Ice and Rain Protection — Page 257, Lesson 327

We're in the middle of the electrical anti-icing system now, so let's pick up right where the heating element is doing its job. I want to walk you through the control loop that keeps that windscreen at the right temperature, and then the safety nets that stop it from cooking itself. Here's the sequence. When power is applied through the system control switch and the power relay, the resistance element heats the glass. Now, the key part is the control element — its resistance changes with temperature. When the glass reaches a temperature that's pre-determined for normal operation, that change in resistance causes the control device or circuit to isolate the power supply to the heater element. In some systems, instead of fully isolating, it reduces the power supply. Either way, the heating drops off. Then, as the glass cools through a certain range of temperature, power is applied again, and the whole cycle repeats. So you've got a continuous on-off or high-low cycling that holds the glass at the normal operating temperature. That's the normal temperature control circuit. Now, what happens if the controller fails? This is where the overheat sensing element comes in. If the controller fails, the glass temperature will rise until it reaches the setting of that overheat sensing element. At that setting, an overheat control circuit cuts off the heating power supply completely, and it illuminates a warning light. So you get a clear cockpit indication that something's wrong. The power is restored and the warning light extinguishes only when the glass has cooled through a specific temperature range. So it's not just a momentary blip — the glass has to cool down by a defined amount before the system will re-energise. Now, some systems go further and incorporate a lock-out circuit. In that case, the warning light will remain illuminated, and power will only be re-applied by cycling the system control switch to 'OFF' and then back to 'ON'. So the pilot has to manually reset the system — it won't recover on its own. That's a deliberate design choice to force the crew to acknowledge the overheat condition. Let me also cover the high-power mode, because that's a separate circuit. In addition to the normal temperature control circuit, it's usual to incorporate a circuit that supplies more heating power under severe icing conditions, when heat losses are high. When the high power setting is selected, the supply is switched to higher voltage output tappings of an auto transformer. That auto transformer also forms part of the anti-icing system circuit, so it's integrated into the same system. The purpose is to maintain the normal operating temperature despite the higher heat loss. And the temperature is controlled in a manner similar to the normal control temperature circuit — so the same cycling principle applies, just at a higher power level. Finally, there's a ground testing consideration. For ground testing purposes, the heating power supply circuit may also be controlled by landing gear shock-strut microswitches. The way it works is that when the aircraft is on the ground, those microswitches are actuated, and they arrange for the voltage applied to the resistance elements to be lower than what's normally available in flight. So you can test the system on the ground without generating the full heat output you'd need in the air. So to tie it together: you have the normal temperature control circuit that cycles power to hold the glass at operating temperature, an overheat sensing element with a warning light and a lock-out option for manual reset, a high-power circuit using auto transformer tappings for severe icing, and ground-test control via the landing gear shock-strut microswitches that reduce the applied voltage. Each one is a distinct layer of the same electrical anti-icing system.