Basic Hydraulics — Page 67, Lesson 93

Let’s pick up right where we left off — we’ve got the accumulator’s gas side sorted, and now I want to walk you through the two big things that follow from it: what happens when the pre-charge pressure is wrong, and then the heart of the system, the hydraulic jacks, or actuators. First, that pre-charge. The gas side of the accumulator is normally inflated through a charging valve. That valve may be attached directly to the accumulator, or it can be installed on a remote ground servicing panel and connected to the accumulator by a pipeline. The charging valve usually takes the form of a non-return valve — meaning it lets gas flow in one direction only — and it may be depressed by means of a plunger in order to relieve excessive pressure. So if you need to bleed off pressure, you push that plunger. Now, here’s the critical operating procedure: to pre-charge or to check the gas pressure, the system pressure should be released — off loaded. Why? Because releasing the system pressure allows the gas pressure to move the floating piston to the bottom of the accumulator. That’s the reference position — the piston sits at the bottom, and only then can you get a true reading of the gas pre-charge. And here’s the consequence of getting that pre-charge wrong. If the pre-charge pressure of the main accumulator is incorrect, it can cause the ACOV — the automatic cut-out valve — to cut in and out too frequently. That rapid cycling produces rapid fluctuations of system pressure, and you can feel and hear that as ‘hammering’ in the system. So an incorrect pre-charge isn’t just a nuisance — it’s a symptom you can detect by feel and by sound, and it directly stresses the cut-out valve. Now let’s move to the actuators — the hydraulic jacks. Their purpose is to convert fluid flow into linear or rotary motion. That’s the whole job: you push fluid in, you get mechanical movement out. Construction: they vary in size and construction depending on the operating loads, but all of them consist of the same basic elements. You have an outer cylinder, and inside that cylinder slides a piston and seal assembly. Attached to the piston is a piston rod — also called a ram — and that rod passes through a gland seal fitted into the end of the cylinder. So the gland seal is what keeps fluid from leaking out where the rod exits the cylinder. Now, there are three types of jack used for different purposes in an aircraft system, and the book is clear that details of a particular jack should be obtained from the relevant maintenance manual — you don’t memorize every variant, you go to the manual. The first type is Single Acting. This is normally used as a locking device. The lock is engaged by spring pressure and released by hydraulic pressure. A typical application is a landing gear downlock. So think of it as a spring-loaded latch that hydraulic pressure pushes out of the way. The second type is Double Acting Unbalanced. This is used in most aircraft systems. Here’s the key geometry: because of the presence of the piston rod, the area of the top of the piston is greater than the area under it. That’s the ‘unbalanced’ part — the two sides don’t have equal area. Consequently, more force can be applied during extension of the piston rod. And that leads to the design rule: the operation which offers the greater resistance is carried out in the direction in which the piston rod extends. The classic example is raising the landing gear — that’s the high-resistance operation, so you do it in the direction of extension, where you have the greater force. The third type is Differential Areas. And this is really just spelling out the same geometry more precisely: the area of the upper side of the piston is greater than the area of the lower side by an amount exactly equal to the area of the piston rod. Therefore the force acting on it will be greater on the larger area. So the rod area is the difference between the two piston faces, and that difference is exactly what gives you the force advantage on extension. So to tie it together: the unbalanced and differential jacks both exploit that area difference caused by the rod, and the single-acting jack uses spring-versus-hydraulic force for locking duties. That’s the actuator family.