Carburetor icing, and carb heat in Cessna 152/172

Carburetor icing and heat in Cessna 152/172

Carburetor icing, or carb icing, is a problem that can happen in the aircraft’s fuel system due to the formation of ice from water vapor in the atmosphere, causing a partial or full blockage that can prevent air/fuel mixture to enter the engine.

To countermeasure this issue in aviation, every aircraft engine uses a Carburetor fuel system equipped with Carburetor heat, or Carb heat, preventing the ice formation and melting it in case of its formation.

Carburetor icing development

The development of ice in the Carburetor can be caused by the temperature drop in the carburetor, because of fuel vaporization in the venture tube canal, and the temperature drop associated with the pressure drop to suck the fuel out of the fuel nozzle.

The Venturi effect can drop the ambient air temperature by 70 Fahrenheit (F), or 38.9 Celsius (C). So the possibility of carburetor icing exists, when the ambient air temperature is below 70 degrees F (21 degrees C) and the relative humidity is above 80 percent, this condition is most favorable to the development of carburetor icing.

Icing is also possible during warm air temperatures, when ice forms on the surface of the carburetor throat, increasing the Venturi effect and dropping the temperature furthermore, carburetor icing is most severe at the formation of ice around the mechanical parts of the carburetor like the Throttle butterfly, causing the pilot to lose control of the engine power.

Carburetor icing temperature range

The temperature range for possible ice formation consists of two envelops:

  1. The first envelope, the Average possibility of ice formation, starts from 20 F (-7 C) to 100 F (38 C), with air relative humidity from 50% to 80%.
  2. The second envelope, High carburetor icing potential, starts from 20 F (-7 C) to 70 F (21 C), with air relative humidity from 80% to 100%

Indications of carburetor icing   

Carburetor icing indications are the effects of the air blockage aftermath, when the ice starts to build pilots can start to notice a loss in engine power because of the decreased air mass entering the combustion chamber in the cylinders.

Ice continues to build up, preventing more air to flow, and causing higher fuel/air mixture leading to running richer, increasing the engine temperature. After building a high amount of ice in the carburetor, ice starts to cover mechanical parts like the throttle valve causing the valve to jam and lose control of the engine power/throttle.

Continuation of ice building without intervention will decrease the airflow to a critical point, where no chemical reaction could happen in the cylinder, which will lead to engine misfire ignition causing the engine to stall and shut down.

Carburetor heat Cessna 172

Carburetor heat or Carb heat used in the Cessna 172 is a subsystem of the fuel system in an aircraft piston engine, it works as a valve, controlled by the pilot to change the amount of hot air, drawn from a heat exchanger( sometimes from carb heat shield) connected to the exhaust system, to flow to the carburetor to the engine.

This hot air is used to prevent or melt ice developed in the carburetor, by raising the temperature in the venturi section high enough to prevent or remove any ice build-up during engine running.         

Applying carburetor heat will change the fuel/air mixture entering the engine because hot air is less dense than cold air, engine power will drop when carburetor heat is used.

A heated carburetor will make the venture effect less efficient in lowering the pressure to draw the fuel from the fuel nozzle, decreasing the amount of fuel sucked to the intake. Lower fuel/air mixture will make the engine run lean causing more loss of power.

A pilot should use carburetor heat in these circumstances to prevent any sudden stall or losing power/speed control:

  • During Takeoff
  • During approach and landing
  • Specific weather conditions may cause icing

How to detect carburetor ice in Cessna 152?

When carburetor ice has formed, The Cessna 152 – Is a fixed-pitch propeller aircraft, carburetor icing will be detected by a decrease in engine RPM, engine hesitation, and running rough.

Engine manifold temperature may also increase due to the rich mixture caused by the air blockage.

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