Basic Hydraulics — Page 59, Lesson 81

I want to walk you through the basics of hydraulic systems, starting from the very foundation. Every hydraulic system, no matter how complex, is built around six main components, and I want you to know each one by name and by job. First, we have the reservoir of oil. This is the storage tank. It delivers oil to the pump and, importantly, it receives oil back from the actuators when they finish their work. So it's the source and the return point. Second, the pump. This can be driven in one of three ways: by hand, by the engine, or electrically. Its job is to move the fluid and create the flow. Third, the selector or control valve. This is the pilot's interface. It lets the operator select the direction of fluid flow to the required service, and it also provides a return path for the oil back to the reservoir. So it directs where the fluid goes. Fourth, the jack, or a set of jacks, which we also call actuators. These are the components that actually do the physical work—they actuate the component, like moving a landing gear leg or a flap. Fifth, the filter, which keeps the fluid clean. Contamination is the enemy of hydraulics, so this is critical. And sixth, the relief valve. This is the safety device. Its job is to relieve excess pressure so the system doesn't burst or damage itself. Now, let's talk about how we classify these systems. Active hydraulic systems are generally classified as low pressure or high pressure. Low pressure is up to 2000 psi—that's pounds per square inch. High pressure is anything above that, with working system pressures averaging around 3000 psi. Why would you want high pressure? The main advantage is that the size of the actuators can be reduced. Smaller actuators need less fluid, and the pipes can be made smaller too. The combination of all that leads to a reduction in weight and saves space. That's a huge deal on an aircraft. Now let's look at a specific type: the open-centre system. I want you to understand its character. The main advantage is that it's simple. The main disadvantage is that only one service can be operated at a time. So if you're moving the flaps, you can't simultaneously move the landing gear. Here's how it works. When no services are being operated, fluid is passed directly back to the reservoir. This allows the engine-driven pump to run in what we call an 'off loaded' condition. Little pressure is generated, but there is still a flow of oil through the pump, and that flow is what cools and lubricates the pump. So the pump is never starved of oil, even when it's not doing work. The working pressure of these open-centre systems is usually up to 2000 psi. Now, when you select a user system—say you select the landing gear—the fluid is directed to the actuator, and the actuator will move. When the actuator reaches the end of its travel, pressure will build up to a value. At that point, the selector is returned to neutral in order to off load the pump and allow alternative selections to be made. So you neutralise the selector to relieve the pressure and free up the system. But here's the safety net: if the selector does not return to its neutral position, the relief valve will relieve the excess pressure. That's its job in this system. This type of system is popular in many light aircraft which do not require a constant pressure to be maintained all the time. Think about it—items like landing gear and flaps are only powered for short periods of time each flight. So you don't need a system that's constantly pressurised. The open-centre design fits that need perfectly. Let me show you the basic layout so you can see how these six components connect. And here's the open-centre system in action, showing that direct return path to the reservoir when nothing is being operated. So to tie it together: reservoir stores and returns oil, pump moves it, selector directs it, jacks do the work, filter keeps it clean, and the relief valve protects against overpressure. And in an open-centre system, the pump runs off-loaded with flow but little pressure until you actually select a service. That's your foundation.