Basic Hydraulics — Page 59, Lesson 83

Let’s start with the power pack, because it’s the simplest way to get hydraulic power without a full engine-driven system. A power pack is a self-contained unit — it has its own pump, reservoir, and motor all in one package. On light aircraft, it might be fitted to operate the landing gear retraction system. On large aircraft, you’ll find them used as emergency systems, or to operate things like freight doors. So think of it as a standalone hydraulic power source you can bolt in wherever you need it. Now, the big distinction I want you to hold onto is between an open-centre system and a closed system. We’ve already touched on open-centre — where fluid flows freely through the selector valve back to the reservoir when nothing is being actuated. But here we’re looking at the closed system, and this is the one that maintains pressure. In a closed system, operating pressure is maintained in that part of the system which leads to the selector valves. That means the pressure is always there, waiting at the valve, ready to be directed to a jack the moment you move the selector. But if pressure is always maintained, you have a problem: the pump would keep building pressure until something gives. So the system needs some method to prevent over-loading the pump. There are three ways this is done, and you need to know all three. First, in systems that use a fixed volume pump — also called a constant delivery pump — an automatic cut-out valve is fitted. When pressure has built up to normal operating pressure, that valve diverts the pump output back to the reservoir. So the pump keeps running, but its output is dumped instead of being forced against a closed system. Second, some systems use a variable volume pump — that’s a constant pressure pump. Here, delivery is reduced as pressure increases. The pump senses the rising pressure and simply delivers less fluid, so it never over-pressurises the system. Third, in some simple light aircraft systems, you have an electrically-driven pump, and its operation is controlled by a pressure-operated switch. When pressure drops, the switch turns the pump on; when pressure reaches the set value, the switch turns it off. So the pump cycles on and off to hold pressure. A simple closed system is illustrated in Figure 2.7 — that’s the one I want you to look at on screen. Now let’s move to the reservoir, because that’s the heart of fluid storage. A reservoir provides storage space for the system fluid. It supplies a head of fluid for the pump — that means a positive pressure at the pump inlet so the pump can draw fluid easily. And it compensates for small leaks in the system. But the reservoir also has to allow for variations in fluid volume, and there are two causes you need to remember. First, jack ram displacement. A jack — that’s your actuator — has a smaller capacity when contracted than when extended. So as the ram extends, it pushes fluid out of the jack and back toward the reservoir; as it retracts, it draws fluid back. The reservoir has to absorb that change. Second, thermal expansion — the volume of oil increases with temperature, so the reservoir needs air space to accommodate that expansion. Most reservoirs are pressurized. Why? Two reasons. First, to provide a positive fluid pressure at the pump inlet — that helps the pump prime and prevents cavitation. Second, to prevent air bubbles from forming in the fluid at high altitude. At altitude, the ambient pressure drops, and dissolved air can come out of solution and form bubbles, which would be bad for the system. Pressurising the reservoir stops that. Now, the fluid level in the reservoir will vary — and you need to know the three factors. It varies according to the position of the jacks, whether the accumulators are charged, and temperature. So if a jack extends, the level drops; if an accumulator is charged, it holds fluid under pressure and the level changes; and temperature changes the oil volume. Where does the pressurising air come from? Normally, it’s supplied from the compressor section of the engine, or from the cabin pressurization system. So the reservoir is kept under pressure by bleed air from one of those sources. Now let’s look at what’s actually inside a reservoir — refer to Figure 2.8 on screen. A reservoir contains a relief valve, to prevent over-pressurization. It has connections for suction pipes to the pumps — those are the lines the pumps draw fluid from. And it has return pipes from the system — where fluid comes back after doing its work. It also contains a contents transmitter unit — that’s what tells you the fluid level on a gauge in the cockpit — and a filler cap for topping up. In some cases, there’s also a temperature sensing probe. One more important detail: in systems fitted with a hand pump, the main pumps draw fluid through a stack pipe in the reservoir. That stack pipe sits higher in the reservoir. Why does that matter? Because if fluid is lost from that part of the system supplying the main pumps — or supplied solely by the main pumps — the fluid below the stack pipe level is still available. So a reserve of fluid for the hand pump would still be available. That’s a safety feature: even if the main pump loses its supply, you can still hand-pump the system because the hand pump draws from a lower point in the reservoir. So to tie it together: the power pack is your self-contained unit; the closed system maintains pressure at the selector valves and uses one of three methods to protect the pump; and the reservoir stores fluid, provides a head for the pump, compensates for leaks and volume changes, is pressurised to prevent bubbles at altitude, and has a stack pipe to protect the hand pump supply.