Piston Engines - Icing — Page 120, Lesson 151

Let’s start with the big picture. I want you to understand that icing in a piston engine is not one single problem — it’s three distinct forms, and each one forms in a different place for a different reason. As a pilot, you need to know all three, because the way you react to each one, and the way you apply heat, depends on which type you’re dealing with. The first form is impact ice. That’s ice that forms on the air filters and on the bends in the induction system. Think of it as ice literally impacting — striking — the surfaces it meets as the air is drawn in. It builds up where the airflow first hits something, like the filter or a change in direction in the ducting. The second form is refrigeration ice, which is also called carburettor icing. This one forms inside float type carburettors. The cause is the low temperatures produced by two things working together: fuel vaporization and low pressure acting on the moisture in the atmosphere. I’ll come back to this in detail in a moment, because it’s the one you’ll hear most about. The third form is fuel icing. This is caused by moisture that is already in the fuel itself. That moisture comes out of suspension — meaning it was dissolved or suspended in the fuel, and now it separates out — and then it gets frozen by the low temperatures in the carburettor. This ice tends to stick to the inlet manifold around the corners, and as it builds up it reduces the air/fuel flow into the engine. So notice the contrast: impact ice and refrigeration ice involve moisture from the air, but fuel icing involves moisture from the fuel. Now let’s focus on carburettor icing, because that’s the heart of this section. If you’re flying an aircraft fitted with a carburettor and you’ve failed to anticipate the problem, the first indication you’ll see is a gradual drop in rpm. That drop may be accompanied by engine rough running and vibration. But here’s an important distinction: if the aircraft is fitted with a constant speed propeller, the indication changes. Instead of an rpm drop, you’ll see a drop in manifold pressure, or a reduction of airspeed in level flight. Why the difference? Because with a constant speed propeller, the propeller governor is holding the rpm constant, so the engine can’t show you the problem through rpm — it shows up as a loss of manifold pressure instead. That’s a key contrast to remember. So why does the ice form in the first place? There are two contributing causes. First, there’s rapid cooling in the throat of the carburettor, because heat is absorbed from the air during the vaporization of the fuel. When fuel vaporizes, it takes heat from the surrounding air — that’s the refrigeration effect. Second, there’s the low pressure area in the Venturi tube. The Venturi is the narrowing in the carburettor that speeds the air up and drops its pressure, and that low pressure also cools the air. Both effects together drive the temperature down. The result is dramatic. The temperature in this area of the carburettor can drop as much as 22°C, which is 70°F, below the temperature of the incoming air. So even on a day when the outside air is well above freezing, the air inside the carburettor can be cold enough to freeze moisture. If the air contains a large amount of moisture, this cooling process may be sufficient to cause ice to form in the area of the throttle “butterfly” — that’s the throttle plate, the valve that controls how much air enters the engine. Now here’s the danger sequence. As ice forms on the throttle butterfly, it reduces the area of the induction intake. That means less air can get through. It may also prevent operation of the throttle plate itself — the ice can physically jam the plate so it can’t move. The result is a loss of power. And if you don’t correct it, the ice may accumulate sufficiently to block the intake completely and stop the engine. So this isn’t just a performance nuisance — it can be a full engine stoppage. Finally, there’s a critical temperature limit you must know. At temperatures of -1°C, which is 14°F, or below, any moisture in the air will already be frozen. It will pass straight through the carburettor as ice crystals, and it won’t form the kind of icing we’ve been describing. So at those temperatures, heat should not be used. That’s a hard rule — below -1°C, applying carburettor heat is not appropriate, because the moisture is already frozen and the heat serves no purpose. So let me pull that together. Three forms of icing: impact ice on filters and bends, refrigeration or carburettor icing in float carburettors from vaporization and low pressure, and fuel icing from moisture in the fuel freezing and sticking to the inlet manifold. The carburettor icing indications differ depending on whether you have a fixed pitch or constant speed propeller. The temperature can drop 22°C below the incoming air, ice forms at the throttle butterfly, reduces intake area, can jam the throttle, causes power loss, and can block the intake completely. And below -1°C, don’t use heat. That’s the full picture of induction icing in a piston engine.