Piston Engines - Icing — Page 120, Lesson 154

I want to walk you through the operational side of carburettor icing now — what you actually do when you suspect it, and the engine considerations that govern how you use the heat. This is the part where the theory turns into cockpit action, so let's take it step by step. First, the moment you suspect icing, the carburettor heat control goes to fully hot, and you leave it there. Not partially, not briefly — fully hot, and you hold it there long enough to actually clear the ice. That could take up to one minute, or longer, depending on how severe the build-up is. The key word is patience. The ice has to melt, and that takes time. Now, why fully hot and not partial? Partial heat should not be used unless the aircraft is equipped with a carburettor air temperature gauge. Without that gauge, you have no way of knowing what temperature you're actually delivering, and a partial setting can sit in the danger band where ice still forms. So the rule is simple: without the gauge, full hot or nothing. Let me explain what the heat actually is. The carburettor heat control provides heated air from around the exhaust pipe into the induction system. That hot air melts the ice, and the melted water then passes through the engine as water — it goes straight through the induction system and into the cylinders. And here's the trap: as that water passes into the cylinders, you may get engine roughness and further power loss. That's normal. That's the meltwater doing its job. Pilots should not be tempted to return the heat control to OFF — that's the cold position — thinking the situation has become worse since applying heat. It hasn't. The roughness is the ice melting, and if you pull the heat off now, you'll just re-freeze it and make things worse. Ride it out. Now, there's a timing factor. Icing is more likely during long periods of flight at reduced power — think glide descent, or the letdown for approach and landing. Why? Because the heat is derived from the engine. During a long descent, the engine temperatures gradually cool, and that reduces the effectiveness of the hot air system. You're trying to melt ice with a heat source that's itself getting cold. So the technique is: where icing conditions exist, select full hot air before reducing power. That way you gain the benefit of the hot engine before the engine temperature starts to drop. And to keep that heat source alive, you need to increase power periodically during the descent — to a cruising setting, at intervals of between 500 and 1000 feet. That periodic power increase does two things: it maintains engine temperatures so there's a sufficient heat source to melt any ice, and it prevents lead fouling of the spark plugs. That's a real operational rhythm — every 500 to 1000 feet down, a burst of cruise power. Carburettor icing can also occur on the ground — during taxiing at small throttle settings, or when the engine is at idle rpm. In those circumstances, the procedure is: use hot air before take-off to clear any ice, but then select cold air before opening the throttle to full power, and check that the correct take-off rpm or manifold pressure is obtained. And the absolute rule: under no circumstances should carburettor heat be used during take-off. Hot air at full power is a recipe for detonation and power loss right when you need maximum performance. Now let's move to the engine considerations — the factors you must understand when using carburettor heat. First, the application of hot air reduces the power output by approximately 15 percent. That's a significant hit. It also creates a richer mixture, which may cause rough running. So you trade power and smoothness for ice clearance. Second, heat should not be applied at power settings greater than 80 percent. Above that, there's a danger of detonation and engine damage. But here's the reassuring part: intake icing should not occur at power settings involving a wide throttle butterfly opening. Wide open throttle means high air velocity and high temperature rise through the venturi — conditions that don't favour ice formation. So the high-power regime is both where you don't need heat and where you must not use it. Third, continuous use of carburettor heat should be avoided. Why? Because of the change of mixture and the increase of engine temperatures. Heat should be used only for a sufficient period of time to restore engine power to its original level. And here's how you know it's worked: you'll see an increase of rpm or manifold pressure above the original setting when the control is returned to cold. That overshoot tells you the ice is gone and the engine is back to full capability. Finally, once you're clear of icing conditions, do not use carburettor heat — but check periodically that ice has not reformed. It's a monitoring task, not a one-time fix. So the whole picture is a cycle: suspect ice, go full hot, wait for the melt, accept the roughness and power loss, then return to cold and confirm the rpm or manifold pressure comes back above where it was. And throughout the descent, keep the engine warm with periodic power bursts every 500 to 1000 feet. That's the complete operational picture for carburettor icing.