Carburetor heat is a system that preheats the air before it reaches the carburetor and thereby prevents or clears carburetor icing in automobile and piston-powered light aircraft engines. The icing itself occurs inside the carburetor when air accelerates through the venturi, causing temperature to fall, water vapor to freeze onto the throttle valve and other internal surfaces, and a build-up that reduces the fuel/air flow to the engine. Carburetor heat counters this hazard by ducting heat from an area near the exhaust back to the carburetor venturi, keeping the interior surfaces above the freezing point and maintaining normal engine operation.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is carburetor icing in aviation?
Carburetor icing occurs when the moisture inside an engine's carburetor drops below freezing, and this build up of icing inside the carburetor causes a reduction of the fuel/air flow through the carburetor and into the engine. Carburetor icing occurs due to fuel evaporation within the carburetor and a drop in pressure, and it can affect any carburetor under certain atmospheric conditions.
Carburetor icing is the most common of several types of induction system icings. It exists when water vapor freezes onto the throttle valve and other internal surfaces of the carburetor. The cooling caused by the throttle's Venturi effect lowers the air's temperature and condenses the water vapor, so ice forms even on a warm day and at any power setting. Because carburetor icing is an inherent risk when flying piston-driven aircraft equipped with carburetors, it affects nearly every pilot and is a common cause of general-aviation accidents.
What causes aircraft carburetor icing?
Carburetor icing is caused by the temperature drop in the carburetor. This drop results from two effects: pressure drop in the venturi and fuel vaporization. Air cooling caused by the venturi restriction draws heat away from the surrounding metal, while fuel evaporation further chills the mixture. At reduced engine power a partly closed throttle valve and lower intake temperature raise the risk of ice formation. Temperature plunges as much as 30°C (86°F) inside the carburetor, so icing occurs even when ambient temperatures reach 100°F (37.8°C) and humidity is only 55. Serious icing occurs from 20°F (-6.7°C) to over 90°F (32.2°C). In warm, humid conditions the probability of icing increases with the relative humidity of the atmosphere. Sufficient moisture makes water vapour freeze onto the throttle valve and other internal surfaces, and the resulting ice reduces fuel/air flow through the carburetor.
What does carburetor heat do in a plane?
Carburetor heat prevents ice from forming in the venturi and melts any ice already present. Redirected hot air raises the temperature inside the carburetor; the raised temperature melts the ice, and the melted ice is then sucked up into the cylinders. Warm air is less dense than cold air, so the same action enriches the mixture slightly and causes up to a 15-percent drop in power accompanied by a slight RPM decrease. Once the ice is cleared, the engine runs smoother. If the accumulation was heavy, temporary roughness and vibration are noticed while the ice melts.
A pilot turns on carb heat whenever icing conditions are probable: during flight in very heavy rain, during any descent if icing conditions are suspected, and any time power is reduced below 1800 RPM on Continental-powered aircraft or below the green arc on the tachometer in Cessna aircraft. Remos GX Light Sport manuals recommend turning on carb heat before any descent. In cruise flight, use the minimum amount of heat necessary to prevent ice from forming, and verify that the system is functional during engine run-up by observing the expected RPM drop.
What is the purpose of the carburetor heat control in a piston engine aircraft?
Carburetor heat control is a push-pull knob in the cabin used to apply carb heat. When there is danger of induction system icing the flight deck carburetor heat control is moved to the hot position. Pulling the cockpit knob closes a valve that shuts off the standard cool, filtered air intake, and a moveable flap draws hot air into the engine intake. The heat source prevents ice formation and melts any existing ice. Warmer air is routed from a shroud surrounding the hot exhaust manifold, keeping the fuel-air mixture above freezing so the carburetor can maintain the proper air-to-fuel ratio for combustion.
How does carburetor heat work in aviation?
The carburetor heating system starts within the cockpit. Pulling the cockpit knob closes the standard cool filtered air intake and the engine draws in warm air routed from a shroud surrounding the hot exhaust manifold. Heated air is warmer than the free air and less dense than colder air, so redirected hot air raises the temperature inside the carburetor. Raised temperature melts the ice, and melted ice is ingested by the engine.
To start the engine, the carburetor heat control is placed in the cold position so that the engine draws through the ram-air intake. Heat is applied only after the engine is running and ice is suspected. After clearing the ice, the carb heat knob is removed and the throttle is readjusted, because the enriched mixture is due to hotter and less-dense air that has less oxygen per volume to burn.
Carburetor heat works by sending warm air from the exhaust pipe into the air horn, which boosts the temperature of the air-fuel composition and thwarts the arrangement of ice. When the pilot hauls the carburetor temperature handle, the engine may show a small dip in RPM, then smooth away as power is revived.
