Use of carb heat
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... Regarding application of Carb Heat - I seem to remember that how and when this was applied would often depend on who was the
instructor of the day. Later I was very fortunate to gradujte to
a Rheims 172J (fuel injection) that had the advantage of no longer having to continually bear in mind carb. heat. Thereafter whenever looking to changing an a/c I would never consider going back to any non f/inj. a/c. - along the lines that once many car drivers have driven with automatic gears then most would probably be reluctant going back to manual shift.
Apologies if this adds little to the debate
...
instructor of the day. Later I was very fortunate to gradujte to
a Rheims 172J (fuel injection) that had the advantage of no longer having to continually bear in mind carb. heat. Thereafter whenever looking to changing an a/c I would never consider going back to any non f/inj. a/c. - along the lines that once many car drivers have driven with automatic gears then most would probably be reluctant going back to manual shift.
Apologies if this adds little to the debate
...
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I agree with the likes of Genghis that we should make people more aware of the aircraft operation in question rather than teach a one size fits all. I do not buy the argument that states the manual for a PA28 says not to use carb heat - well it does, but read carefully as I think someone already here has elluded.
I do not have a copy infront of me BUT, when this was argued before on here and at my school I read several manauls again to check my understanding. It says something along the lines of carb heat is only used on approach when icing conditions exist. Well, as we have already discussed earlier, 99.9% of the time in the UK icing conditions do exist according to CAA safety sense leaflets and AIC's etc . . . oh now there is a point - how many students have heard of those or even read them . . !?!?!?!?! Let alone the aircraft manuals.
I do not have a copy infront of me BUT, when this was argued before on here and at my school I read several manauls again to check my understanding. It says something along the lines of carb heat is only used on approach when icing conditions exist. Well, as we have already discussed earlier, 99.9% of the time in the UK icing conditions do exist according to CAA safety sense leaflets and AIC's etc . . . oh now there is a point - how many students have heard of those or even read them . . !?!?!?!?! Let alone the aircraft manuals.
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Carburetor ice is a function of physics, not checklists.
You're faced with two issues when confronted with a checklist calling for carburetor ice. One is the utter lack of defense you'll have if you incur a power loss due to carburetor icing, if you didn't use the carburetor heat. Explain that to your insurance carrier when they seek litigation for damages to to aircraft, passengers, and property on the ground. The other issue is more personal to you; the actual operation of the airplane, your safety, and of course, your legal requirement as pilot in command to ensure the safe outcome of the flight.
First of all, why touch the control if you don't know how to use it, or what it's for? It's there to heat induction air. Why, and by how much?
Without a carburetor air temperature gauge, you have no idea how much heat you're putting into the induction air, let alone the temperature value of air passing through the carburetor venturi. That air must be within a certain range to prevent icing. Too high or too low, and you enter ranges that are either conducive to icing, or that are detrimental to engine operation (particularly at high power settings). Further, by unnecessarily using carburetor heat, you limit available engine power, and substantially alter your fuel-air mixture...which can lead to plug fouling and power loss in some cases. So...before you touch that control, do you have any clue what you're doing, or are you parroting what a low-time instructor told you to do, because that's what he or she was told to do? Worse, are you doing it because you read about it on the internet? Bad form.
What types of carburetor icing are there, and how do they differ? Did anybody ever discuss this with you before advising you when and where to apply carburetor heat?
Carburetor ice, or more accurately induction ice, tends to form in three ways. Two of them occur in the carburetor, and one of them doesn't (and can't be affected by carburetor heat). Induction ice at the air intake (typically around the air filter) occurs when flying in freezing conditions and visible moisture, and can't be removed by carburetor heat. It can only be prevented by not entering conditions that cause such icing.
Carburetor icing, inside the carburetor, however, is more insidious, and occurs on warm summer days well outside of visible moisture. It occurs in two ways; at or close to idle, and with an open throttle. Too often students are mislead to believe that carburetor icing is a product of operating at low power settings, when this is patently untrue.
Your carburetor has two fuel jets, or inlets, which provide fuel to air flowing through the carburetor, and give your engine life. One is an idle jet, which serves the carburetor only when the throttle plate is closed. The engine, acting as a heavy vacuum cleaner or suction machine, draws air through the air filter and carburetor. When the throttle plate is closed, this is the same as placing one's hand over the hose on a vacuum cleaner; the pressure in the hose drops, and the machine (engine) sucks away at your hand...or in this case, the closed throttle plate.
Some air gets past the throttle plate, and this happens at a point where a controlled leak is allowed...this is precisely where the idle jet is placed, and also where carb icing occurs at idle. Block that small controlled leak at idle, and one cuts off the airflow to the engine. Cut off the airflow and one cuts off the pressure drop in that same location that draws fuel into the airstream, and subsequently the engine quits from lack of airflow and lack of fuel. Opening the throttle makes this problem go away; the carburetor isn't choked off from icing...just the edge by the idle jet. Carburetor heat prevents this icing in the first place.
During a prolonged descent with idle power, the engine cools, and carburetor heat becomes less effective. Carburetor heat in most light piston powered airplanes is little more than a small door in a box ahead of the carburetor which is moved to allow air from inside the cowling to be drawn into the carburetor. This air is warmer than ambient air outside the cowling. How warm this air is largely depends on things such as the position of cowl flaps (where installed), the airspeed and airflow of the nacelle (how fast it's receiving cooling air, and how much), and the power setting of the engine. An engine operating at a high power setting is hotter than an engine at idle, and makes for hotter air inside the cowl. Closed cowl flaps make for hotter air than open ones, and so forth.
One must realize that in a small piston engine installation without a carburetor air temperature gauge, one is taking a shotgun approach to removing or preventing icing. One has no idea where in the carb temp range one is placing the carb air temp, when applying carburetor heat. It's what one might call a swag, or scientific wild-assed guess...hardly appropriate in the artistic science we call flying, but often all one has when attempting to control carburetor icing.
Most airplanes don't come with an ice detector, either, so one has no way of knowing if one has ice, or has removed ice, unless one understands the formation of icing, it's removal, and how to tell the difference by power response, vibration, and sound.
The other kind of carburetor icing occurs with an open throttle, which may be partial, or wide-open throttle (WOT); it can occur at high power settings in cruise or on takeoff...or on the go-around.
Both kinds of carburetor ice form as a result of a drop in pressure and temperature at the carburetor venturi, or by the calibrated leak at the idle jet with a closed throttle. Pilots are often mistakenly taught that carburetor icing only occurs at low power settings, when in fact a high power setting produces the greatest airflow and temperature drop, and can produce the fastest power loss. Think about it. This occurs at wide open throttle or partial throttle...such as a takeoff or go-around. Given some of the comments about the lack of training that's provided, I have to wonder how many have considered the possibility or likelihood of carb icing during those critical phases of flight.
Don't be lulled into thinking that the only times to apply carburetor heat are when one is approaching to land. Carburetor heat, like the mixture control, is there to be used. One does not need to rope one's self into a constraining and artificial mental limit of thinking that one cannot use it liberally as required. One can, and must. I've seen pilots land shaken and unsure about the engine failure they just experienced...when in fact they just lost power due to carburetor icing...and had absolutely no idea. I've seen them look astounded when a short time later we were able to start their airplane and run it normally...they were absolutely convinced that carburetor icing couldn't have happened to them...after all, they were at a high power setting on a hot day. Carburetor ice? Naw...can't be.
It is.
When one has been using carburetor heat during a prolonged descent and approach, and one pushes it in at the moment of go-around, one may suddenly develop carb ice, and get a rough engine. Or one may already have carburetor ice, shed some of it off the throttle plate as the throttle is advanced, and get a rough engine. Or one may see no response at all. Knowing which is which, and what to expect, may make the difference between abandoning the go-around and landing on the remaining runway, the re-application of carburetor heat, or even an unnecessary execution of a forced landing off-field.
If one has been using carburetor heat during the approach, especially without a mixture change, one may have significantly enriched the mixture. With 100LL fuel, which contrary to the name does contain a high concentration of tetraethyl lead, spark plug fouling is a distinct possibility. If one has flown a prolonged descent with carburetor heat and no leaning, one may push the power up and find a rough engine that can't be cleared with additional carburetor heat...because the spark plugs are fouled. I've seen student after student, and instructor after instructor, bring me an airplane repeatedly throughout a single day, with reports of a rough engine. Upon pulling lower spark plugs, I have found completely shorted spark plugs, glazed over with lead and carbon deposits, which must be chipped free using a pick, and the spark plug thoroughly cleaned before being returned to service. The problem wasn't the magneto gap, it wasn't the spark plug, and it wasn't a timing issue...it was a pilot issue and a failure to properly lean, and too much unnecessary use of carburetor heat.
Rather than teach rote use of the carb heat control, one should be teaching proper use, as well as the principles of it's application, and the principles of carburetor function. To fail to do so borders on criminal behavior, and is highly inappropriate.
You're faced with two issues when confronted with a checklist calling for carburetor ice. One is the utter lack of defense you'll have if you incur a power loss due to carburetor icing, if you didn't use the carburetor heat. Explain that to your insurance carrier when they seek litigation for damages to to aircraft, passengers, and property on the ground. The other issue is more personal to you; the actual operation of the airplane, your safety, and of course, your legal requirement as pilot in command to ensure the safe outcome of the flight.
First of all, why touch the control if you don't know how to use it, or what it's for? It's there to heat induction air. Why, and by how much?
Without a carburetor air temperature gauge, you have no idea how much heat you're putting into the induction air, let alone the temperature value of air passing through the carburetor venturi. That air must be within a certain range to prevent icing. Too high or too low, and you enter ranges that are either conducive to icing, or that are detrimental to engine operation (particularly at high power settings). Further, by unnecessarily using carburetor heat, you limit available engine power, and substantially alter your fuel-air mixture...which can lead to plug fouling and power loss in some cases. So...before you touch that control, do you have any clue what you're doing, or are you parroting what a low-time instructor told you to do, because that's what he or she was told to do? Worse, are you doing it because you read about it on the internet? Bad form.
What types of carburetor icing are there, and how do they differ? Did anybody ever discuss this with you before advising you when and where to apply carburetor heat?
Carburetor ice, or more accurately induction ice, tends to form in three ways. Two of them occur in the carburetor, and one of them doesn't (and can't be affected by carburetor heat). Induction ice at the air intake (typically around the air filter) occurs when flying in freezing conditions and visible moisture, and can't be removed by carburetor heat. It can only be prevented by not entering conditions that cause such icing.
Carburetor icing, inside the carburetor, however, is more insidious, and occurs on warm summer days well outside of visible moisture. It occurs in two ways; at or close to idle, and with an open throttle. Too often students are mislead to believe that carburetor icing is a product of operating at low power settings, when this is patently untrue.
Your carburetor has two fuel jets, or inlets, which provide fuel to air flowing through the carburetor, and give your engine life. One is an idle jet, which serves the carburetor only when the throttle plate is closed. The engine, acting as a heavy vacuum cleaner or suction machine, draws air through the air filter and carburetor. When the throttle plate is closed, this is the same as placing one's hand over the hose on a vacuum cleaner; the pressure in the hose drops, and the machine (engine) sucks away at your hand...or in this case, the closed throttle plate.
Some air gets past the throttle plate, and this happens at a point where a controlled leak is allowed...this is precisely where the idle jet is placed, and also where carb icing occurs at idle. Block that small controlled leak at idle, and one cuts off the airflow to the engine. Cut off the airflow and one cuts off the pressure drop in that same location that draws fuel into the airstream, and subsequently the engine quits from lack of airflow and lack of fuel. Opening the throttle makes this problem go away; the carburetor isn't choked off from icing...just the edge by the idle jet. Carburetor heat prevents this icing in the first place.
During a prolonged descent with idle power, the engine cools, and carburetor heat becomes less effective. Carburetor heat in most light piston powered airplanes is little more than a small door in a box ahead of the carburetor which is moved to allow air from inside the cowling to be drawn into the carburetor. This air is warmer than ambient air outside the cowling. How warm this air is largely depends on things such as the position of cowl flaps (where installed), the airspeed and airflow of the nacelle (how fast it's receiving cooling air, and how much), and the power setting of the engine. An engine operating at a high power setting is hotter than an engine at idle, and makes for hotter air inside the cowl. Closed cowl flaps make for hotter air than open ones, and so forth.
One must realize that in a small piston engine installation without a carburetor air temperature gauge, one is taking a shotgun approach to removing or preventing icing. One has no idea where in the carb temp range one is placing the carb air temp, when applying carburetor heat. It's what one might call a swag, or scientific wild-assed guess...hardly appropriate in the artistic science we call flying, but often all one has when attempting to control carburetor icing.
Most airplanes don't come with an ice detector, either, so one has no way of knowing if one has ice, or has removed ice, unless one understands the formation of icing, it's removal, and how to tell the difference by power response, vibration, and sound.
The other kind of carburetor icing occurs with an open throttle, which may be partial, or wide-open throttle (WOT); it can occur at high power settings in cruise or on takeoff...or on the go-around.
Both kinds of carburetor ice form as a result of a drop in pressure and temperature at the carburetor venturi, or by the calibrated leak at the idle jet with a closed throttle. Pilots are often mistakenly taught that carburetor icing only occurs at low power settings, when in fact a high power setting produces the greatest airflow and temperature drop, and can produce the fastest power loss. Think about it. This occurs at wide open throttle or partial throttle...such as a takeoff or go-around. Given some of the comments about the lack of training that's provided, I have to wonder how many have considered the possibility or likelihood of carb icing during those critical phases of flight.
Don't be lulled into thinking that the only times to apply carburetor heat are when one is approaching to land. Carburetor heat, like the mixture control, is there to be used. One does not need to rope one's self into a constraining and artificial mental limit of thinking that one cannot use it liberally as required. One can, and must. I've seen pilots land shaken and unsure about the engine failure they just experienced...when in fact they just lost power due to carburetor icing...and had absolutely no idea. I've seen them look astounded when a short time later we were able to start their airplane and run it normally...they were absolutely convinced that carburetor icing couldn't have happened to them...after all, they were at a high power setting on a hot day. Carburetor ice? Naw...can't be.
It is.
When one has been using carburetor heat during a prolonged descent and approach, and one pushes it in at the moment of go-around, one may suddenly develop carb ice, and get a rough engine. Or one may already have carburetor ice, shed some of it off the throttle plate as the throttle is advanced, and get a rough engine. Or one may see no response at all. Knowing which is which, and what to expect, may make the difference between abandoning the go-around and landing on the remaining runway, the re-application of carburetor heat, or even an unnecessary execution of a forced landing off-field.
If one has been using carburetor heat during the approach, especially without a mixture change, one may have significantly enriched the mixture. With 100LL fuel, which contrary to the name does contain a high concentration of tetraethyl lead, spark plug fouling is a distinct possibility. If one has flown a prolonged descent with carburetor heat and no leaning, one may push the power up and find a rough engine that can't be cleared with additional carburetor heat...because the spark plugs are fouled. I've seen student after student, and instructor after instructor, bring me an airplane repeatedly throughout a single day, with reports of a rough engine. Upon pulling lower spark plugs, I have found completely shorted spark plugs, glazed over with lead and carbon deposits, which must be chipped free using a pick, and the spark plug thoroughly cleaned before being returned to service. The problem wasn't the magneto gap, it wasn't the spark plug, and it wasn't a timing issue...it was a pilot issue and a failure to properly lean, and too much unnecessary use of carburetor heat.
Rather than teach rote use of the carb heat control, one should be teaching proper use, as well as the principles of it's application, and the principles of carburetor function. To fail to do so borders on criminal behavior, and is highly inappropriate.
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Excellent post, thank you.
A DC3 in Aus many years ago had carb icing in cruise on a nasty night. The crew applied full carb heat, disregarding the Carb Temp indicator. They continued on, with the engines both detonating badly due to the intake temperature (well over 40 degrees C) until both engines failed and they crashed, killing all on board.
Many years later, also in a DC3 I was flying over the Simpson desert on a search with an OAT of 45C. Both engines backfiring every 30 minutes or so. What I needed there was carburetor cooling!
A DC3 in Aus many years ago had carb icing in cruise on a nasty night. The crew applied full carb heat, disregarding the Carb Temp indicator. They continued on, with the engines both detonating badly due to the intake temperature (well over 40 degrees C) until both engines failed and they crashed, killing all on board.
Many years later, also in a DC3 I was flying over the Simpson desert on a search with an OAT of 45C. Both engines backfiring every 30 minutes or so. What I needed there was carburetor cooling!
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That really clears up a lot.
If only there were more engineers with the experience, knowledge, and time to help pilots better understand the engines that keep us aloft.
I have been guilty of teaching on the same fly by numbers routine that I was once taught, until I myself learned better. Befriending a few engineers myself has helped me right a few of my own shortcomings.
We're due to be getting a Cessna to go alongside our PA28s, so I just hope the other instructors welcome teaching different use of carb heat for different engines, or I will have a battle.
If only there were more engineers with the experience, knowledge, and time to help pilots better understand the engines that keep us aloft.
I have been guilty of teaching on the same fly by numbers routine that I was once taught, until I myself learned better. Befriending a few engineers myself has helped me right a few of my own shortcomings.
We're due to be getting a Cessna to go alongside our PA28s, so I just hope the other instructors welcome teaching different use of carb heat for different engines, or I will have a battle.
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As a fairly new student I have been following this thread with interest. My usual instructor only uses heat in known icing conditions. If needed it goes on before turning crosswind and off at about 300ft. This is a C172 in Australia.
The other instructors have different versions.
The manual says to put it on on approach and take off before applying full power.
The other instructors have different versions.
The manual says to put it on on approach and take off before applying full power.
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Off at 300'
I have flown exclusively in PA28s since my training days and was always taught to use full carb heat on approach and then switch it off at 300'. However, upon reading this thread I may try to leave it on as late as possible to avoid any chance of carb ice build up during the round out and landing run. Having read the entire thread I have to say the debate regarding use of carb heat seems endless!
Grouse
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In the UK "known icing conditions" are most days of the year according to the graphs we have in the PPL ground school books (regardless of aircraft type), and therefore we are taught to do a FREDA check every 15 minutes, which includes turning the carb heat on for 15-30 seconds. Apart from this we are normally taught to always use carb heat in descent (normally 2000rpm or lower).
What's described above is probably the correct use of carb heat certain days of the year and/or in certain aircraft types, but for "safety" reasons we follow the procedures at every flight, or do we?
I'm going flying tomorrow, and yes, according to the graph it will be SERIOUS ICING - Any Power
What's described above is probably the correct use of carb heat certain days of the year and/or in certain aircraft types, but for "safety" reasons we follow the procedures at every flight, or do we?
I'm going flying tomorrow, and yes, according to the graph it will be SERIOUS ICING - Any Power
Last edited by Intercepted; 23rd Oct 2010 at 14:25.
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If one has been using carburetor heat during the approach, especially without a mixture change, one may have significantly enriched the mixture. With 100LL fuel, which contrary to the name does contain a high concentration of tetraethyl lead, spark plug fouling is a distinct possibility. If one has flown a prolonged descent with carburetor heat and no leaning, one may push the power up and find a rough engine that can't be cleared with additional carburetor heat...because the spark plugs are fouled.
The R22 helicopter has a PA28-style 180hp Lycoming engine (though it's derated) and burns 100LL. I was taught (in the UK, so seldom hot or high) to use Full Rich mixture all the time, and always use full Carb Heat for descents. Unlike Fixed-Wing, every Rotary descent to land ends with something like a 'go-around' because you need nearly maximum power just to hover.
Landing an R22 requires serious attention to several details, but I have never been warned about this one. I am very low hours rotary, so I will ask next time I see my old instructor. Though if anyone here wants to chip in...?
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R22 vs. PA28 differences:
Derated engine
Different intake manifolds etc
Variable CH - have a gnatty little gauge to tell you the actual carb temp.
Gradual change of power - you don't 'yank' on the collective. Final approach is probably at about 15in MP with this being slowly increased to about 23/24in in the hover. All that takes quite a while as you decelerate from 50-60kts and about 300ft to the hover. I don't think this is anything like the fixed wing go-around scenario.
Derated engine
Different intake manifolds etc
Variable CH - have a gnatty little gauge to tell you the actual carb temp.
Gradual change of power - you don't 'yank' on the collective. Final approach is probably at about 15in MP with this being slowly increased to about 23/24in in the hover. All that takes quite a while as you decelerate from 50-60kts and about 300ft to the hover. I don't think this is anything like the fixed wing go-around scenario.
I have flown exclusively in PA28s since my training days and was always taught to use full carb heat on approach and then switch it off at 300'. However, upon reading this thread I may try to leave it on as late as possible to avoid any chance of carb ice build up during the round out and landing run. Having read the entire thread I have to say the debate regarding use of carb heat seems endless!
Grouse
Grouse
Or did I miss something?
G
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Originally Posted by FamousGrouse21
However, upon reading this thread I may try to leave it on as late as possible to avoid any chance of carb ice build up during the round out and landing run.
And as Genghis said - if manual says only to use it when there is evidence of icing and you do not have CAT gauge, I suggest you do as manual suggests.
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CGB,
I agree it is not a yank, but most of the power goes on in the last few seconds of the transition to hover, (as evidenced by the left foot suddenly needing to wake up).
15in MAP is typically set during downwind, so the profile is roughly 3 minutes of low power followed by a 3-5 second change up to 22-24in MAP, which sounds quite like the plug-fouling scenario mentioned previously. So I am a litle surprised I haven't heard of it happening.
Presumably icing is the bigger risk, so carb heat goes on, and the fouled-plug engine failure is the less likely of the two evils. Also, under 500' you don't have a spare hand to change the Carb Heat setting, so you must choose one setting for the whole descent.
Gradual change of power - you don't 'yank' on the collective. Final approach is probably at about 15in MP with this being slowly increased to about 23/24in in the hover. All that takes quite a while as you decelerate from 50-60kts and about 300ft to the hover. I don't think this is anything like the fixed wing go-around scenario.
15in MAP is typically set during downwind, so the profile is roughly 3 minutes of low power followed by a 3-5 second change up to 22-24in MAP, which sounds quite like the plug-fouling scenario mentioned previously. So I am a litle surprised I haven't heard of it happening.
Presumably icing is the bigger risk, so carb heat goes on, and the fouled-plug engine failure is the less likely of the two evils. Also, under 500' you don't have a spare hand to change the Carb Heat setting, so you must choose one setting for the whole descent.
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We were taught 45 years ago to select carb heat (Austers) when reducing power for the final descent, and keep it there until the end of the landing roll or when applying power for a go-around or touch-and-go.
As someone said, in some aircraft you can deselect carb heat with the same hand that opens the throttle.
We did it the same in the club's Piper and Cessna aircraft, and I've done it since in every piston I have flown.
Works for me. I would imagine that reverting to cold air for the last 300' of a proper glide descent, on the wrong day, could pretty much guarantee a certain lack of response for a go-around. And knowing if it's a "wrong day" is quite difficult.
If you're going to drag a single-engine piston in under a high power setting then that might change things. But maybe you need to learn to fly properly, so that you can land on the airfield from any point of the circuit if it all goes quiet. At least that's what we were taught.
As someone said, in some aircraft you can deselect carb heat with the same hand that opens the throttle.
We did it the same in the club's Piper and Cessna aircraft, and I've done it since in every piston I have flown.
Works for me. I would imagine that reverting to cold air for the last 300' of a proper glide descent, on the wrong day, could pretty much guarantee a certain lack of response for a go-around. And knowing if it's a "wrong day" is quite difficult.
If you're going to drag a single-engine piston in under a high power setting then that might change things. But maybe you need to learn to fly properly, so that you can land on the airfield from any point of the circuit if it all goes quiet. At least that's what we were taught.
G
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Cirrus (Minor?), as it happens, but much the same propensities....
I did own a Gipsy Queen for a while....it wasn't just icing that stopped it; cloud had the same effect on occasions, it didn't like damp.
I did own a Gipsy Queen for a while....it wasn't just icing that stopped it; cloud had the same effect on occasions, it didn't like damp.
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Also, under 500' you don't have a spare hand to change the Carb Heat setting, so you must choose one setting for the whole descent.
The right hand is permanently occupied with the cyclic. Anywhere near the ground the left hand holds the collective, in order to lower it quickly if the engine fails.
Fixed wing, of course I agree, you usually do have one hand free. Most of the time, you have two hands free!
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Shouldn't the Carb Heat control been designed out of pistion aircraft by now?
The rotax powered microlights have managed to do it so why not the continental and lycoming fleet.
There can't be a month that goes by without an accident report related to carb icing.
The rotax powered microlights have managed to do it so why not the continental and lycoming fleet.
There can't be a month that goes by without an accident report related to carb icing.
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Follow the POH.
period.
You may think you know more than the manufacturer of your airplane about physics, weather, etc. etc., but I doubt you do, even if you have a PHD in physics and/or weather, the manufacturer probably had several people with PHD's involved with many decades of combined experience involved in the development of the POH.
Besides the notes about always doing it the exact same way, so that it's ingrained, there's the liability situation, and a level of comfort in knowing that if you're at X stage the controls will be at Y. If not, you know something is wrong. There's the fact that someday you may be flying in different conditions than you do at your home airport in the middle of summer when this student learned what to do with the controls. Additionally, having the carb heat on fully is a reduction of what, 50 RPM, but having had to use carb heat on a few occasions I can tell you that with the carb heat off, you lose much more than 50 RPM.
period.
You may think you know more than the manufacturer of your airplane about physics, weather, etc. etc., but I doubt you do, even if you have a PHD in physics and/or weather, the manufacturer probably had several people with PHD's involved with many decades of combined experience involved in the development of the POH.
Besides the notes about always doing it the exact same way, so that it's ingrained, there's the liability situation, and a level of comfort in knowing that if you're at X stage the controls will be at Y. If not, you know something is wrong. There's the fact that someday you may be flying in different conditions than you do at your home airport in the middle of summer when this student learned what to do with the controls. Additionally, having the carb heat on fully is a reduction of what, 50 RPM, but having had to use carb heat on a few occasions I can tell you that with the carb heat off, you lose much more than 50 RPM.