What is the purpose of the Carb heat

Discussion in 'Pilot Training' started by Allison Riccardi, Nov 13, 2017 at 10:06 AM.

  1. Harold Rutila

    Harold Rutila Pre-takeoff checklist

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    This is a total myth. I'm conducting an analysis of Cherokee carb ice occurrences that were reported to the NASA ASRS as we speak, and as it stands now, over 50% of the reports I've analyzed are from PA28s, many of which involved engine failures and off-airport landings because the pilots did not believe carb ice was a factor in that model, and in some cases didn't even bother to pull the carb heat when the engine failed. I'm not criticizing you personally, but I am certainly on a mission to get rid of this myth that Cherokees are not prone to carb ice.
     
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  2. Harold Rutila

    Harold Rutila Pre-takeoff checklist

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    Not according to NASA ASRS. http://asrs.arc.nasa.gov
     
  3. Tom-D

    Tom-D Taxi to Parking

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    In the little aircraft we fly, that's all you really need to know.
     
  4. Tom-D

    Tom-D Taxi to Parking

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    No, I don't care if the spelling is right, you understood it well enough to criticize ?
     
  5. Tom-D

    Tom-D Taxi to Parking

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    I hope every one knows there are bolt on multi port fuel injection and electronic ignition for almost every engine in GA, but our stupid PMA rules say we can't use them.
    Just think,, no more carb icing, or mag problems.
    a regular old 0-320 will produce 180 horses with these devices.
    But Hell no, GA will be dead before the FAA will do anything about the deaths the antique stuff causes.

    Rant over.
     
  6. bobmrg

    bobmrg Pattern Altitude PoA Supporter

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    Relative to the Cessna/Piper/Lycoming/Continental carb heat question, the NTSB recognized long ago that different procedures for different planes was a recipe for disaster and recommended that full carb heat be used whenever power was reduced below cruise in all planes so equipped (as I recall it..I may have the triggering condition slightly wrong). In their recommendation to the FAA they asked that this procedure be widely disseminated in the pilot/instructor community and that operating handbooks and FAA pubs be changed. As is often the case with NTSB recommendations, the FAA did nothing.

    Following this practice regardless of engine or airplane model would not have a deleterious effect on anything.

    Bob Gardner
     
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  7. Clark1961

    Clark1961 Touchdown! Greaser!

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    I don’t even have carb heat on my PA-28,what should I do?
    -scared of ice in Colorado
     
  8. Sundancer

    Sundancer Pattern Altitude

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    Carb heat is short-hand slang, for carbonara, a nice way to prepare chicken; you apply carb heat in a Cessna to keep your infrared signature large enough to be detected by the seeker heads installed on the roofs of Carrabas Italian Grills; as you fly over, the Chef hears a tone in his hat, sort of similiar to a "growling" noise, and he knows to start browning the panchetta, you being within the restaraunt's serving parameters. Some pilots keep carb heat on all the time, until their HDL/LDL levels go bad; or they get fat. As technology has advanced, the seeker heads can "see" you well in advance, and get a good tone for the Chef even as you approach head-on; no rear-aspect required. To help control their weight, some pilots dispense hot buttered rolls to confuse and defeat detection, though the technique is becoming less effective.

    Piper aircraft pilots can't see the restaurant, and so rarely use carb heat; they are often hit in the face by Mach 3 servings of pasta, and end up with big, wide butts and terrible cholesterol.
     
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  9. Tom-D

    Tom-D Taxi to Parking

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    The real cure or carb ice, is a carb temp gauge, and use it.
     
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  10. Sundancer

    Sundancer Pattern Altitude

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    Plunge from the sky in an ice-encased aluminum coffin, howling in terror, with the sickening certainty that 26 trillion tons of Earth is seconds from crushing you like a grape in a granite quarry.
     
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  11. Clark1961

    Clark1961 Touchdown! Greaser!

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    Can I just say something like “well that didn’t work out favorably” instead of howling?
     
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  12. GRG55

    GRG55 En-Route

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    And if there's time, resolve in the next life to buy a Cirrus with a chute. ;)
     
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  13. denverpilot

    denverpilot Taxi to Parking

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    I think y’all got taken by a troll. But since we’re trolling...

    That’s one really iced up carb, Bill. :)
     
  14. Bill Jennings

    Bill Jennings Final Approach

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    Meh, whatever. Breaks up the monotony. It IS interesting that this pops up right after the 6pc shaken not stirred video.
     
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  15. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Final Approach PoA Supporter

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    Carb heat myths:

    1 ) "Never use Partial carb heat because can make it worse by moving you into a danger zone." Or, words to that effect.
    Look at the diagram in the quote - the vertical axis is dew point - this is directly related to the absolute humidity, or the amount of water vapor in the air as a fraction of the total mass. In the absence of visible moisture (snow, rain, cloud) adding heat to the air changes the temperature, but does not add or subtract water. So, as a result, as you add heat, you move horizontally to the right on the chart - there is NO up and down. And, no matter where you start, as you add any heat at all, you move to a less likely to ice place, not a more likely to ice.

    Exceptions: Your intake has a bunch of water from snow/rain, the heat melts evaporates the water and you can have some upward movement. In a cloud, it is possible to move up the 100% relative humidity line for a few degrees.

    2 ) "If you don't apply carb heat before the engine quits, it's too late."
    The claim is that the exhaust cools so fast that there won't be any heat available. I can tell you from experience that this isn't true - carb heat applied after a moment of silence can result in resumption of the noise. If you have any doubts, right after you shut down next time after flying, reach into the cowl and grab an exhaust pipe with your bare hand. Don't worry, just grab it. Grab it hard. We all know it will be cool since it's just thin stainless and can't hold any heat. (If your car has a stick shift, use your left hand so you can still shift with your right hand on the way to the emergency room.)
     
  16. Dan Thomas

    Dan Thomas En-Route

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    Grabbing that exhaust after shutdown on the ground isn't the same as its cooling in flight. There's no air movement through the cowl or heat muff after shutdown as there is in flight. I am a full-time aircraft mechanic, and I can shut down an engine, outside in the breeze, and within a very short time I can grab the tailpipe. Cool already.

    And we're not talking about a "moment of silence." We're talking about gliding in an approach or practice forced approach, where the carb can ice up and put the fire out well before one notices that the engine is dead. It will windmill pretty much as normal. In a carb ice event from normal cruise RPM, the RPM will fall (or airspeed, with constant-speed prop) and if the pilot sits there and waits until the engine is totally dead, he wasn't trained too well. The engine doesn't fail suddenly.
     
  17. George Mohr

    George Mohr Pre-takeoff checklist

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    Disclaimer: I'm not trying to convince anyone NOT to use carb heat, just trying to understand the frequency.

    I'm very interested in this, so I did an ASRS query on PA-28 with (('carburetor' or 'carb') and 'ice'). I received 15 cases total. Here's my quick analysis:
    • Of the 15, 6 of them had identified other causes.
    • The remaining 9 were at least 'likely' caused by carb ice.
    • Of those, only 4 were able to restore power using the carb heat.
    Note that the earliest report was from back to 1995. So from this data only, 9 cases of likely carb ice over 22 years certainly seems like a very rare event to me. 4 cases of carb heat solving the issue makes it even more rare.
     
  18. coloradobluesky

    coloradobluesky En-Route

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  19. Adam Weiss

    Adam Weiss Pre-Flight

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    Took a long IFR XC KDEC to KFAR last week in a C182.
    On the way back, planned for 7000, but at 6400 got icing on the windshield less than 30 seconds.
    Requested 5000.
    Kept a close eye on the carb temp (the 182 has a handy gauge for that), but added some heat periodically just to make sure there was no ice build up.

    Bottom line, if you're even moderately suspecting there might be carb ice, it doesn't hurt to slowly add some heat...don't do it fast in case there is ice, and you blast a bunch of water into your engine.
     
  20. PaulS

    PaulS Final Approach

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    I suspect ice, that carb heat is going full in, no screwing around here, the engine stumbling from the ice being melted is the sound of a problem being solved.
     
  21. Vance Breese

    Vance Breese Line Up and Wait

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    In my opinion a successful application of carburetor heat would likely not be reported anywhere.
    A report would come from a mishap where the pilot failed to apply carburetor heat.
     
  22. George Mohr

    George Mohr Pre-takeoff checklist

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    That's certainly true.
     
  23. Tarheelpilot

    Tarheelpilot Cleared for Takeoff PoA Supporter

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    Partial carb heat is appropriate if the system is designed to be operated at partial application. A carb air temp gauge is normally part of those systems. If you have a gauge showing the air temp in the Venturi it makes using carb heat much easier.
     
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  24. WannFly

    WannFly Pattern Altitude PoA Supporter

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    not directly related to carb heat but...
    jamming the throttle in carbureted engines is never a good idea. some people have made it to NTSB list by doing that... when you jam the throttle, there is a chance that more air rushes in than the fuel and the mixture could become excessively lean for a very short period of time, leading to a sputter and not getting full power for a couple of second and then suddenly getting full power. very bad combination if are in a go around situation close to ground
     
  25. WannFly

    WannFly Pattern Altitude PoA Supporter

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    carb temp gauge will only tell you the temp, but the pilot have to decide whether to use card heat or not. not questioning you, asking. I have a carb temp gauge, when it shows around 32F, and there is visible moisture, I use carb heat. if it shows 20 degrees....no moisture that I can see... I don't. PA 28 flyer
     
  26. Jeff Oslick

    Jeff Oslick En-Route PoA Supporter

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    It was 20+ years ago, but it was likely in the mid-60s, with moderate humidity, typical for the location (New Bedford, MA).

    If carb ice is so rare in PA-28s, why did the owner spend money on a carb ice detector?

    BTW, this was during my first checkout in the plane, and among my first takeoffs. The owner of the plane was giving me the check out. My hand beat his to the carb heat knob. It wasn't a long check out flight.

    Frankly, I wouldn't be opposed to an AD requiring a carb temp gauge on Conti-powered 182s. Fortunately, many have one already. They are pretty easy to install (even if a stand-alone analog instrument), and not expensive. We have carb temp on our UBG-16 engine monitor, and I usually set it to primarily monitor carb temp on takeoffs.
     
  27. George Mohr

    George Mohr Pre-takeoff checklist

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    Hard to say, but it sounds like a great tool to have. I've never seen another PA-28 with one though. C182's on the other hand, seem to all have one!
     
  28. Sinistar

    Sinistar Pre-takeoff checklist

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    This will probably stir pot...but I'd much rather have a precision (digital & accurate) MP gauge than our damn Carb temp guage. We've had carb ice twice now.

    Once when my wife (PIC) and me riding shotgun. It was during winter, some clouds above us carb temp was in the yellow (like it always is on a 182), wife said something didn't sound quite right after leveling off to cruise. Carb heat in, engine eats some water then all good. If we had to guess on that one, it was the long engine warm up and slow taxi that started it off.

    Second time, carb heat gauge in yellow (like it always is on a 182) me having a lesson with CFI in right seat. Visible moisture, light mist he says we're gonna have a good lesson. He never even looked at the carb temp the entire time. We very carefully observed the MP needle and sure enough it was very, very slowly dropping. Pushed in carb heat, MP drops more than normal, engine chomps water, MP back to normal carb heat difference. Did it two more times during flight. Was a lesson I will not forget.

    The problem up here in the north is that during fall, winter and early spring that carb temp reading is always in the yellow unless youre way up where it's really cold.

    I really think a carb ice sensor (if it exists) would be worth way more than that temp gauge.

    Also, why is there no good guidance on when to use carb heat during cold wx engine warm up, idling and taxi? Those times freak me out way more.
     
  29. Clark1961

    Clark1961 Touchdown! Greaser!

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    Carb heat is avoided during warm up and taxi because the intake air is not filtered. Check card heat as part of the run-up. Ice should be cleared at that point.
     
  30. Sinistar

    Sinistar Pre-takeoff checklist

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    What about 8 minutes after a run-up while waiting for landing traffic and other departing planes ahead of you?

    I think this whole carb heat thing is a bit more complex than just avoiding it during warm up and taxi (due to lack of filtering) and using it for the run up.

    My wife's previous mechanic (now retired) was a engine mechanic during WWII working on bombers. Later he went on to be a ASI(A&P) and did some occasional work for friends on older planes. He told my wife to always use carb heat on the ground for her previous plane (1946 Chief). She did that for going on 20 years with no damage to the engine. He said just too many accidents over the years attributed to carb ice which includes takeoff. I'm guessing he believed unfiltered air was much less a risk than carb ice on takeoff. Obviously during really warm wx this is moot, but where you have these significant seasonal temperature changes around 20F...60F and lots of close temp/dewpoint spreads I guess I can see his point.

    So this makes me wonder. Can it hurt to use carb heat for like 15 seconds prior to takeoff. Just to burn out any ice that might be forming. Then back to normal, do the normal pre-take off checklist and go?
     
  31. Clark1961

    Clark1961 Touchdown! Greaser!

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    Use your judgement. If you want to check for carb ice at any time then do so. If you noticed carb ice during the run-up check then fer sure apply heat to melt ice before going to full throttle. Check engine instruments on the roll and look for more than just “green” note fuel flow, mp, and rpm.
     
  32. Dan Thomas

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  33. Half Fast

    Half Fast Line Up and Wait

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    The Tecnams I fly don't have carb heat. Is there something that makes Rotax engines less prone to carb ice? I suspect the air intake location causes the engine to breathe warm air all the time.
     
  34. Dan Thomas

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    One of the items in a METAR is the temperature and dewpoint. Look closely at those when checking the weather before your flight, and if the spread is small, carb ice could be an issue. If you're flying certain Continentals, carb ice WILL be an issue. If the spread is wide, ice is much less likely. If you look at the chart posted earlier you can see that the biggest icing risk comes where the temp/dewpoint differences are small. That's because the air entering the carb is at the temp mentioned in the METAR, and as the air temp drops in the carb due to evaporative cooling and pressure drop, it will easily get lower than the dewpoint if the spread is small, and water vapor in the air will condense. Enough vapor and a big enough temp drop, and that condensation starts turning into ice sublimating directly from the vapor. Like frost.

    I will often give the engine a blast of carb heat right before takeoff if the conditions warrant it. And sometimes it tells me it was icing up.

    Unfiltered carb heat air really isn't that big a deal unless there's airborne dust. You should see the big gaps in some airboxes. Made that way at the factories. They can suck dirty air through those gaps all the time. The flapper valves in many of them have plenty of clearance between the valve sides and the sides of the box; the carbureted 172s are classic examples of that. They're sucking some heated, unfiltered air all the time past that thing. It's old technology. Cars would never get away with that at all; too much dust on the roads. Boats don't have any filters at all. Pretty hard to raise dust out on the water.
     
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  35. Sinistar

    Sinistar Pre-takeoff checklist

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    I figured it was safe to apply some carb heat prior to takeoff. Just make sure its back out (pre-takeoff checklist) before takeoff.

    However, I then started wondering if that carb heat right before takeoff might have some other short term effect that would be a gotcha. Since it makes the engine run rich. It doesn't seem desireable to run a engine rich just before takeoff. What if it starts fouling the plugs. Or maybe what if the linkage didn't quite return all the way and so the engine ended up lacking some power during takeoff. Or, if you do use carb heat prior to takeoff...what RPM should be used - IDLE, RUN-UP rpm, something else? For me I have been bumping the IDLE to about 1200, pull carb heat, watch to see if it increases a bit, then back to normal IDLE (800rpm). Let things stabilize, run the pre-takeoff checklist and then quick checking the MP and RPM at full power during takeoff roll.
     
  36. Tom-D

    Tom-D Taxi to Parking

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    on your PA-28 isn't the carb bolted directly to the hot oil sump? does that make a difference ?
     
  37. George Mohr

    George Mohr Pre-takeoff checklist

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    It certainly does. Btw, Rotax engines have a pressure carb. It makes icing pretty much impossible.
     
  38. Tom-D

    Tom-D Taxi to Parking

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    Did we just change the subject?
     
  39. JAWS

    JAWS Pre-takeoff checklist

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    My opinion about Pipers? The pilot manual was partially written by the marketing department, trying to differentiate the cherokee from the 172. Same basic engine between them, very different operating guides.
    I like using carb heat while on base and initial final, off during GUMP check.
    Either full on or full off.
    Run up check includes leaving it on and ensuring that the engine will idle with heat on.
     
  40. mscard88

    mscard88 Touchdown! Greaser!

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    That never happens on POA Tom, you know that. ;)