Training Tip: Charting the carb-ice risk

July 15, 2013

There’s nothing like a summer heat wave to take your mind off all things cold and icy. In aviation, however, that’s not entirely warranted, as some pilots discover when experiencing a sudden engine-choking encounter with carburetor ice on a hot and humid day.

Then, if a pilot doesn’t respond correctly by applying carburetor heat as the aircraft’s operating checklist recommends, the encounter can progress to a complete engine failure—and an off-airport landing.

Student pilots learn that under the right conditions, moist air passing through a float-type carburetor throat can chill below the freezing point, depositing ice that chokes off the flow of the fuel-air mixture. Student pilots also learn that generally, a high-risk temperature range is between 50 and 70 degrees Fahrenheit.

Accidents indicate that some pilots fail to recognize conditions conducive to carb-ice formation, and its symptoms. But there’s no reason to be caught completely off guard about the risk.

When accident investigators review conditions encountered by an aircraft that lost power in flight, one of the tools they use in determining an accident cause is a carburetor icing probability chart. Note how this valuable graphic illustrates that an increase in the air’s relative humidity elevates the carb-icing risk in two flight phases: cruise power and glide power. Note also how risk rises at higher ambient temperatures in both phases.

Just as important as knowing when carb ice is likely to form is the ability to recognize symptom of icing’s onset.

“It is imperative for a pilot to recognize carburetor ice when it forms during flight because a loss of power, altitude, and/or airspeed will occur,” explains Chapter 6 of the Pilot’s Handbook of Aeronautical Knowledge. “These symptoms may sometimes be accompanied by vibration or engine roughness. Once a power loss is noticed, immediate action should be taken to eliminate ice already formed in the carburetor, and to prevent further ice formation. This is accomplished by applying full carburetor heat, which will cause a further reduction in power, and possibly engine roughness as melted ice goes through the engine.”

Applying carburetor heat can reduce power by as much as 15 percent. Strong evidence that ice was present is a gradual increase in power to an rpm or manifold-pressure value higher than just before heat was applied.

Start with the chart and avoid the carb-ice hazard!