Ice and Rain Protection — Page 257, Lesson 325

Let’s pick up with the rain repellent system, because it’s the first of three windscreen protection methods we’re covering here. This one works alongside the wipers, not instead of them. The idea is simple: during heavy rain, you spray a special fluid onto the windscreen, and that fluid forms a thin film that makes water bead up and blow away, so the wipers can clear the screen more effectively. Here’s how the pilot actually operates it. There’s a rain repellent button switch on the captain’s or first officer’s wiper control panel. When you momentarily press it, fluid is sprayed onto the respective windshield. Now, here’s a key detail: each actuation of the switch opens the container valve for approximately one third of a second, regardless of how long you hold the switch in. So even if you hold it for two seconds, the valve only opens for about a third of a second. That’s a deliberate design to give a fixed dose every time. How long does that dose last? Depending on airspeed and rain intensity, each actuation should be adequate for 2 to 5 minutes. So at higher speed or heavier rain, it might only last two minutes; in lighter conditions, closer to five. A fully charged container holds about 75 applications. So you can count roughly 75 presses before you need to refill. One important caution: repellent applied to a dry windscreen will reduce visibility. So you only use it when the screen is wet. And you should avoid using both systems simultaneously — that is, don’t run the rain repellent and the wipers at the same time, because the repellent film can smear or interfere with the wiper action. See Figure 12.15 for the layout of that system. Now let’s move to the second method: the Fluid De-icing System. This one is different in purpose — it’s for ice, not just rain. The method is to spray the windscreen panel with a methyl-alcohol based fluid. Methyl alcohol, or methanol, is the key agent here because it lowers the freezing point of water, so it melts ice and prevents it from re-forming on the glass. The principal components are: a fluid storage tank, a pump — which may be hand-operated or electrically operated — supply pipe lines, and a spray tube unit. So fluid sits in the tank, a pump pushes it through the supply lines, and it exits through the spray tubes aimed at the windscreen. Figure 12.16 shows a typical system where fluid is supplied to the spray tubes by two electrically operated pumps. The system may be operated using either of the pumps or both, according to the severity of icing. So in light icing, one pump is enough; in severe icing, you run both to get more flow. Now the third method, and this is the most sophisticated: the Electrical Anti-icing System. This one doesn’t use fluid at all. It employs a windscreen of special laminated construction that is heated electrically. The purpose is twofold: to prevent the formation of ice and mist, and also to improve the impact resistance of the windscreen at low temperatures. So it’s not just about keeping the glass clear — it’s about keeping the glass strong when it’s cold and brittle. The heating element is a film-type resistance element. It’s heated by alternating current supplied from the aircraft’s electrical system. So it’s AC, not DC. The power required for heating varies according to the size of the panel and the heat required to suit the operating conditions. Bigger panel, more power; colder conditions, more power. Now here’s the critical part: the circuit embodies a controlling device. Its function is to maintain a constant temperature at the windscreen and also to prevent overheating of the vinyl interlayer. That vinyl interlayer is the middle layer of the laminated windscreen, and if it overheats, it suffers permanent damage — specifically vinyl ‘bubbling’ and discolouration. So the controller has two jobs: keep the temperature steady, and never let it get hot enough to ruin the vinyl. How does the controller know the temperature? In a typical anti-icing system, shown schematically in Figure 12.17, the controlling device is connected to two temperature sensing elements laminated into the windscreen. These elements are usually in the form of a fine wire grid, and the electrical resistance of that grid varies directly with the windscreen temperature. So as the glass heats up, the resistance rises; as it cools, resistance falls. That resistance change is what the controller reads. Now, why two sensing elements? Because they have different jobs. One sensing element is used for controlling the temperature at a normal setting — that’s the primary control loop. The other is used for overheat protection — it’s a safety backup that catches a failure in the primary loop and prevents the vinyl from being damaged. Finally, the system includes a set of warning lights and, in some cases, magnetic indicators, as part of the control circuit. These provide visual indications of circuit operating conditions — for example, ‘normal’, ‘off’, or ‘overheat’. So the pilot can see at a glance whether the system is running normally, is switched off, or has detected an overheat condition. So to summarise the three systems: rain repellent for heavy rain, used with wipers; fluid de-icing with methyl alcohol for ice; and electrical anti-icing with a heated laminated windscreen, which also strengthens the glass. Each has its own operating principle and its own limitations.