Basic Hydraulics — Page 80, Lesson 106

I want to walk you through the heart of a hydraulic system — the reservoir, the pumps, and the protection devices that keep the whole thing alive. This is the part where we go from "fluid in a pipe" to a real, working aircraft system. Let's start with the reservoir. Think of it as the storage tank for the hydraulic liquid. It does several jobs at once. It stores the liquid, it provides a head of liquid for the pump — that's the pressure from the weight of the fluid itself that helps feed the pump — and it compensates for small leaks in the system. Above the liquid there's a reservoir air space, and that space allows for variations in liquid volume. Those variations come from three sources: jack ram displacement — when a hydraulic actuator's piston rod moves in and out, it changes the volume of fluid in the system — thermal expansion as the fluid heats up, and the accumulator liquid charge, which we'll get to in a moment. Now, the reservoir also has a pressurizing air connection. That provides a positive liquid supply at the pump inlet, and it prevents air bubbles from forming in the liquid at high altitude. At altitude, the ambient pressure drops, and without that pressurization the fluid could boil or cavitate — so this keeps the pump fed with solid liquid. There's a separator piston inside the reservoir. Its job is to seal between the gas and the liquid. That's critical, because if gas gets into the liquid, you get air bubbles, and air bubbles destroy pump performance. Now let's look at the accumulator. This is a pressure storage device. It has an air side and a liquid side, separated by a piston. The accumulator does six things, and I want you to remember all of them. First, it stores liquid under pressure. Second, it damps out pressure fluctuations — that's the "hammering" you see in the figure. When a valve slams shut, the fluid momentum creates a pressure spike, and the accumulator absorbs that. Third, it allows thermal expansion — as the fluid heats up, it expands, and the accumulator gives it somewhere to go. Fourth, it provides an emergency supply of liquid if the pump fails. Fifth, it provides the initial liquid supply when a selection is made — so when you move a selector valve, the accumulator gives that first surge of fluid before the pump catches up. And sixth, it prolongs the period between cut-in and cut-out of the ACOV, if fitted. ACOV is the Automatic Cut-Out Valve — it's the device that stops the pump when the system reaches full pressure. The accumulator smooths that cycling. Now, the pumps. There are six types listed here, and each has its own power source. First, the hand pump — and note it's double acting, meaning it pushes fluid on both the forward and return strokes. Second, the engine driven pump. Third, the electric pump, which can be DC or AC. Fourth, the pneumatic pump — that's an air turbine driven pump. Fifth, the ram air turbine, called the HYRAT — that's the Hydraulic Ram Air Turbine. And sixth, the hydraulic pump — that's a pump driven by a motor, so one hydraulic system powers another. Now, protection devices. Let's go through them. The non-return valve, or NRV — also called a one-way check valve — closes and stops flow when the inlet pressure is less than the outlet pressure. So it only lets fluid flow one way. The full flow relief valve protects the system from excess pressure if the ACOV or pump control fails. The firewall valve shuts off the liquid supply to the pump in the event of an engine fire — it's operated by the fire handle, and it protects the pump. The cooler reduces pump wear and liquid overheating — it's a heat exchanger that cools the fluid before it returns to the reservoir. Now, the ground service coupling. This allows the systems to be tested on the ground without the engines running. You connect an external hydraulic test rig to it, and you can exercise the system on the ground. There's also a transfer device — the "another hydraulic system" connection. This enables power to be transferred from another system without transferring liquid. So you can use one system's pressure to drive another system's pump, but the fluids never mix. Now, look at Figure 2.29 — this is the full system layout. You can see the high pressure filter and the low pressure filter. The HP filter cleans the fluid on the pressure side, and the LP filter cleans it on the return side. The flight deck gauges show you the liquid pressure and the temperature. And note the label "CONSTANT VOLUME ONLY SYSTEM" — this is a constant volume system, meaning the pump delivers a fixed volume of fluid per revolution, and pressure is built up by resistance to that flow. Now let's move to Figure 2.30, which shows the distribution of that pressure to the services. This is a typical layout. The supply comes in, and it goes to the essential services — the flying controls, the flaps, the brakes, the landing gear. And there's a priority valve — also called a pressure maintaining valve — which ensures that the essential services get pressure first, before the non-essential ones. Look at the HYRAT again. It only supplies the flying controls. And it's fitted in the U/C UP line — the landing gear up line — so that free-fall speed will be restricted. That's a clever detail: when the gear is selected down and free-falls, the HYRAT in that line acts as a restriction to control the descent speed. There's a shuttle valve in the system. That's a valve that allows two different sources to supply the same service, but only one at a time — so if the main system fails, the standby or alternate system can take over. There's a sequence valve, which ensures operations happen in the right order — for example, the gear doors open before the gear extends. There's a restrictor valve, which limits flow rate. There's a brake modulator valve, which controls the rate of brake application. And there's a pressure reducing valve, which steps the pressure down for a service that needs less than system pressure. There's also a mechanical overload protection fuse — that's a device that isolates a line if the flow exceeds a safe limit, protecting the system from a burst. And there's a control flow valve for the flaps. Now, the last figure, Figure 2.31, shows the F27 high pressure pneumatic system. That's a different aircraft type, but it illustrates the same principles applied to a pneumatic — air-based — system. So the whole picture is this: the reservoir stores and conditions the fluid, the pump pressurizes it, the accumulator smooths and stores that pressure, the filters clean it, the valves direct and protect it, and the services consume it. Every component has a specific job, and they all work together to deliver precise, reliable hydraulic power to the flight controls and landing gear.