Ice and Rain Protection — Page 247, Lesson 312

Let’s pick up right where the boots themselves left off — because now we’re looking at what actually feeds them. I want to walk you through the air supplies and distribution for a pneumatic de-icing system, then the controls and indicators that run it. First, the air supply. The tubes inside the boot sections are inflated by air from one of three sources. The first is the pressure side of an engine-driven vacuum pump. The second is a high-pressure reservoir. And the third, on some turbo-propeller aircraft, is a tapping at an engine compressor stage — that means they bleed compressed air straight off the engine’s compressor. Now here’s the key contrast: inflation is pressure, but deflation is vacuum. At the end of an inflation stage of the operating sequence — and also whenever the system is switched off — the boots are deflated by vacuum. That vacuum comes from the vacuum pump, or, in systems that use the engine compressor tapping, from the Venturi section of an ejector nozzle. So the same pump that pushes air in can also suck it out, and the ejector nozzle’s Venturi creates the suction for the compressor-fed systems. Next, how the air gets distributed to the boots. The method depends on the de-icing system fitted to a particular aircraft type, but in general there are three methods in use. The first uses shuttle valves, which are controlled by a separate solenoid valve. The second distributes air to each boot through individual solenoid-controlled valves — so each boot gets its own valve. The third method uses a motor-driven valve to do the distribution. There’s a schematic diagram of this whole pneumatic de-icing system — let me show you that. Now the controls and indicators. What you need depends on the aircraft type and the particular arrangement of its de-icing system. In the basic arrangement, the controlling section includes a main on-off switch, pressure and vacuum gauges, or indicating lights. So you’ve got a master switch, and either gauges showing you the pressure and vacuum, or lights to tell you the state. Here’s the operating principle: pressure and vacuum are applied to the boots in an alternating timed sequence. The methods adopted usually vary with the method of air distribution I just described — so the shuttle-valve system times differently from the individual-valve system, and so on. But in most installations, timing control is effected by means of an electronic device. And one important professional note: for the details of the appropriate controlling system and the time cycles, you should always refer to the relevant aircraft Maintenance Manual. The exact cycle times aren’t universal — they’re specific to each installation, so the manual is the authority. So to tie it together: you’ve got three possible air sources, three possible distribution methods, and a control section with a main switch plus pressure/vacuum gauges or lights, all timed electronically in an alternating pressure-vacuum sequence. That’s the complete picture of how a pneumatic de-icing system is supplied and controlled.