Carburetor ice has caused sweat to break out on many a pilot’s brow. The Puget Sound area is ripe for carb ice conditions so we will review how carb ice forms and how to predict if you may get carb ice your next flight.
As air enters the venturi it accelerates to mix with the atomized fuel before splitting to the individual intake valves. While the air accelerates it also drops in pressure and temperature. If sufficient moisture exists in the air and is cooled to below freezing temperatures, the moisture will freeze to any solid surface, namely the walls of the venturi and butterfly valve.
It is important to remember that the more you accelerate the air, the colder it gets. If you are flying an aircraft like the Cessna 162 that has a Carb Temp gauge, try different power settings in the climb and see what effect it has on the Carb Temp. You will notice that as you add power, the temperature drops dramatically. This helps to explain why some aircraft and weather conditions are actually more susceptible to ice in a full power climb, like this pilot…. you can skip to 4:00 minutes.
To melt the ice the intake air must be raised to higher temperatures. Most airplanes accomplish this through a simple heat exchanger where intake air is directed over the exhaust manifold before entering the carburetor. Because hot air is less dense we do not operate with carb heat on under normal conditions. However, it is an effective method for melting ice when necessary.
When experiencing carburetor ice, it is not unusual for the engine to run worse when initially applying the carburetor heat. This is because as the hot air melts the ice, bits of ice and water are being ingested into the engine reducing performance and causing roughness. Typically this smooths out and resumes normal operation within a minute of applying heat. Care must be taken not to use carb heat for short intervals because melted ice may refreeze further up the venturi and exasperate the icing condition. When in doubt, leave the carburetor heat on for a longer period because the only downside is reduced engine performance from the less dense air.
Here is a YouTube video demonstrating carb ice,
Now most importantly, how do you know when to anticipate carb icing conditions? Here is a useful chart that shows to the probability of carb ice formation. You can see it is a function of outside air temperature and dew point to determine relative humidity.
In Puget Sound temperatures and dew points are routinely in the 40-60 degree range for the majority of the year. Seattle’s average daily high humidity is 90%. That puts you smack in the center of serious carburetor icing at cruise and idle power! Aircraft engine and carburetor design also play an factor. A quick internet search for your engine type and carb ice will tell you what other pilots are saying about their experiences. Here is a link to an NTSB report describing how carb ice affected an unsuspecting student and instructor during a normal training flight: http://www.ntsb.gov/AviationQuery/brief2.aspx?ev_id=20120111X05220&ntsbno=ERA12FA143&akey=1
Preparation is key so make a habit to include a carburetor ice prediction as part of your preflight planning and fly safe!