Basic Hydraulics — Page 73, Lesson 104

Let's pick up right where we left off — we've covered the main power and control components, and now I want to walk you through the instruments and the servicing hardware that keep a hydraulic system honest. First, pressure switches. These are used to illuminate a warning lamp, and to indicate loss of fluid pressure, or loss of air pressure in a reservoir. So think of them as the system's pressure sentinels — if pressure drops where it shouldn't, they light up a lamp in the cockpit. Next, flow indication. A flow indicator valve is often fitted in the outlet line from a constant delivery pump, and it's used to provide warning of pump failure. A constant delivery pump, as the name suggests, is designed to deliver a steady flow — so if that flow stops or drops, the flow indicator valve tells you the pump has failed. Then there's temperature indication. Warning of fluid overheating is normally provided by a temperature sensing element in the reservoir. And warning of overheating of electrical motors — the ones used to operate emergency pumps — is normally provided by fitting a similar element in the motor casing. So one sensor type, two locations: one in the reservoir for the fluid, one in the motor casing for the emergency pump motor. Now let's move to the components for servicing purposes. These are included specifically to make maintenance easier, and they're normally located in the hydraulic equipment bay. First, quick-disconnect and ground servicing couplings. In positions where you frequently need to disconnect a coupling for servicing, a self-sealing, quick-disconnect coupling is fitted. The key feature: it enables the line to be disconnected without loss of fluid, and without the need for subsequent bleeding. So you don't spill fluid, and you don't have to bleed air out afterward. Next, pressure release valves or off load controls. These are fitted to enable pressure to be released from the system for servicing purposes. They're manually operated, and used prior to checking and setting pre-change pressures or reservoir levels. So before you go adjusting anything, you release the pressure first. Then drain cocks, or drain valves. These are generally simple manually operated spherical valves, located in the hydraulics bay at the lowest point in the system, to enable the fluid to be drained. Lowest point — that's so gravity does the work and you get all the fluid out. Shut-off valves. These are fitted at the engine bulkhead — that's the firewall — and they enable the fluid supply to the engine-driven pumps to be stopped in the event of engine fire or component replacement. They're usually spherical ball cocks, or valves, which allow unrestricted flow when open. So in a fire, you shut these off to starve the pump and stop feeding the fire. Bleed points. Air in the system causes loud bangs and erratic operation. Bleed points are provided throughout the system to allow air to be removed. That's why we bleed — trapped air makes the system behave badly. Fluid sampling points. These are suitably positioned in the suction and pressure lines, to enable samples of fluid to be removed for analysis. And here's the diagnostic part: overheated fluid will appear darker than normal. A "milky" appearance indicates water. So the fluid itself tells you what's wrong — dark means heat damage, milky means water contamination. Now, the last piece — powered flying controls. This is a sub-system for the operation of the flying controls. It's often fed through a priority valve or pressure maintaining valve, which ensures that fluid under pressure is always available. The sub-system may also have a separate accumulator. Most modern aircraft will have alternate hydraulic supplies available for flight controls — two, three, or even four independent hydraulic systems can simultaneously supply power for primary flying controls. So redundancy is built in at the system level. A complete system is shown in Figure 2.29 and Figure 2.30, overleaf, where the position and purpose of the major components are illustrated. That's the full picture — instruments for monitoring, servicing hardware for maintenance, and the priority-fed flying control sub-system with its redundancy.