Ice and Rain Protection — Page 253, Lesson 317

Let's pick up with the temperature control side of the pneumatic anti-icing and de-icing systems. We've already covered how bleed air is taken from the engines and ducted to the leading edges. Now I want to walk you through how that heated air is actually regulated, because you can't just blast full-temperature bleed air at the structure all the time — you'd overheat the skin and cause damage. In a typical compressor bleed system, control is effected by temperature sensing units located at various points in the leading edge ducting, and by valves in the main air supply ducting. So we have two families of components working together: the sensing units that measure temperature, and the valves that actually throttle the airflow. The sensing units and valves are electrically interconnected, which means the valves are automatically positioned to regulate the flow of heated air to the system, maintaining the temperature within a predetermined range. That's the key idea — closed-loop control. The sensor reads the temperature, sends an electrical signal, and the valve moves to keep the air within a set band. Now, how does the crew see what's happening? Indications of air temperature conditions are provided by resistance type temperature sensing elements and indicators, temperature sensitive switches, and overheat warning lights. Let me unpack those. A resistance type temperature sensing element is a device whose electrical resistance changes with temperature — as the air heats up, the resistance changes, and the indicator converts that into a temperature reading on the flight deck. Temperature sensitive switches are simpler — they're basically snap-action switches that change state at a preset temperature. And the overheat warning lights are your alert — if the temperature exceeds the safe limit, the light comes on to warn you. There's an important operational detail here. On some aircraft, the electrical supplies to the valves are interrupted by landing gear controlled relays when the aircraft is on the ground. So when you're on the ground, the automatic valve control is disabled — the relays, which are tied to the landing gear position, cut the power. Under these conditions, valve operation is accomplished by holding the system control switch to a 'TEST' position. So the pilot physically holds the switch to manually drive the valves for testing, because the automatic control is inhibited on the ground. Now let's look at a different arrangement — systems that use heat exchangers. When heat exchangers are employed, temperature control is usually obtained by the use of adjustable flaps and valves to decrease or increase the supply of heating and cooling air passed across the exchangers. So instead of directly throttling bleed air, you're mixing hot and cold air across a heat exchanger, and the flaps and valves regulate how much of each passes across it. The method of controlling those flaps and valves varies with different aircraft, but a typical system incorporates an electric actuator, which is operated automatically by an inching device controlled by a temperature sensing element fitted in the duct on the warm air outlet side of the heat exchanger. Let me break that down. The actuator is the motor that moves the flaps. The inching device is what drives the actuator in small steps — "inching" it forward or backward — rather than running it continuously. And that inching device is controlled by a temperature sensing element located in the duct on the warm air outlet side of the heat exchanger. So the sensor reads the temperature of the air leaving the exchanger, and tells the inching device to nudge the actuator a little at a time until the temperature is right. In some systems, actuators are directly controlled by thermal switches, so that the flaps or valves are automatically closed when a predetermined temperature is reached. That's a simpler, more binary approach — no inching, just a thermal switch that closes the flaps when the temperature hits the limit. And again, indications of air temperature conditions are provided by resistance type temperature sensing elements and indicators, temperature sensitive switches, and overheat warning lights — the same suite of indications as before. Finally, let's look at systems incorporating combustion heaters. These are heaters that burn fuel to produce the hot air, rather than using engine bleed. Here, the temperature is usually controlled by thermal cyclic switches located in the heater outlet ducts. A thermal cyclic switch is a switch that cycles on and off based on temperature. So when the temperature reaches a predetermined maximum, the fuel supply to the heaters is automatically switched off. That's the safety cut-off — the switch senses the high temperature and shuts off the fuel, stopping the combustion and preventing overheating. So to tie it together: we have three control architectures. Compressor bleed systems use temperature sensing units and valves, electrically interconnected for automatic regulation. Heat exchanger systems use flaps and valves driven by actuators, either inching devices with a sensing element or direct thermal switches. And combustion heater systems use thermal cyclic switches that cut the fuel at a maximum temperature. In every case, the goal is the same — hold the heated air within a predetermined temperature range to protect the structure while still providing effective anti-icing.