Ice and Rain Protection — Page 257, Lesson 321

We're starting a new topic now: Ice and Rain Protection, and I want to walk you through the fluid systems used to prevent ice from sticking to the aircraft's surfaces. The core idea here is simple but clever. Instead of trying to break ice off after it forms, this system prevents the ice from adhering in the first place. It does this by pumping a freezing point depressant fluid—we call it FPD—to panels mounted in the leading edge of the aerofoil. The fluid then gets carried back over the surface by the airflow. So the FPD lowers the freezing point of the water hitting the surface, meaning it can't freeze solid and stick. Let me walk you through the plumbing. The fluid is stored in a storage tank. From there, it's supplied to the pump, and on its way it passes through an integral filter—that's a filter built into the system, not a separate add-on. The pump itself has a single inlet and a number of delivery outlets. That's a key detail: one inlet pulls fluid in, but multiple outlets feed the distributors mounted on the aerofoil leading edges. Now, the pump construction is worth understanding. It's built around a main casting that incorporates three things: a pump body, a filter chamber, and a gear casing. When the pump is installed and operating in the system, both the pump body and the filter chamber are flooded with de-icing fluid. And here's a neat point—that fluid acts as the lubricant for the pump. So the same fluid that does the anti-icing job also keeps the pump running smoothly. There's a safety device built into the pump, and I want you to remember why it's there. Its job is to protect the pump and the whole system from damage caused by things like a blocked pipe. When abnormal pressure builds up, this safety device relieves that pressure by reducing the flow. So instead of letting pressure spike and damage something, it simply throttles back the flow. Now let's talk about the distributors, because there are two types: strip and panel. The panel distributors are the ones that cover a large area of the aerofoil leading edge, and they're more economical and efficient than the strip type. But they have a limitation—they're not suitable for surfaces with double curvature. Think of a fin, for example. A fin curves in two directions, so you can't fit a rigid panel to it, and that's where you have to use the strip distributor instead. Let me describe the panel distributor in detail, because the construction is specific. It's fitted over, or let into, the leading edges of the mainplanes and the tailplane. It's made up of three layers: a porous outer panel, a microporous sheet, and a back plate. The porous panel is the outermost layer, and it extends beyond the edges of the porous sheet underneath it. Screws pass through the panel to secure the whole distributor to the aerofoil surface. There's also an entry connector, and this is where the fluid actually comes in. The entry connector accommodates a metering tube—that's the tube that controls how much fluid flows through. The connector passes through the backplate, and it's bolted to that backplate. So the fluid enters through the connector, passes through the metering tube, and then seeps out through the microporous sheet and the porous outer panel onto the surface. I want to show you the layout of this system so you can see how the tank, pump, and distributors all connect. That figure shows the fluid system diagrammatically, and there's a sectional view of the panel distributor shown inset in that same figure, so you can see the porous panel, the microporous sheet, the back plate, and the entry connector with its metering tube all in cross-section. So to tie it all together: the FPD fluid sits in the tank, gets filtered, pumped through a single inlet and out through multiple delivery outlets, reaches the distributors on the leading edges, and seeps through the porous layers onto the surface where the airflow carries it back over the aerofoil. The pump is lubricated by the fluid itself, and a safety device protects everything from overpressure by cutting the flow. And remember, if the surface has double curvature like a fin, you switch from a panel distributor to a strip distributor.