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ALC-33: Inflight Icing
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Induction Ice

There are two major types of icing:  induction and structural.

Induction icing consists of any ice accumulation that blocks the venturi, air filter, ducting, and/or fuel metering device. Impact ice, a type of induction icing, can occur anywhere that temperatures are near to, or colder than, the freezing point of 0° C. Impact ice can block the air filter and rob the engine of air needed for combustion, even on a fuel injected engine. If you suspect impact ice, activate alternate air or carburetor heat as directed by your POH/AFM.

Carburetor icing (ice that forms in the carburetor) occurs when the drop in air pressure inside the venturi causes rapid cooling of moisture-laden air, or due to vaporization of fuel. 

The resulting ice accumulation in the carburetor intake tube can greatly reduce engine performance.  In severe cases, it can reduce intake flow to the point that the engine may stop.

Carb heat is both an anti-ice and a de-ice system.  By pre-heating air before it enters the carburetor and preventing ice formation, it is an anti-ice system.  By melting ice that has already accumulated inside the carburetor, it is a de-ice system.  Remember, though, that carb heat cannot eliminate a large ice accumulation.  In addition, use of carb heat results in decreased performance since the warmer air is less dense.

Carburetor icing is a deceptive hazard.  Unlike impact ice, carburetor icing often occurs when outside air temperatures are well above freezing.

Application & Risk Management

  • A fuel injected engine does not prevent impact ice.
  • If you suspect impact ice, activate carb heat or, for fuel injected engines, alternate air.
  • Expect carb icing when relative humidity is high and temperatures are between 20°F and 70°F.  Indications of carb ice include rough running engine, and loss of RPM (fixed pitch propeller) or loss of manifold pressure (constant speed propeller).
  • In general, apply carb heat or alternate air immediately if you suspect carb icing.   Be prepared for an initial additional decrease in engine performance as the ice melts and moves through the system.
  • Always follow specific manufacturer's recommendations.