Ice and Rain Protection — Page 253, Lesson 316

I want to walk you through the air supply side of thermal ice and rain protection. This is the part of the system that actually produces the hot air that keeps the leading edges of the wings and tail clear of ice. There are three distinct ways to get that heated air, and I want you to hold all three in your head because they're the backbone of this whole chapter. The first is compressor bleed air. Here, hot air is tapped directly from a stage of the turbine engine's compressor. That air is extremely hot, so it gets mixed with a supply of cooler air in a mixing chamber before it passes into the main ducting. The reason for that mixing is simple — you need to bring the temperature down to something the ducting and structure can tolerate. In some systems, you'll find equipment like safety shut-off valves fitted. Their job is to guarantee that an air mass flow sufficient for all de-icing requirements is supplied, and that it stays within pressure limits acceptable to both the duct and the structural limitations. So you're balancing two constraints at once — enough flow to do the de-icing job, but not so much pressure that you burst the ducting. The second method is the heat exchanger system, and this one is typically found on aircraft powered by turbo-propeller engines. The heat exchanger unit is positioned so that exhaust gases can be diverted to pass between tubes, and outside air flows through those tubes and into the main supply ducts. So the ram air gets heated by the engine's exhaust, not by direct bleed. The supply of exhaust gases is usually regulated by a device such as a thermostatically controlled flap, fitted in the ducting between the exhaust unit and the heat exchanger. That flap modulates how much hot exhaust gas reaches the exchanger, which in turn controls how much heat gets transferred to the ram air. The third method is combustion heating. Here, ram air is passed through a cylindrical jacket that encloses a sealed chamber. Inside that sealed chamber, a fuel/air mixture is burned, and the ram air is heated by contact with the chamber walls. Now, note the detail — the air for combustion is not the same air being heated. It's derived from a separate air intake and is supplied to the chamber by means of a blower. So you have two separate air paths: one for combustion, one for the heated air that actually goes to the de-icing system. Now, before we move on, I want to flag the temperature control aspect, because it's the bridge to the next part of the system. Controlling the air temperature within the ducting and the leading edge sections is an important aspect of thermal de-icing system operation. And the methods adopted depend on the type of system — which is exactly what we're about to look at next. Let me show you the schematic of a pneumatic de-icing system so you can see how these controls and indicators fit together in practice. And here's a figure showing which areas of the aircraft get heated by anti-icing air, so you can see where all this hot air actually ends up. So to summarise where we are: three ways to make hot air — compressor bleed with a mixing chamber, heat exchanger using exhaust gases with a thermostatically controlled flap, and combustion heating with a separate combustion air supply. And the temperature control of that air is the critical piece that ties it all together. That's where we'll go next.