Most of us understand the basics of carburetor ice. Moisture condenses in the carburetor to form ice, blocking airflow to the engine, reducing engine power and eventually causing engine failure. Operating at less than full power increases the risk of carburetor icing (such as on approach to land).
Here are a few facts so that we can understand this further:
- air temperatures can drop by as much as 33 degrees celcius in the carburetor (due to the pressure drop as well as the fuel mixing and vaporizing into the air coming in).
- Even a small amount of carburetor ice can result in power loss.
- If enough ice builds up, the engine may cease to operate.
- Carburetor ice can be detected by noticing a drop in RPM (fixed pitch propeller), a drop in manifold pressure (constant speed propeller), the engine running rough, or a combination.
- It is possible for carburetor icing to form even when the skies are clear and the OAT (outside air temperature) is as high at 90F if the relative humidity is 50% or more.
- Carburetor ice is most likely to occur when temperatures are below 70 degrees Fahrenheit (°F) or 21 degrees Celsius (°C) and the relative humidity is above 80 percent. Due to the sudden cooling that takes place in the carburetor, icing can occur even in outside air temperatures as high as 100 °F (38 °C) and humidity as low as 50 percent.
It’s a warm September evening as you fly your Cessna 150 (carbureted, fixed-pitch propeller) towards Savanah, Georgia. You expect to land a few minutes before sunset. You complete your normal “flow” when you are abeam your touchdown point on downwind and begin your descent to land (except you forget to apply carburetor heat as you reduce power as the POH calls for). While on short final you begin sinking below glide path and attempt to add power to correct your glide path, but you don’t get the power increase you expect and the engine runs a little rough.
It would have been better for you to notice the gradual decrease in RPM to recognize the signs of carburetor ice earlier on, but with changing airspeeds and your attention focused on the runway ahead it is easy to miss the signs of carb ice forming. While you may be just fine here applying carburetor heat and full power to gain enough energy to make it to the runway safely, other times carburetor ice can cause such a loss of power that you are unable to maintain altitude or land the aircraft where you want it to go. Always being ready and watching for the signs of carburetor icing can be a good risk mitigation technique. Remember it is rarely too warm or too cold outside for carburetor ice not to form, and it can form during ANY phase of flight. Always fly your airplane giving yourself room for errors and the unexpected (i.e. more runway, more altitude, and using carb heat liberally when the potential for carburetor icing exists).