Basic Hydraulics — Page 67, Lesson 92

Right, let's get into the hydraulic accumulator. This is the component that gives your hydraulic system its memory and its emergency backbone. An accumulator is fitted for six distinct reasons, and I want you to hold all of them. First, it stores hydraulic fluid under pressure. Second, it dampens pressure fluctuations — it smooths out the spikes and troughs in the system. Third, it allows for thermal expansion, so when the fluid heats up and expands, there's somewhere for it to go without bursting a line. Fourth, it provides an emergency supply of fluid to the system in the event of pump failure. Fifth, it prolongs the period between the cut-out and cut-in time of the ACOV — that's the automatic cut-out valve we looked at — and by extending that cycle, it reduces the wear on the pump. And sixth, it provides the initial fluid when a selection is made and the pump is cut-out, so the very first instant a service is demanded, the accumulator answers before the pump even wakes up. Now, the plumbing. There's a non-return valve fitted upstream of the accumulator. That valve prevents fluid from being discharged back to the reservoir. So once the accumulator is charged, that pressure is trapped and held for the system, not allowed to bleed backwards. Look at Figure 2.16 — it shows two different types of accumulator, and these are the most commonly used, though many other types exist. The key principle is the same for both. The accumulator has a gas side, and that gas side is charged to a predetermined pressure with air or nitrogen. So you pre-load the gas to a set value before the system even runs. Here's the operating sequence. As hydraulic pressure builds up in the system, the gas gets compressed. The fluid pushes against the gas until the fluid pressure and the gas pressure equalize at normal system pressure. At that point, the pump commences to idle — it stops delivering — and system pressure is now maintained by the accumulator alone. The accumulator is holding the line. Then, if a service is selected, a supply of fluid under pressure is available from the accumulator, and it keeps supplying until the pressure drops sufficiently to bring the pump back on line. So the pump only runs when it's actually needed. Now the sizing logic, and this is important for your exam thinking. The initial gas charge of the accumulator is greater than the pressure required to operate any service. So the pre-charge alone is enough to move any single service. And the fluid volume is usually sufficiently large to operate any service once. But there's a special case: brake accumulators. Those are sized to permit a guaranteed number of brake applications, or the ability to stop the aircraft during a rejected take-off. So the brake accumulator isn't just a one-shot — it's certified to give you a defined number of brake applications, or enough energy to bring the aircraft to a halt in an RTO. So the whole picture: the accumulator is a pressure reservoir that stores energy, smooths the system, covers pump failure, and lets the pump rest between cycles. The non-return valve keeps it charged, the gas pre-charge sets the floor, and the sizing rules guarantee that a single service can always be operated — with the brakes getting the extra margin for a rejected take-off.