Ice and Rain Protection — Page 247, Lesson 310

Let me walk you through the start of the Ice and Rain Protection chapter. This is a big subject, so I want to begin by giving you the full picture of what gets protected, and then we'll dive into the first mechanical de-icing system in detail. First, the list of protected components. This isn't meant to be exhaustive, but it gives you an idea of the variety of systems and components that are protected against the effects of ice and rain. Let me go through them by category. On the engine side, we protect the intakes, the IGVs — that's inlet guide vanes, the vanes that straighten the airflow entering the compressor — and the struts or webs that support those vanes. We also protect oil cooler intakes and fuel system filters. Then there are ram air intakes, which are used for generator cooling or engine bay ventilation. Moving to the aerofoils — that's the technical term for the wing and tail surfaces. We protect the wing and tail leading edges, the slats, and the propellers. On the airframe itself, we protect the aerials, the waste water outlet horns, and the large fences and bullets — those are the aerodynamic fairings you see on some aircraft. In the instrument systems, we protect the pitot heads and probes, which are critical for airspeed and altitude sensing. And finally, the cockpit windows. Now, the first system we look at is mechanical de-icing, specifically pneumatic de-icing. These systems are employed in certain types of piston-engined aircraft and twin turbo-propeller aircraft. The number of components varies from system to system, as does the method of applying the operating principle. The arrangement of a typical system is shown schematically in Figure 12.9. Let me be precise about what a de-icer boot is, because this is the heart of the system. The de-icer boots, also called overshoes, consist of layers of natural rubber and rubberized fabric. Between these layers are disposed flat inflatable tubes that are closed at the ends. The tubes themselves are made of rubberized fabric and are vulcanized — that means chemically bonded — inside the rubber layers. Here's the key construction detail. In some boots, the tubes are arranged so that when the boot is in position on a wing or tailplane leading edge, the tubes run parallel to the span — that's the lengthwise direction of the wing, from root to tip. In other boots, the tubes run parallel to the chord — that's the direction from the leading edge back toward the trailing edge. So you have two possible tube orientations, spanwise or chordwise, depending on the boot design. Now, how do these tubes get their air? The tubes are connected to the air supply pipelines from the distribution valves system. This connection is made by short lengths of flexible hose, which are secured to connectors on the boots on one end, and to the pipelines by hose clips on the other end. There's one more important construction detail. The external surfaces of the boots are coated with a film of conductive material. This is there to bleed off accumulations of static electricity. That's a critical safety feature — as the aircraft flies through the air, friction can build up static charge on the rubber surface, and that conductive film gives the charge a path to bleed away safely. Finally, how is the boot attached to the leading edge? Depending on the type specified, a boot may be attached either by screw fasteners, which are called rivnuts, or by cementing them directly to the leading edge. So two attachment methods: mechanical fastening with rivnuts, or adhesive bonding with cement. So to summarize the operating principle: you have a rubber boot on the leading edge with inflatable tubes inside. When ice builds up, you inflate those tubes with air from the distribution valves, the boot surface expands and flexes, and that mechanical action cracks and breaks off the ice. That's why we call it mechanical de-icing — it's a physical flexing action, not heat. That's the foundation of pneumatic de-icing. When you're ready, we can move on to the next system in the chapter.