Carb heat on Grumman AA1B, yes or no

Discussion in 'Pilot Training' started by jd21476, Aug 13, 2019.

  1. jd21476

    jd21476 Pre-Flight

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    I just recently purchased my first airplane, a 1976 Grumman AA-1B with the Lycoming O-235. I have mostly been flying Cessna 172's but I do have some hours in Cherokees and a Grumman AG5B.

    My question is, when I did the pre-purchase check flight, the guy i bought it from said he does not use carb heat on the plane. This was odd to me because coming from C172's you always use carb heat below 2000 rpm or at least that what I was taught. I live in sunny southern California but is this correct? Is it true that I should very rarely use carb heat in the AA-1?
     
  2. pigpenracing

    pigpenracing Pattern Altitude

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    I use carb heat on every airplane that has carb heat.
    It is there for a reason...
     
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  3. Ravioli

    Ravioli Final Approach

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    What’s the POH say?

    In Cherokee and my RV it is only when there is suspected carb ice. In the contis it is normal with the power reduction. The rpm gauge has the line on when to apply.

    @Ted DuPuis can be more authoritive. And oddly this was a chat convo this morning.
     
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  4. jd21476

    jd21476 Pre-Flight

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    I read the POH and it doesn't seem to specifically say. Again, I am used to C172's so this is kind of new to me.
     
  5. pigpenracing

    pigpenracing Pattern Altitude

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    So do you use it on the 172? I do on mine every time I land.
    Do you want to wait till you are over trees on short final and need power to find out your carb is iced up?
    What will it hurt using it if it isn't needed?
    What if you don't use it and you do need it and its to late?
     
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  6. Ravioli

    Ravioli Final Approach

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    If you were asking me. What it hurts is unfiltered air and a slight performance loss at a bad time.

    I do follow the POH on cessnas and I also follow the POH on pipers. Don’t know what the Grumman says. But if isn’t called out and marked on the tach I’d probably not do it
     
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  7. Ted DuPuis

    Ted DuPuis Administrator Management Council Member

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    Because Lycomings attach the carb to the oil pan (and run the intake air through it) they are not very prone to icing. I don't know what Grumman said, but Piper basically said you never needed carb heat but Cessna says you do, and that's for the same engine. Continental has a different design on the O-200/300 and it is much more prone to carb icing. I've even seen O-300s carb ice on takeoff.

    Really the unfiltered air isn't too big of a deal at altitude unless you're flying through some kind of a storm. Some airplanes (like the M20F I used to fly) have a "ram air" option that gets you a little more manifold pressure by bypassing the filter and opening up an inlet in the front of the plane in a high-pressure area. I'm not going to tell you to do one way or the other. I haven't flown a plane with a carb in 8 or 9 years. On the Archer (Lycoming O-360) I never used carb ice. One of the other people who flew that plane did end up getting carb ice one day, which surprised him as well as everyone he told, as nobody in the club had ever gotten carb ice in that plane before.
     
  8. Grum.Man

    Grum.Man En-Route

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    I'm curious why someone would not use it on an airplane with it? I can't think of a single reason why you shouldn't use it when rpm is low. There is plenty of power to adjust your glide path on landing and if properly trained pushing it in on a go around becomes instinct.
     
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  9. Weekend Warrior

    Weekend Warrior Pre-Flight

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    I've got about 500 hours in my Warrior, and in those 500 hours I've used carb heat twice (not counting testing the carb heat, which I do during run-up on every flight). My POH says to only use carb heat when necessary. I believe I read somewhere that on some planes, due to a different carb, or intake, or carb/engine mounting location, etc, that they are more prone to carb ice, so the POH says to use it every time on those planes.
    I personally don't use it because I try to minimize the unfiltered air (carb heat is unfiltered)
     
  10. SkyDog58

    SkyDog58 Touchdown! Greaser! PoA Supporter

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    Define “when necessary”.

    Is it when you actually have symptoms of carb ice such as power loss?

    Is it when conditions exist that are conducive to the formation of carb ice?

    How do you define it?
     
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  11. C-1 PILOT

    C-1 PILOT Pre-takeoff checklist

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    Had my Tiger 5 years flew over 500 hours and NEVER had to use carb heat. Must be a Grumman thing.
     
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  12. Weekend Warrior

    Weekend Warrior Pre-Flight

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    I was paraphrasing or over simplifying. The actual wording is something to the affect of "use when you suspect carb ice is present".
     
  13. Skip Miller

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    600 hours in PA-28s and I used carb heat in flight exactly once. It was a high dewpoint day (75 or so) and the engine began to get rough on short final. Carb heat on, landed. Engine stalled on the runway. Restarted promptly. CFI and I agreed it was prolly carb ice. -Skip
     
  14. SkyDog58

    SkyDog58 Touchdown! Greaser! PoA Supporter

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    Got it. Thanks for explaining.
     
  15. bflynn

    bflynn Final Approach

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    Doesn't matter, apparently you're still going to be paying a huge annual every so many years.
     
  16. Dana

    Dana Line Up and Wait

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    From the Lycoming O-235/290 operator's manual:

    (1) Take-Off- Take-off should be made with carburetor heat in full cold position. The possibility of icing at wide throttle opening is very remote.

    (2) Flight Operation - The carburetor air heat control should be left in the cold position during normal flight operations. On damp, cloudy, foggy or hazy days, regardless of outside temperature, keep a sharp lookout for loss of power. This loss of power will be shown by unaccountable loss in manifold pressure or RPM or both, depending on whether a constant speed or fixed pitch propeller is installed on the aircraft. When this situation arises, apply full carburetor air heat and open the throttle to limiting manifold pressure. This will result in a slight additional drop in manifold pressure which is normal and this drop will be regained as the ice is melted out of the induction system. When the ice has been melted from the induction system, the carburetor heat control should be returned to the cold position. In those aircraft equipped with a carburetor air temperature gage, partial heat may be used to keep the mixture temperature above the freezing point (32°F.).

    WARNING
    Caution must be exercised when operating with partial heat on aircraft that do not have a carburetor air temperature gage. Moisture in crystal form that would ordinarily pass through the induction system, can be raised in temperature by use of partial heat to the point where the crystals are melted into liquid form. This moisture can form carburetor ice due to the temperature drop as it passes through the venturi of the carburetor. It is advisable, therefore, to use either full heat or no heat in aircraft that are not equipped with a carburetor air temperature gage.


    (3) Landing Approach - In making an approach for a landing, carburetor air heat should usually be in the "Full Cold" position. However, if icing conditions are known or suspected, then "Full Heat" should be applied. In the case that full power need be applied under these conditions, as for an aborted landing, the carburetor heat should be returned to "Full Cold" prior to power application. See aircraft flight manual for specific instructions.
     
  17. jetedrick

    jetedrick Pre-Flight

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    I fly a Piper Warrior, I follow my POH, which is use carb heat when carb ice is suspected, usually just open it up during run up and that is it.
     
  18. PaulS

    PaulS Final Approach

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    If it has it I'd use it. The problem with carb ice, if it's happening on say an approach or descent is that if the engine stops running and you don't notice it, there will be no heat left to melt the ice when you figure it out. The other problem with carb heat is when you need it and you put it on, it can make the engine run even worse for a period of time as the ice melts. If you make the mistake of turning it off for this the engine could stop running. If you think you need it, put it in and keep it in.
     
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  19. danhagan

    danhagan Pattern Altitude

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    Air intake is through the cowling and behind cylinder #3 on Tigers (meaning air is being heated over the cylinders before it gets to the carb). Dave Fletch (FletchAIR) told me when I picked mine up prior to flying it for 10 years that I should never need carb heat, and that there has never been a Tiger that suffered carb ice (not sure of the accuracy of that statement). I'm in the desert, so I only used it in high humidity low RPM trips (which wasn't often).
     
  20. jd21476

    jd21476 Pre-Flight

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    I see you have Grumman AA1 so do you use carb heat?
     
  21. Ryanb

    Ryanb Touchdown! Greaser! PoA Supporter

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    There was an aviation related conversation? That’s out of the norm.
     
  22. jd21476

    jd21476 Pre-Flight

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    So from what I am hearing and reading it doesnt sound like I need to use it or actually should use it unless I suspect that I am in conditions where carb ice may be present. This is the same sentiment that the previous owner told me but i just wanted to hear from others.
     
  23. Ravioli

    Ravioli Final Approach

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    @jd21476 - You should not use it unless you suspect icing, as in, the engine stumbles. Conditions that could cause is the Cessna way. (paraphrasing a lot)
     
  24. Grum.Man

    Grum.Man En-Route

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    I have since sold it but yes every single time I land or am below 2k rpm. It certainly doesn’t help and would rather push it in if I need an expected power increase than to have a surprise and have to pull it out when you are likely to really need the power back.
     
  25. Grum.Man

    Grum.Man En-Route

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    That is probably true coming from the masters mouth. If it were me I would use it regardless. If you were on final approach and noticed the engine trying to quit from carb ice, you now have a potential go around situation where you need max power but instead you have the carb heat pulled to burn off the ice. Where as with it already on the worst case if you need a go around is you get max power back when you close it and go full throttle.
     
  26. Dan Thomas

    Dan Thomas Final Approach

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    In any airplane that uses air cooling, that air above the engine is at ambient temperature. It doesn't get heated as it passes over the top of the cylinders. The only air getting heated is that which is pushed downward through the cooling fins, and there's enough of that that the air above, on its way to the carb heat or oil cooler or whatever, is insulated from the fins and doesn't pick up any heat. Air is passing through there at a terrific rate, too.
     
  27. Dan Thomas

    Dan Thomas Final Approach

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    There is way too much rote learning about stuff like carb heat, and way too little actual theory, so we get people who never use carb heat because the POH doesn't demand it, people who use it all the time because they're afraid of carb ice sneaking up on them, people who are scared of dirt getting into the intake via the unfiltered carb heat air, people who are convinced Lycomings are ice-proof, and so on.

    First, one of the things the regs demand is that we check the weather before we fly. Two of the items on the METAR are the temperture and dewpoint, and a smart pilot will pay attention to those to determine the ice risk for that flight. If there's a substantial spread between the two temperatures, ice is unlikely. There are dozens of charts available on the 'net showing the risk as it relates to those factors; use them. Way too many engine failures are attributed to carb ice, and one study revealed that more engines quit due to ice than any other factor including running out of fuel.

    A carburetor is a tiny icemaker. As air passes through its venturi, its pressure drops, so its temperature drops as well. And as fuel enters the airstream in the carb, much of it evaporates. Evaporation involves turning a liquid into a gas, and heat is required for that change. The heat comes from the air, which gets really cold. Between the two factors, pressure and evaporation, the temp in the carb can drop as much as 70°F, so we can get ice at an ambient temp of as much as 100°F. It all depends on the moisture content of the air (which is where temp and dewpoint come in) and the particular quirks of an engine and its installation.

    Yes, Lycs bolt their carbs to the oil sump, so the carb stays warmer and is less likely to ice up. But I have had Lycs ice up on me in cruise on nice summer days. We had Lycs ice up on our students and instructors on beautiful summer mornings, first start of the day, when that oil sump wasn't warm yet and the dewpoint was high enough to get some ice. I'd hear the engine start and run at 1000 RPM for a minute or so, then the pilot would close the throttle and the engine would try to quit, so he'd just open the throttle some, not realizing that he had ice already. That has killed folks, taking off with ice in the carb, limiting the power output. I would run out and we'd have a little chat and I'd get the guy to pull the carb heat and see what happened; their eyes would get real big when they realized how insidious this thing is.

    The small Continentals have their carbs on an intake manifold that gets very little heat transfer off the crankcase. The O-300, though, has the same sort of setup as the Lycoming: it's on the sump. Beware of internet legends.

    Dirt in the intake? How often do you see dust in the air, especially at altitude? We ran a bunch of flight training airplanes with carbs, taught the proper use of carb heat, the students used it on every approach, and all those engines ran all the way to TBO and still had compressions in the high 70s. Dirt is a thing to worry about when you're operating off dirt strips, or if some other aircraft has kicked up a bunch of it. And the carb heat box on most airplanes is a sorry affair, having numerous places where air can get in past the filter as well as through the carb heat muff and past the sloppy-fitting control valve plate. You should avoid dust anyhow, even with the carb heat closed. But for Pete's sake, don't avoid using carb heat because you're afraid of dust; that carb ice will kill you dead in minutes, while the dust might only shorten the life of the engine, and it would take more dust than most of us ever see.

    Boat engines, you know, don't use any air filtration at all.

    The POHs for many airplanes are totally inadequate. They don't cover the subject of carb ice well at all. You have to study up on it yourself, especially since your instructors often don't have any handle on it either, and they tend to convey no info or totally wrong info, and the student eventually gets into trouble when his carb ices up. As long as most of us can only afford to fly carb-equipped airplanes, we owe it to ourselves and our passengers to know the stuff and use it.
     
  28. chemgeek

    chemgeek Cleared for Takeoff

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    The POH for Grummans recommends using carb heat as necessary, as in when conditions warrant, or when you experience carb icing symptoms. In both my AA-1A and AA-5 I've used carb heat for more than a few moments only a few times in 35 years, and those were in serious ice-prone conditions, as in descending through lake effect ice making clouds, or in cruise on the worst god awful hazy humid summer days. These instances were completely understandable from looking at the icing chart. Grummans aren't prone carb icers. By all means use carb heat to CHECK for potential icing at any time, especially in favorable wx conditions, but it is not generally needed for extended operations like in other planes.
     
    Last edited: Aug 15, 2019
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  29. farangutan

    farangutan Filing Flight Plan

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    The instructor for our club's Grumman AA-5 instructed us to limit carb heat to testing on runup and when carb ice is suspected or strongly likely. The main concern (if I remember correctly) was fouling spark plugs from a richer mixture. I came from flying C172s and always used carb heat.
     
  30. flyingron

    flyingron Touchdown! Greaser!

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    While the mixture is richer, it's not that much richer. If you're going to foul the plugs with the carb heat on, you're likely to foul them with it off as well.

    In fact, most of the "loss of performance" is due to this enrichment. You'll get it back if you lean it back a bit.
     
  31. flyingron

    flyingron Touchdown! Greaser!

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    The Cheetah book I have only says to use it "if necessary."
     
  32. Dan Thomas

    Dan Thomas Final Approach

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    That's a prime example of rote teaching, a lack of understanding the physics involved, and a cavalier dismissal of the risk.

    Sometimes I ran into students who, during the runup, pulled the carb heat on, saw the RPM drop, and pushed it off again. Took three seconds. I would tell them to pull it out again and leave it out for awhile; they'd see the drop, and more than once they also saw the RPM rise while that carb heat was on as the ice they'd collected was melted out. When they pushed the carb heat off the runup RPM would rise to a number considerably higher than what they started with.

    Carb ice at low power settings like idle/taxi and runup is very common, and airplanes have crashed due to it.
     
  33. Dan Thomas

    Dan Thomas Final Approach

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    An example of a totally inadequate POH. If one looks at the current crop of POHs (now known as AFMs) one sees an awful lot more information and warnings. A 172 AFM is a thick affair in a three-ring binder. This sort of thing is a result of lawsuits by victims or their families after they've crashed due to ignorance of the risks and management of those risks.

    We're not taking groundschool seriously enough. The older airplanes were built and sold in a generation when most folks understood a lot more about engine management and things like carb ice. Back then, every car had a carburetor and a heating system to keep it from icing up. A significant percentage of people did their own car maintenance. Now we have cars that manage themselves, giving us a generation of drivers who have no clue about operating a 1970s aircraft engine; it's all rote, and much of it is plain wrong.
     
    Last edited: Aug 15, 2019
  34. flyingron

    flyingron Touchdown! Greaser!

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    Dan, you might want to read this book: https://www.amazon.com/dp/B00273BHPU/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1
    I think you'll find it in line with your views (and it's a short read).

    I don't think it's inadequate. It does talk about testing the Carb Heat in preflight. It talks about using it in certain situations (emergency descent through clouds, engine failure, etc...).
    Does it really need to tell you when carburetor heat is necessary when the conditions conducive to car ice are supposed to be in your private pilot curriculum?

    You can divide the carb heat rules into three basic categories (four, if you count airplanes that don't have carbs):

    1. Aircraft where you leave the carb heat off unless you suspect ice.
    2. Aircraft where you use the carb heat prophylactically on low power situations.
    3. Aircraft that are prone to ice and you use it even more often (or you install a carb temp guage).

    I can tell you that my "owner's manual" (pretty much hard to describe it as a POH, it predates all that stuff) is pretty mumm on carb heat other than saying what it is and use it when you get carb ice.

    In fact, my plane had a pressure carb which the books tell you are pretty hard to ice up (if you see the humidity/temp graph often shown, it has a tiny window).
    Nothing is impossible. We were flying back from a nearby airport where my wife was doing landing practice. Since our home field is only ten miles away or so, she had reduced power pretty much right after takeoff. Now it was a fairly warm sunny day so we didn't think much of it. We got just about to the field when the engine started to run rough. Since we were in a position to head to the numbers and land, my wife did that rather than flying the pattern. On roll out, it occurred to me what the problem was. I told her to hold the brakes and I applied carb heat and a little power and sure enough it gulped and ran smoothly. Only time I ever experienced carb heat in that plane.

    Now in the O-300-powered 170 I used to fly, I'd seen it on taxi and a few times in flight.
     
  35. Dan Thomas

    Dan Thomas Final Approach

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    Whatever is in groundschool curricula, it's not getting the job done. This thread reflects that, as does the accident record. In speaking at recurrency seminars I have found far too many pilots that haven't got a useable grasp of it. That represents a lethal threat. Carb temp gauges are good, but if they're not monitored they're useless. And they don't indicate ice; they indicate temperature, and in dry air they can read subfreezing and the risk is still zilch. In the winter they'll read subfreezing all the time so that we start ignoring them, yet we can get carb ice down to -20°C (-4°F) or so. Liquid water droplets can stay liquid at such temperatures. A carb ice detector make much more sense. https://www.aircraftspruce.com/catalog/inpages/icedetect2.php
     
  36. chemgeek

    chemgeek Cleared for Takeoff

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    Well, the POH can't be blamed for improper training. Lawyers think that loading up operating manuals with excessive verbiage and warnings will improve safety. It won't. (But the lawyers will point to it during the inevitable lawsuits.) In the Grumman AA-X series it is neither necessary nor recommended to use continuous carb heat for landing, except when warranted (by atmospheric or other factors). That recommendation does not, however, absolve you of properly CHECKING carburetor heat at any time. Of course, checking carb heat is in the takeoff checklist. If the engine stumbles when you apply carb heat, or rpm increases after the application of carb heat, then further investigation is warranted prior to continuing. Same thing applies for the pre-landing checklist, or whenever atmospheric conditions warrant, or the operator suspects rpm decrease. Short-term application of carb heat will identify incipient carb icing. To my knowledge, Grummans have not been falling out of the sky because of poor carb heat management.

    It is indeed frustrating that pilots still manage to crash perfectly operational aircraft by failing to check carb heat during partial or full power loss. We had an accident out of our airport where a low-time pilot in a C-172 likely did not recognize carb icing during cruise and wound up spinning in with the engine sputtering. In 35+ years of flying carbureted airplanes, I've had precisely ONE significant (likely, but not proven) carb icing experience, and that was in my AA-5. Following the power loss checklist, which includes carb heat application, cleared up the problem after a few tense moments. Atmospheric conditions were highly conducive to carb icing, so I should have probably been more proactive in that instance. I've had a few incidents in cruise over the years where I noticed a slow decrease of 50-100 rpm on excessively humid days and not above the haze layer. Application of carb heat revealed incipient icing, which did not return.

    I do agree that pilots should educate themselves about carb icing. It can occur at a wide variety of temperatures if the relative humidity is sufficient. It's not just a cool or cold-weather issue. Be safe out there.
     
    Last edited: Aug 16, 2019
  37. Dan Thomas

    Dan Thomas Final Approach

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    That brings up another situation: the carbed engine with a constant-speed prop. The governor will keep the RPM constant as the carb ices up, until the engine is nearly dead and can't maintain the RPM anymore. The pilot, used to looking for an RPM decrease, misses the developing threat. One has to monitor airspeed instead.
     
  38. Clip4

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    A lot of Piper drivers apply full carb heat on downwind to clear ice if present and turn carb heat off for landing.
     
  39. Clip4

    Clip4 En-Route

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    Cli4ord
    That is true at high power settings, a low Power settings (usually below 15 inches MP) the governor no longer controls propeller RPM.
     
  40. Dana

    Dana Line Up and Wait

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    Dana
    I thought with a CS prop (never flown one myself) you're supposed to watch manifold pressure as an indication of carb ice?

    I learned to fly in Continental powered Cessnas where you use carb heat, but when I started renting Lycoming powered Pipers I was taught to do a "carb heat check" on downwind (pull it, watch the rpm drop, then if it comes back up you may have had ice), dunno what the POH said. The Lycoming powered experimental I fly now has no POH, but I do the same thing if the temp and dewpoint are close together.