depends on your airspeed. In a variable pitch propeller installation, you may see no drop in RPM at all. Without a manifold pressure gauge, you may see no change at all until engine roughness occurs, depending on the speed at which the ice builds up. It can build up rapidly, so you should be constantly listening and looking for it.
You will.
The instructors are wrong. Inexperiened enough to not know what they're saying, or that it's a dangerous thing to teach someone. They're wrong.
A--Keep the carb heat on with a leaned out mixture. Sometimes this is warranted...but you can't tell unless you have a carburetor air temperature gauge. Have you just moved the carb air temperature out of, or into the ideal icing range? Think about that.
If you're contantly applying carburetor heat because you keep getting ice, then leaving it on is probably a good idea. It's also a VERY STRONG indication that you need to get out of those icing conditions, pronto.
Yes, if you leave the carb heat on, the engine should be leaned, accordingly.
B--Look for icing indications such as RPM or MP drops then apply carb heat until cleared. Returning the switch to the off positon.
Look for indications, but your best indication may be a change in sound. If you don't have a manifold pressure gauge, you may not see much change. Try this. In normal visual flight, tweak your throttle back in slight increments, and see if you note much change in the RPM. You may not. Likewise, when you're getting ice, it isn't necessarily precluding airflow substantially yet. Ice tends to "breed" ice. A little can begat a lot. Once it starts to form, it forms faster or more. But until enough has formed, you may not see the indications of carb ice, and sometimes you'll hear them first.
If you have an aircraft equipped with a manifold pressure gauge, you may see a manifold pressure drop, or you may not. RPM tends to stay fairly constant in a constant speed engine installation (and in a fixed pitch installation, your prop RPM isn't just a function of power or throttle setting, but also of airspeed...you won't see much RPM decay until airspeed decays, and that probably won't happen much until you've lost substantial power...by which time you'll definitely know you have carburetor icing, or at least, a problem).
At the first sign of carburetor icing, apply carburetor heat. Don't wait until you have a positive RPM loss of manifold pressure loss. Carburetor ice isn't a deice system, it's an anti-ice system. It can remove small quantities of ice under the right circumstances, but don't count on it. It's there to prevent the formation of ice...it's proper use is in conjunction with a carb air temperature gauge, to put the carb air temperature in the right position to prevent the formation of ice.
Again, carburetor heat is NOT a de-ice system. It's an anti-ice system, and works best in the early stages of carb ice detection. You can always shut it off again. With that said, remember my earlier comments. Don't put yourself in a position where you must shut it off...remember the carb heat control that failed in my hand. That one involved both a carb air box failure and a failure at the attach point for the carb air control cable. Leave yourself an out. My mistake, one of several on that flight, was that my out was a power off descent out of the clouds in mountainous terrain. Don't do that or put yourself in that position. I was fortunate. Will you be?
Don't apply carb heat with the idea that you can just shut it off again. You may find that it's required continuously. Are you prepared to deal with the power loss...the same as just having moved a couple thousand feet in density altitude? If you're not, then you've put yourself in a bad position. Throughout the flight, if you're encountering carburetor icing, then you need to consider getting to a place where you're not getting carburetor icing. Getting out of the conditions that are causing the icing.
Remember, carburetor heat is a limited anti ice tool...it's there to help you while you get out of the conditions that are causing the carburetor ice. Don't stay in them, any more than you should with any minimal anti-ice capability. It's your powerplant you're talking about here. It's warning you. Listen.
C--Perioidcally check for ice digestion by applying the carb heat at intervals.
Also a valid technique. Be careful doing this at high power settings in high compression engines, especially when already leaned out. You can cause detonation by overheating the induction air for the power setting and mixture setting in use. This can destroy an engine in seconds. Use carb heat prudently. Using it to check for ice is appropriate, but remember to check not just for a second or two, but for fifteen seconds or so.
A good rule of thumb is that if you're going to apply carb heat, on the ground or in the air, don't do it for one or two or five seconds. Fifteen seconds. Count them off. Otherwise, you're not giving the heat time to melt anything that's there, and you may be causing ice to form, not removing it...and then you have no way of knowing.
A lot of people will apply carb heat during the runup for only a second or two, long enough to hear a change in the engine, and see a slight RPM drop. This is poor airmanship. The carburetor heat should be left on for fifteen seconds, time enough to see that there isn't ice that's already formed during the taxi, time enough to ensure that none has formed right before takeoff. Usually the carb ice check is the very last thing I do before taxiing onto the runway for takeoff, or if there's time, I'll do it on the runway as I'm holding with power, before applying full power. I've had engines quit, especially on a cool morning right after a rain, right on the takeoff roll as ice developed. Or on a go-around, after carburetor heat has been shut off and power applied.
Be ready for carburetor ice in the summer, winter, and all times. Any time humidity is up or visibile moisture is present, you stand a good possibility for carburetor ice. Apply it long enough to ensure the ice is gone, apply it often enough to ensure it isn't building up unawares, and apply it whenever you need it. If you do apply it and need it, get out of the conditions that are causing it, lest they exceed your ability to handle them (and they can, very easily). Always have a plan that's better than accepting whatever comes out of the cloud at you, and guesswork. And consider getting a carb air temperature gauge if it's your aircraft No aircraft with a carburetor should be without one.
In a fixed pitch airplane, that depends on your airspeed. In a variable pitch propeller installation, you may see no drop in RPM at all. Without a manifold pressure gauge, you may see no change at all until engine roughness occurs, depending on the speed at which the ice builds up. It can build up rapidly, so you should be constantly listening and looking for it.
You will.
The instructors are wrong. Inexperiened enough to not know what they're saying, or that it's a dangerous thing to teach someone. They're wrong.
A--Keep the carb heat on with a leaned out mixture. Sometimes this is warranted...but you can't tell unless you have a carburetor air temperature gauge. Have you just moved the carb air temperature out of, or into the ideal icing range? Think about that.
If you're contantly applying carburetor heat because you keep getting ice, then leaving it on is probably a good idea. It's also a VERY STRONG indication that you need to get out of those icing conditions, pronto.
Yes, if you leave the carb heat on, the engine should be leaned, accordingly.
B--Look for icing indications such as RPM or MP drops then apply carb heat until cleared. Returning the switch to the off positon.
Look for indications, but your best indication may be a change in sound. If you don't have a manifold pressure gauge, you may not see much change. Try this. In normal visual flight, tweak your throttle back in slight increments, and see if you note much change in the RPM. You may not. Likewise, when you're getting ice, it isn't necessarily precluding airflow substantially yet. Ice tends to "breed" ice. A little can begat a lot. Once it starts to form, it forms faster or more. But until enough has formed, you may not see the indications of carb ice, and sometimes you'll hear them first.
If you have an aircraft equipped with a manifold pressure gauge, you may see a manifold pressure drop, or you may not. RPM tends to stay fairly constant in a constant speed engine installation (and in a fixed pitch installation, your prop RPM isn't just a function of power or throttle setting, but also of airspeed...you won't see much RPM decay until airspeed decays, and that probably won't happen much until you've lost substantial power...by which time you'll definitely know you have carburetor icing, or at least, a problem).
At the first sign of carburetor icing, apply carburetor heat. Don't wait until you have a positive RPM loss of manifold pressure loss. Carburetor ice isn't a deice system, it's an anti-ice system. It can remove small quantities of ice under the right circumstances, but don't count on it. It's there to prevent the formation of ice...it's proper use is in conjunction with a carb air temperature gauge, to put the carb air temperature in the right position to prevent the formation of ice.
Again, carburetor heat is NOT a de-ice system. It's an anti-ice system, and works best in the early stages of carb ice detection. You can always shut it off again. With that said, remember my earlier comments. Don't put yourself in a position where you must shut it off...remember the carb heat control that failed in my hand. That one involved both a carb air box failure and a failure at the attach point for the carb air control cable. Leave yourself an out. My mistake, one of several on that flight, was that my out was a power off descent out of the clouds in mountainous terrain. Don't do that or put yourself in that position. I was fortunate. Will you be?
Don't apply carb heat with the idea that you can just shut it off again. You may find that it's required continuously. Are you prepared to deal with the power loss...the same as just having moved a couple thousand feet in density altitude? If you're not, then you've put yourself in a bad position. Throughout the flight, if you're encountering carburetor icing, then you need to consider getting to a place where you're not getting carburetor icing. Getting out of the conditions that are causing the icing.
Remember, carburetor heat is a limited anti ice tool...it's there to help you while you get out of the conditions that are causing the carburetor ice. Don't stay in them, any more than you should with any minimal anti-ice capability. It's your powerplant you're talking about here. It's warning you. Listen.
C--Perioidcally check for ice digestion by applying the carb heat at intervals.
Also a valid technique. Be careful doing this at high power settings in high compression engines, especially when already leaned out. You can cause detonation by overheating the induction air for the power setting and mixture setting in use. This can destroy an engine in seconds. Use carb heat prudently. Using it to check for ice is appropriate, but remember to check not just for a second or two, but for fifteen seconds or so.
A good rule of thumb is that if you're going to apply carb heat, on the ground or in the air, don't do it for one or two or five seconds. Fifteen seconds. Count them off. Otherwise, you're not giving the heat time to melt anything that's there, and you may be causing ice to form, not removing it...and then you have no way of knowing.
A lot of people will apply carb heat during the runup for only a second or two, long enough to hear a change in the engine, and see a slight RPM drop. This is poor airmanship. The carburetor heat should be left on for fifteen seconds, time enough to see that there isn't ice that's already formed during the taxi, time enough to ensure that none has formed right before takeoff. Usually the carb ice check is the very last thing I do before taxiing onto the runway for takeoff, or if there's time, I'll do it on the runway as I'm holding with power, before applying full power. I've had engines quit, especially on a cool morning right after a rain, right on the takeoff roll as ice developed. Or on a go-around, after carburetor heat has been shut off and power applied.
Be ready for carburetor ice in the summer, winter, and all times. Any time humidity is up or visibile moisture is present, you stand a good possibility for carburetor ice. Apply it long enough to ensure the ice is gone, apply it often enough to ensure it isn't building up unawares, and apply it whenever you need it. If you do apply it and need it, get out of the conditions that are causing it, lest they exceed your ability to handle them (and they can, very easily). Always have a plan that's better than accepting whatever comes out of the cloud at you, and guesswork. And consider getting a carb air temperature gauge if it's your aircraft No aircraft with a carburetor should be without one.
NYCPilot
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Thanks avbug.
Very informative post.
As for the temp. range, I mixed that up. I meant to say somewhere around 20 - 70 degrees and as high as 90 with high humidty. I realize the +5/-15 is for structural icing.
One other topic about carb heat during a runup. I once read that you should expect at least a 200 RPM drop when applied. I don't think I've ever seen more than say 100, if that much.
Very informative post.
As for the temp. range, I mixed that up. I meant to say somewhere around 20 - 70 degrees and as high as 90 with high humidty. I realize the +5/-15 is for structural icing.
One other topic about carb heat during a runup. I once read that you should expect at least a 200 RPM drop when applied. I don't think I've ever seen more than say 100, if that much.
Thanks for clarifying that. The next obvious question is how fast you exited icing conditions after noting the structural icing. It's time to stop holding at to get out of ice...especially if you're not equipped for known ice.
As far as the RPM drop to be seen on the runup...you should be seeing the book value that the manufacturer specifies. The actual amount of loss is going to be somewhat dependent on the outside air temparture and ambient conditions, but you should still see close to the book loss. If you're not getting that loss, you may have a problem in the carb air box. Common problems include improperly adjusted linkage, and damage to the carb air butterfly, or it's seal material. This can be serious; if the seal material is damaged it can come off and enter your induction and cause an engine failure or blockage in the carburetor. Carb air boxes crack...they're notorious for it. Once the box begins to flex, the carb heat control doesn't work the same.
In many installations, your induction air filter attaches through the cowling, and also attaches to the carb air box. The motion of the cowling against the motion of the carb air box can cause cracking. It also can mean your air filter isn't secure, and can lead to cracking in the cowl, too. Your carb air box is bolted to the carburetor, in most cases.
If you're performing engine runs and not getting the manufacturer-specified results, then maintenance should be performed to find out why.
As far as the RPM drop to be seen on the runup...you should be seeing the book value that the manufacturer specifies. The actual amount of loss is going to be somewhat dependent on the outside air temparture and ambient conditions, but you should still see close to the book loss. If you're not getting that loss, you may have a problem in the carb air box. Common problems include improperly adjusted linkage, and damage to the carb air butterfly, or it's seal material. This can be serious; if the seal material is damaged it can come off and enter your induction and cause an engine failure or blockage in the carburetor. Carb air boxes crack...they're notorious for it. Once the box begins to flex, the carb heat control doesn't work the same.
In many installations, your induction air filter attaches through the cowling, and also attaches to the carb air box. The motion of the cowling against the motion of the carb air box can cause cracking. It also can mean your air filter isn't secure, and can lead to cracking in the cowl, too. Your carb air box is bolted to the carburetor, in most cases.
If you're performing engine runs and not getting the manufacturer-specified results, then maintenance should be performed to find out why.
avbug said:
No problem. Yeah it was immediate. The examiner and I agreed without speaking that the hold was satisfactory and we were inbound on the approach.
Funny how stuff works like that isn't it?
I don't mess with ice. My first experience (then) freaked me out. If I'm gonna fly in it, I want an aircraft capable of handling it until I can get out of it.
...anywho...(structural) ice discussion over.
-mini
gkrangers said:Man..all you guys with your fancy ice experience...I'm instrument rated and don't even have any real legitimate actual.
While I love my actual time (reeeeeeally love actual for some sick reason), I do not like actual when it's even close to icing conditions...I can't tell you how many trips I cancelled because the freezing level was at 4000' (MEA 3000'), etc...
Now if you're in something like a Malibu...well...have at it!
-mini
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