Over the years I have written a lot on this subject, but a couple of test stand photos can do wonders to illustrate a point that flight instructors rarely teach correctly anymore. For a much deeper look at the subject, follow this link: Critical Understanding #10 – Carb Ice.
Mike Loevin’s Corvair, running on my stand at Corvair College #43. The white section of the intake manifold above the carb is solid white ice, and it is 48F outside.
The digital Thermometer shows the temp has dropped 30F going through the carb. There is identical ice on the inside of the manifold also. This is simple physics in action, and this is why planes have carb heat. Contrary to popular myth, injected engines do this also, they are jest less prone to quitting when the throttle is advanced when the pilot notices the rpm drop. No carb should be assumed to be immune to this. Popular hangar talk says Elison’s are not vulnerable, but cast right into the Elison body is the phrase “CARB HEAT REQUIRED”.
Another look at the engine. Look at the prop blade and the balancer; its running. the power setting is 2,000 rpm and 20″ MAP, to break in the cam. This is about 50% power. If anyone tells you to wait until the engine is reduced to idle to use carb heat, they don’t know what they are speaking of.
The picture above addresses one more myth, that it somehow need to be cloudy or rainy to get carb ice. Look at the sky in the picture, understand that anyone who claims you have to see dense clouds, fog or rain for ice to form is clearly mistaken.
Just to reiterate it one more time, this isn’t a Corvair issue. Rotax and to some extent, Lycoming, avoid this somewhat by having full time carb heat, but every competent pilot should understand the fact it can happen to any plane, and they should take the engine manufacturers recommendations very seriously and ignore they hangar myths.
9 Replies to “Carb Ice example from CC #43”
William Why not put the carb on top of the engine like it is in the car?
Sent from my iPhone Dennis McGuire Et padre, et filiu et spiritu sanste. Amen’
Dennis, if you look at the side view of the planes, you will see any carb on top of the engine won’t fit in the cowl. Having it on bottom is the traditional aircraft location for a number of reasons, led by the ability to use a gravity feed fuel system without pumps. In terms of carb heat, the carb below is actually on the hot side of the motor. In flight, the cooling air entering the cowling would further chill a carb on top. William
Additionally, a carb mounted under an engine can’t leak fuel down onto hot exhaust manifolds if there’s a problem.
I would add to what William wrote to point out that if one has a leaky carburetor internally, it can flood the engine or, in an extreme case, if the electrical fuel pump is left on, cause hydraulic cylinder lock. If the carburetor leaks externally, the leaked fuel can drip down on the engine, increasing the danger of a fire. When the fuel hits the hot engine, it vaporizes and it becomes possible, at the worst, to have an explosion, or even a possible blow torch effect with the propwash once ignited.
That being said, I have seen a V-8 auto engine conversion with the automotive downdraft carburetor on top. Needless to say, the builder was VERY CAREFUL about insulating the fuel line to prevent vapor lock from the heat rising and radiating from the engine, as well as the way in which the fuel system was assembled to prevent any leaks.
My Corvair test stand engine iced up just like this in Tuscon when it was 78 degrees outside and not a cloud to be seen.
Truly excellent stuff. Having experienced carb ice even on a Cessna 172 with an O-320 Lycoming. I can appreciate the problem.
My preliminary instruction was in an ’86 or earlier 172 w/carb. One thing bothered me..instruction to use carb heat was by event/rpm reduction, such as entering slow flight or just before the first reduction in power entering the pattern from the training area…I still wonder why and there must be a reason, we don’t have temp indicators for the manifold at the carb juncture? Even an ‘idiot’ (I hate the designation) light would be helpful. It would gut all the risk lover’s myths, anyway.
On my way to Oshkosh this past summer, I was cruising at 8000 feet at 3100 RPM. I noticed a drop in the RPM and a noticeable sound from the engine. I pulled car heat and it immediately went away. Proof that carburetor ice can happen anywhere and anytime.
My beater (100mi/day) commuter Toyota had an aftermarket carb on it. more than once it iced up just going along with the traffic. Sitting on the side of the freeway for 15 min long enough to pop the hood and wonder WTF the first few times it happened. Missing carb heat tube from manifold. Physics works with cars too evaporation ( vaporization) of the gas cools everything downstream. Any water vapor in the air will freeze directly without a liquid phase. People at sea level often do not realize that waters freezing point goes up as pressure goes down!!! Let me think increasing altitude cruising along with high intake vacuum from partial throttle. Read this :
Now pull on the carb heat and adjust the mixture when at less than full throttle. Just remember to push it off before going full throttle again. Reduced performance from using carb heat will not hurt you like carb ice will.