Basic Hydraulics — Page 59, Lesson 85

Let’s start with filters, because they are the first thing the system touches in this part of the book, and they protect everything downstream. Filters are fitted in both the suction and pressure lines — that means on both sides of the pump — and sometimes in the return line to the reservoir. The suction filter protects the pump itself, and the pressure filter ensures the fluid stays clean during use. Their job is to remove foreign particles from the fluid, which protects the seals and the working surfaces inside the components. On top of that, individual components often have a small filter fitted to their inlet connection, and constant pressure pumps will have what’s called a “case drain filter” to help monitor pump condition. Now, some filters have a device that senses the pressure differential across the filter element. When the element becomes clogged, that pressure differential increases, and the device releases a visual indicator — either a button that pops out or a warning lamp that illuminates. There’s a clever detail here: a false indication of clogging can happen if the fluid is very viscous at low temperature, so a bi-metal spring is fitted to inhibit the indicator button movement at low temperatures. That prevents a false alarm. Other filters are fitted with a relief valve instead. When the element becomes clogged, the relief valve allows unfiltered fluid to pass to the system — so the system keeps working, but the element must be changed at regular intervals. Paper filter elements are usually discarded when removed, but elements made of wire cloth may usually be cleaned. Cleaning by an ultrasonic process is normally recommended, but if a new or cleaned element isn’t available when the element is due for check, the old element may be cleaned in trichloroethane as a temporary measure. Now let’s move to pumps. Pumps draw oil from the reservoir and deliver a supply of fluid to the system. They may be hand operated, engine driven, electric motor driven, pneumatically driven by an air turbine motor — that’s the ATM — or by a ram air turbine, which is the HYDRAT or RAT, or hydraulically driven, where a hydraulic motor drives a hydraulic pump, and that arrangement is known as a Power Transfer Unit, or PTU. In most cases, the ATM, RAT, or PTU is used to provide an alternate supply as part of the redundancy provision for the safe operation of the aircraft. Hand pumps may be the only source of power in a small, light aircraft hydraulic system, but in larger aircraft they are employed for three specific jobs: to allow ground servicing without the need for engine running, so that lines and joints can be pressure tested, and so that cargo doors and similar items can be operated without power. The hand pump is usually a double acting pump — it delivers oil on both strokes — in a very compact body. It incorporates non-return valves, the NRVs, and a relief valve which can be set to relieve at any required pressure, typically about 10% above normal system pressure. Let me show you the basic layout. This is the basic hydraulic system — you can see the reservoir, the pump, the filters on both sides, and the return line. And here is the open-centre system, which we’ll look at next. And this is the closed system. So to recap what we’ve covered: filters protect the system on both sides of the pump, with a case drain filter on constant pressure pumps, and they have either a pressure differential indicator with a bi-metal spring to prevent false warnings, or a relief valve that bypasses unfiltered fluid when clogged. And pumps come in several drive types — hand, engine, electric, pneumatic ATM, ram air turbine, and hydraulic PTU — with the hand pump being double acting and set to relieve at about 10% above normal system pressure.