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Thread: Does the plane HAVE to have carb heat to pass inspection?

  1. #1

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    Does the plane HAVE to have carb heat to pass inspection?

    I'm building a small LSA and using a snowmobile engine. I know planes use carb heat for icing issues.
    I have ridden both snowmobiles and dirt bike in the winter on snow and frozen lakes and never had problems.
    To pass the airworthiness inspection does the LSA Have to have carb heat?
    Thanks for your time.

  2. #2
    Dana's Avatar
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    I assume you mean you're building an E-AB that meets LSA specs, because if you're building an LSA kit you have to set it up exactly per the plans.

    On an experimental I'm pretty sure there's no legal requirement for carb heat, but some DARs will insist on it being there anyway.

    But... just because you've never seen carb ice on these engines in the winter doesn't mean anything, carb ice isn't common in winter. It's cool to warm humid days that it can get you. That said, ice is rarely a problem with 2-strokes and slide carburetors... but it can happen.

  3. #3

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    Thanks for the reply. I can add electrical carb heat very easy and will probably do that. The way airplanes do it with heated intake air can be done but is a lot more work I wish not to do.

  4. #4
    Mel's Avatar
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    Many aircraft that use snowmobile engines take the intake air from the plenum around the engine. This air is already warmed and may not need addition heat. OTOH, if you are using external "ram" air for carb intake, additional carb heat may be a good idea. Carb heat is not a "requirement" for certification, but an inspector can deny for any reason he/she feels is a safety factor. BTW, these types of carbs can ice up. It's somewhat rare but has happened.

  5. #5

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    Quote Originally Posted by Mel View Post
    Many aircraft that use snowmobile engines take the intake air from the plenum around the engine. This air is already warmed and may not need addition heat. OTOH, if you are using external "ram" air for carb intake, additional carb heat may be a good idea. Carb heat is not a "requirement" for certification, but an inspector can deny for any reason he/she feels is a safety factor. BTW, these types of carbs can ice up. It's somewhat rare but has happened.
    This engine is a water cooled 2-stroke so to get warm air I would need to make something that goes around the exhaust. I can get electric carb heat to insert on the side of the carbs like on the Yamaha rhino. Hopefully that will be fine because I don't want to do the other way using the exhaust.

  6. #6
    DaleB's Avatar
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    Personally... if I were doing it, I'd think about an electric heater strip wrapped around the carb throat, with an automatic thermostatic control. Then you have carb heat, when it's needed, without having to do anything yourself. Let the electrons do the work, they don't charge anything. Well, they do, but... you know what i mean.
    Measure twice, cut once...
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    Flying an RV-12. Building a Fisher Celebrity.

  7. #7
    rwanttaja's Avatar
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    Quote Originally Posted by DaleB View Post
    Personally... if I were doing it, I'd think about an electric heater strip wrapped around the carb throat, with an automatic thermostatic control. Then you have carb heat, when it's needed, without having to do anything yourself. Let the electrons do the work, they don't charge anything. Well, they do, but... you know what i mean.
    Dunno if it's my being a bit of a luddite, or all the 737 Max discussion, but I'm a bit skittish about systems that automatically turn themselves on or off. I'd rather have a big warning light on the panel and a manual control.

    Besides, one normally doesn't use carb heat at full power, since the butterfly is all the way open. A system based solely on carb temperature might heat the air when you really would prefer it didn't. You could add a second sensor plus logic to inhibit the heater when the throttle is full open, but now you're departing from a simple system to a more-complex one. Pretty soon you've got multiple sensors for redundancy, digital logic that needs validation, cats and dogs living together, etc.

    Ron "It's true...the man has no carb heat" Wanttaja

  8. #8
    DaleB's Avatar
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    Easy peasy, Ron. Microswitch to disable it above whatever throttle position you want. We're not really talking about a complex system here; just a thermostatically controlled heater for the carburetor itself. But why even worry about that? We're not heating the incoming air. Who cares if the air coming in is cold and damp? What we care about is not letting it form ice or frost in the carb, right? So warm the carburetor, not the air. You'll still feed cold air to the engine, so you actually probably could leave it on all the time.

    I'd give a system like that a probability of failure only slightly worse than that of a Bowden cable, and significantly better than that of the gray mush between the pilot's ears.

    Just an idea. Maybe a crazy one, I don't know... just an idea.
    Measure twice, cut once...
    scratch head, shrug, shim to fit.

    Flying an RV-12. Building a Fisher Celebrity.

  9. #9
    Dana's Avatar
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    The commercially available electric carb heat systems I've seen for Rotax (or any engine with a Bing carburetor, I guess) sandwich the electric heater between the carb and the manifold, so it heats the carb directly, not the air. Mostly I think they're left on all the time, running directly from the lighting coil.

  10. #10

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    The atmospheric conditions that are most likely to promote carb ice are temp 25-65f and humidity 65-100%. Keep in mind that it's not just a matter of the cool air/venturi, it's also the fuel being vaporized that cools the inside of the carb to below freezing. The ice forms on the butterfly as well as the walls of the carb, so if your going to use a system that heats the outside of the carb body, it must get the heat all the way to the butterfly and heat it up above freezing. Systems that heat the incoming air are more efficient because it puts heated air directly on all the insides surfaces. Heating up the inside of the carb from the outside is going to be very inefficient. If ice forms inside the carb to a point that it causes a reduction in power, it is unlikely an external system would heat up the carb fast enough to deal with the emergency. That leaves a system that operates all the time, monitoring the temp of the carb and either a manual control like a rheostat to adjust the temp or a thermostat to automatically control it. I've flown planes with carb temp indicators and a manual control to maintain a certain temp range in the carb. It's a nuisance.

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