Basic Hydraulics — Page 80, Lesson 108

Let’s start with the big picture. On modern transport aircraft, we don’t generally use high-pressure pneumatic systems for heavy jobs like raising and lowering the landing gear — hydraulics do that more efficiently. But you’ll still find these pneumatic systems on aircraft like the F.27, so we need to understand them properly. First, the term pneumatic means using compressed air or gas, as opposed to hydraulic, which uses liquid. Now, why would anyone choose compressed air? Let’s list the advantages the book gives us. Air is universally available and it’s free. It’s lighter than hydraulic fluid. There’s no fire hazard. There are no viscosity problems with changes of temperature — viscosity is how thick or thin a fluid is, and air doesn’t get thick or thin the way oil does. And the system is lighter because no return lines are required — in a hydraulic system you need lines to bring the fluid back to the reservoir, but air is just vented. Now the major disadvantage: air is compressible. That means it squashes under pressure, so it doesn’t give the same firm, precise control that an incompressible liquid does. That’s the trade-off. Let’s look at the actual system on the F.27, which is a high-pressure, closed-centre system. Closed-centre means that when no component is being operated, the pressure is held in the system rather than being circulated back. The heart of it is a four-stage compressor — four stages of compression to build up the high pressure — and it’s driven from the accessory gearbox of the turboprop engines. The accessory gearbox is the gearbox that drives auxiliary systems off the engine. There’s an unloading valve, and its job is to ensure the system pressure is maintained at 3300 psi. Psi means pounds per square inch — so 3300 psi is a very high pressure. The unloading valve basically dumps the compressor output when the pressure is high enough, so it doesn’t over-pressurise. There’s also a shuttle valve, and that enables the system to be charged from an external source. A shuttle valve is a type of valve that lets you feed pressure from either of two sources — here, either the compressor or an external ground supply — into the same system. Now, a critical problem with compressed air is water. When air is compressed, water vapour can condense, and if that water freezes, it can block the system. So there are two components protecting against water freezing. First, a moisture separator, which removes 98% of the water present in the air. Second, a dryer, which removes the remaining water using a desiccant — a drying agent — such as silica gel or anhydrous aluminium silicate. Anhydrous means without water, so it’s a substance that readily absorbs moisture. Before the air enters the system, it passes through a 10 micron filter to ensure it’s clean. A micron is a millionth of a metre, so 10 microns is a very fine filter — it catches tiny particles. Then we have three air bottles — also called reservoirs or accumulators — which store the high-pressure air ready for instant use. The sizes matter: the main system has a 750 cubic inch bottle, the brakes have a 180 cubic inch bottle, and there’s another 180 cubic inch bottle for emergency use. So you have dedicated storage for the main system, for the brakes, and for emergencies. Here’s the key point about pressure levels. Most of the components operate with a pressure of 1000 psi, not the full 3300 psi. So the air is passed through a reducing valve — a valve that steps the pressure down — before it’s used by the landing gear, the passenger door, the nose wheel steering, and the propeller brake. So let me tie it together. The four-stage compressor, driven by the engine accessory gearbox, builds pressure up to 3300 psi, held there by the unloading valve. The shuttle valve lets you charge from an external source. The moisture separator and dryer remove water to prevent freezing. The 10 micron filter cleans the air. Three bottles store it — 750 cubic inches for the main system, 180 for brakes, 180 for emergency. And then a reducing valve drops the pressure to 1000 psi for the landing gear, passenger door, nose wheel steering, and propeller brake. That’s the complete high-pressure pneumatic system on the F.27. The diagram in Figure 2.31 shows this layout — the compressor, the valves, the bottles, and the reducing valve feeding those components.