Helicopter Flight Information

Airframe, Carburetor, and Induction Icing

Confused, Misinterpreted, and often Mis-taught

Airframe Icing

All that is required for airframe icing to occur is visible moisture, and temperatures conducive to it. This does not mean that the temperatures have to be at freezing (0°C or 32°F), only near it. As the airfoil moves through the air, the venturi effect that creates lift also cools the air as it flows over the airfoil. Just add moisture, and you have ice. Moisture also adds to the cooling effect and as a result temperatures are cooler still.

On an airplane, this can be seen on the wings. On a helicopter, icing on the rotor blade is not visible, but you can be sure that it is there under the right conditions. If you see ice build on the windshield or anywhere else, find a landing area and wait this one out. Don't risk an accident thinking you can fly clear of the icing conditions.

Airframe icing can kill you, make no mistake about that. Icing of any king is about as serious a condition as you can ever find yourself in, in aviation. Although you will hear some pilots brag of flying in ice until they were pulling so much power they had to land, don't let this lead you into a similar situation.

Induction Icing

Induction icing is when the air intake ices over, or plugs up with ice that has built up on the surface similar to airframe icing. The conditions conducive to it are also the same as airframe icing as this is just another form of the same.

Carburetor Icing

Carburetor icing is the one that gets a lot of unsuspecting pilots in trouble because it is so misunderstood. If the temperatures are cold, and there is rain or snow, icing is understandable, but when the sun is shining, and there is not a cloud in the sky, how can the engine stop from ice?

First it is necessary to understand how the carburetor works, and how it affects the air flowing through it. In an exaggerated form, the carburetor is shaped similar to an hour glass; it is smaller in the middle than it is on either end. There are four basic sections in the carburetor as follows.

The narrow middle section is the venturi section, which is also the low pressure area. Inboard (engine side) of the venturi is the suction area which contains the throttle plate, obviously varied by the throttle control. Outboard of the venturi is the intake end of the carburetor where there is sometimes a choke plate, and always a filtering system of some sort even if it is just a course screen to keep out boulders. Below the carburetor is a float bowl where a small amount of fuel is collected for operation.

Remember Bernoulli's principle, just as there is a low pressure above a wing, there is a low pressure at the venturi in the carburetor. Imagine a small tube extending from the filter area to the top of the float bowl allowing atmospheric pressure into the area above the fuel. Then imagine a small tube extending from the venturi to the bottom of the float bowl. The higher atmospheric pressure above the fuel in the float bowl pushes fuel up the tube into the venturi where it is vaporized as it mixes with the inflowing air.

As the air passes through the venturi, the moisture contained in the air condenses on the inside of the carburetor in the area of the throttle plate. Also as this air passes through the venturi it is cooled tremendously especially as the fuel is introduced into the flow. Once the temperature inside the carburetor drops to freezing, ice begins to accumulate. The engine will begin to lose power, and at some point it will stop running.

It is often taught that carburetor icing occurs at any temperature between +20 and +70 degrees F, with visible moisture present. The problem with that teaching is that visible moisture is not necessary for carburetor icing to occur. 50 percent relative humidity is plenty for carburetor icing, and you cannot see that. Also, according to the the ASF, carburetor icing can occur at outside temperatures as high as 90 degrees F. I do not dispute that icing can occur at temperatures as high as 90F, but I think it would be a rare case.

I do maintain the I would not bet my life on an outside temperature gauge to the point that I would fly without carburetor heat applied at temperatures indicated cooler than 75 degrees F. Especially when I am flying at an altitude where if I did have carburetor icing, I would be on the ground before I could do anything about it.

Remember first that the carburetor heat is a preventative measure, not a cure. Remember also that when you have carburetor icing you will, at the very least, experience a significant power loss, and possibly even complete engine shutdown (it happened to me once).

Remember that it will likely take many hundreds of feet to recover full power, and you need to conduct your flying as you have been trained in the even of a power failure. You must also fly at altitudes (at all times) that will permit sufficient glide distance should you need it.

Remember that from an altitude of 2500 feet AGL, you will be on the ground in just 1.4 minutes (most helicopters with a sink rate of 1800 fpm). You need to do everything right, and the place to start is to apply carburetor heat anytime the outside air temperature is indicated less than 75 degrees F, as well as fly at altitudes appropriate to the conditions.

Remember that in a helicopter, you must apply carburetor heat at all times to keep the temperature gauge either in the green, or out of the yellow as appropriate. Also remember that if you have carburetor icing in a helicopter, the engine will shutoff, and there will not be time for an air-restart (unless your altitude is higher than 4000 feet AGL). If you have engine stoppage due to carburetor ice, it will take several minutes for the ice to melt enough for a restart.

If you ever have an engine failure whether it is from ice or otherwise, you must remember to follow engine failure procedure, and establish a stable autorotation while focused on your landing zone, all else is secondary.

Fly safe!!

Send email to rb@helicopterflight.net with questions or comments about this web site.
Copyright © 2003-2010 All Rights Reserved.
Last modified: 08/24/10