Carburetor Ice
By: Bob Robertson & Bud Connolly

Although carburetor ice is not a common occurrence on Rotax engines, it can happen when conditions of temperature and humidity/dew point come together perfectly. Here is a photo sent to us by Bob Robertson who runs a Rotax Service Center, Light Engine Services Ltd, and also sells engine controls, Aero Controls Ltd, in Alberta, Canada.

The photo is of a Bing carburetor on a Rotax engine. The engine in question was on a powered parachute operating in the Pacific Northwest, Washington State, USA. You can see the ice on the front of the slide valve as well as around the diffuser located in the bottom of the carburetor intake throat. It's the ice around that diffuser that causes the real problem in most cases. It disturbs the air flow around that section and reduces the amount of fuel mix that can be drawn up through the needle jet opening.

Typically, carburetor ice problems will show up first during partial throttle operation. The engine will begin losing power and the RPM will slowly drop back. The immediate response for the pilot is to push the throttle full forward which will open the slide valve and help to shed a little of that ice. If the throttle responds to full power, or nearly full power, you have a little more time to make a decision about landing. Descending to a lower (warmer) altitude will often help melt the ice.

Find a good airstrip though, and make a precautionary landing. As quickly as possible, shut down the engine after landing and look at the carbs. If you see frost on the outside of the carb, or even a lot of water dripping off the outside, you can be pretty sure you just experienced carb ice. You can let the plane sit there for 15 minutes or more, during which the ice can melt from the engine heat. Then start it up again and check for full throttle response. If all seems well again, you can fly. But be aware that you may very well get more carb ice, especially at the altitude you were previously flying. If you are at your home field it's probably best to just wait until the temperature rises, or the humidity is less, or both.

For further information on when to expect carburetor icing conditions, see the links below. Notice that even at 80 degrees F. you can get carb ice if the humidity is above 50%.