Carburetor Icing: PHAK Chapter 6

Handbook Reference

PHAK Ch 6

6.carburetor-and-carb-icing. Carburetor Icing

The float-type carburetor is one of two common fuel-metering systems found on reciprocating aircraft engines, the other being fuel injection. The carburetor mixes incoming air with vaporized fuel in the proper ratio for combustion and delivers that mixture to the cylinders. Outside air first flows through an air filter at the front of the cowling, then through the carburetor venturi, where the constriction accelerates the airflow and produces a localized drop in pressure. Atmospheric pressure in the float chamber forces fuel through the discharge nozzle into the low-pressure venturi, where it atomizes and mixes with the air. A throttle valve (butterfly) downstream of the venturi controls the volume of the fuel-air mixture delivered to the cylinders, while the mixture control regulates the fuel-to-air ratio.

The greatest disadvantage of the float-type carburetor is its susceptibility to induction icing. Two cooling effects occur inside the carburetor:

Vaporization cooling: as liquid fuel changes to vapor, it absorbs heat from the surrounding air.
Pressure-drop cooling: as air accelerates through the venturi, its temperature drops in accordance with the gas laws.

The combined cooling can lower the temperature of the air inside the carburetor by as much as 30 to 40 °F (approximately 70 °F in extreme cases) below ambient. If the air contains moisture, that moisture can freeze on the throttle plate, venturi walls, and discharge nozzle. The result is restricted airflow, a leaner mixture, and a loss of power. Left uncorrected, ice accumulation can cause complete engine stoppage.

Conditions favorable to carburetor ice. Because the cooling is internal to the carburetor, icing can occur even when ambient conditions seem benign. The classic danger zone is:

Outside air temperature (OAT) between 20 °F (-7 °C) and 70 °F (21 °C), with the highest risk between 50 °F and 70 °F.
Relative humidity above 80%, although icing has been reported at humidities as low as 50%.
Visible moisture is not required — humid summer air on a clear day is sufficient.
Most common at low power settings (descent, approach, taxi) because the partially closed throttle plate provides additional surface area for ice to form and reduces engine heat.

Indications. In an airplane equipped with a fixed-pitch propeller, the first sign of carburetor ice is usually a gradual loss of RPM, often accompanied by engine roughness. In an airplane with a constant-speed propeller, the governor masks RPM changes, so the first indication is a drop in manifold pressure (MP), with RPM remaining steady until the situation worsens. In either case, pilots should suspect carb ice anytime engine performance deteriorates unexpectedly, particularly during descent with the throttle reduced.

Carburetor heat system. To prevent and remove icing, aircraft are equipped with a carburetor heat (carb heat) system. Pulling the carb heat control routes induction air around or through a shroud over the exhaust manifold, where it is heated before entering the carburetor. Key operating considerations:

Carb heat air is unfiltered, so it should not be used routinely on the ground, where dust and debris can damage the engine.
Heated air is less dense, producing an enriched mixture and a slight drop in power (50–150 RPM) when applied. This drop confirms that the system is working.
Use carb heat as an on/off control in most light aircraft — partial heat can raise carburetor temperature into the icing range without melting existing ice (unless a carburetor air temperature gauge is installed).

Procedure when carb ice is suspected:

Apply full carburetor heat immediately.
Expect a further decrease in RPM or MP and possibly rougher running as the ice melts and water passes through the engine.
After a short period (typically 15–30 seconds), engine operation should smooth out and power should return to — and often exceed — the pre-icing value.
Return the carb heat to COLD once the ice clears, unless conditions warrant continued use.

Recommended use. Many POHs direct pilots to apply carb heat:

During the before-landing checklist and any prolonged descent at reduced power.
When runup is performed (to check operation; expect a small RPM drop).
Anytime atmospheric conditions are conducive to icing and the throttle is at a low setting.

Fuel-injected engines do not have a venturi or throttle plate icing problem because fuel is injected directly into the intake ports or cylinders, but they remain vulnerable to induction air filter icing, which is addressed by an alternate air source. Understanding the principles of carburetor operation and recognizing the conditions, symptoms, and corrective actions for carburetor icing are critical pilot skills, particularly in older training aircraft equipped with float-type carburetors such as the Cessna 152 and many 172 variants.

Oral Exam Questions a DPE Might Ask

Q1What atmospheric conditions are most conducive to carburetor icing?

Outside air temperatures between roughly 20°F and 70°F with relative humidity above 80%, with the highest risk between 50°F and 70°F. Visible moisture is not required, and icing is most likely at low power settings such as descent or approach.

Q2How would you recognize carburetor ice in a fixed-pitch versus a constant-speed propeller airplane?

In a fixed-pitch propeller airplane the first indication is a gradual loss of RPM, often with engine roughness. In a constant-speed propeller airplane the governor holds RPM constant, so the first sign is a drop in manifold pressure.

Q3What should you expect when you apply carburetor heat if ice is present, and why?

You should expect an initial further drop in RPM or manifold pressure and possibly rougher running as the ice melts and water is ingested by the engine. After 15–30 seconds the engine should smooth out and power should return to or exceed the previous value.