Basic Hydraulics — Page 67, Lesson 89

Let’s start with the automatic cut-out valve, the ACOV. This is a component you’ll find in a hydraulic system that uses a constant delivery pump — that’s a fixed volume pump, meaning it pushes out the same amount of fluid every time it turns, regardless of what the system needs at that moment. The ACOV has two jobs. First, it controls system pressure. Second, it gives the pump an idling circuit when no services have been selected — in other words, when you’re not moving any flight control or landing gear, the pump doesn’t have to keep working against full pressure. It can just circulate fluid at low pressure. Now, here’s the key point: when a cut-out valve is fitted, an accumulator is always fitted as part of the power system. Why? Because even a tiny leak through a component, or from the system itself, would otherwise cause the cut-out to operate very frequently. That would mean the pump is constantly loading and unloading — which is hard on the system. The accumulator’s job is to maintain system pressure while the pump is in its ‘cut-out’ position. So the accumulator is the pressure reservoir that holds things steady during the idle period. Let me walk you through the actual operation, because it’s a beautiful sequence. In the ‘cut-in’ position, the delivery from the pump passes through a non-return valve — the NRV — and pressurizes the system. So the pump is on load, building pressure. Once system pressure has been reached, the piston is forced upwards by the pressure acting underneath it. That upward movement opens a poppet valve. With the poppet valve open, the output of the pump now passes to the reservoir at low pressure. The ACOV is now in its ‘cut-out’ position. The pump is off-loaded — it’s still turning, still circulating fluid, but it’s not building pressure. That circulating flow maintains a lubricating and cooling flow through the pump, which is important because a pump that runs dry or overheats will fail. Now, the non-return valve holds system pressure with the aid of the accumulator. So while the pump is off-loaded, the NRV prevents the pressurized fluid from flowing back, and the accumulator keeps the pressure up. If system pressure falls — say, because a service is selected, like you move a flight control — then the piston falls. That closes the poppet valve. Now the rising pump pressure is delivered through the NRV to the system again. That’s cut-in. So the cycle is: cut-in, build pressure, cut-out, hold pressure with accumulator, and when pressure drops, cut back in. Here’s a really useful diagnostic point. The time between cut-out and cut-in — that’s the periodicity of the ACO valve — is a good indication of the condition of the system. If the system is healthy, the period is long because the accumulator holds pressure well. If the period shortens, you have a problem. Let me give you the two failure cases. External leakage — that’s fluid escaping to the outside — will cause a reduction in the operating period, with frequent loading and unloading of the pump. And you’ll also see a loss of system fluid. So the fluid level drops. Internal leakage is different. It’s usually caused by a piston seal failure. That will also cause frequent loading and unloading of the pumps — note the plural there, because this affects the whole system. But here’s the contrast: with internal leakage there is no fluid loss to the outside. Instead, there could be an increase in fluid temperature. The fluid is leaking internally, doing work, generating heat, but not leaving the system. So the diagnostic distinction is: external leak — fluid loss plus frequent cycling. Internal leak — no fluid loss, but temperature rise plus frequent cycling. Both give you the frequent loading and unloading, but the tell-tale difference is whether you’re losing fluid or gaining heat. Let me show you the valve in its two states. This is the automatic cut-out valve in its ‘cut-in’ position, allowing the delivery from the pump to pass through to the system. And you can see the accumulator in the power system as well. Here are the hydraulic accumulators themselves — these are the pressure reservoirs that hold the system pressure during the cut-out period. So to tie it all together: the ACOV manages the pump’s workload, the accumulator smooths out the pressure, and the periodicity of the valve tells you how healthy the whole system is. External leaks cost you fluid; internal leaks cost you temperature. Both are bad, but they show up differently.