Basic Hydraulics — Page 73, Lesson 100

Let's pick up with the electrically-operated selectors. Sometimes it's just not practical to run long mechanical linkages from the cockpit to a valve that sits deep in the airframe. So we operate the selector electrically instead. It can be either motor driven or solenoid controlled. A solenoid is simply an electromagnetic coil that, when energised, moves a plunger or spool directly. So the pilot's switch sends an electrical signal, and that signal drives the valve remotely. Now, the shuttle valve. This one is a real workhorse in landing gear and brake systems. Its job is to let an alternate system drive the same actuators as the normal system. Think of it as a switch that picks which supply gets through. During normal operation, free flow passes from the normal system to the service, and the alternate line is blocked. If normal system pressure is lost and you select the alternate system, the shuttle valve moves across because of the pressure difference. It blocks the normal line and lets the alternate supply operate the services — brakes, landing gear, and so on. So the shuttle valve is essentially a pressure-operated changeover device. You can see a typical one in Figure 2.24. Next, sequence valves. These are often fitted in a landing gear circuit to ensure correct operation of the landing gear doors and jacks. The idea is order of operation — you don't want the gear coming down before the doors are open, or the doors closing on the gear. The sequence valve makes sure one action completes before the next begins. Figure 2.25 shows a sequence valve. Now, modulators. A modulator works with the anti-skid unit in a brake system. On initial brake application it allows full flow to the brake units, and thereafter a restricted flow. So you get the immediate bite of full pressure, then the modulator throttles the flow down to prevent the wheels from locking up. Flow control valves. A flow control valve may be fitted to maintain a constant flow of fluid to a particular component. It's frequently found upstream of a hydraulic motor that must run at a constant speed. Because a motor's speed depends on flow rate, you need to hold that flow steady regardless of what else the system is doing. Now, fuses. Modern jet aircraft depend on their hydraulic systems for far more than just raising and lowering the landing gear — control system boosts, thrust reversers, flaps, brakes, and many auxiliary systems. That's why most aircraft use more than one independent system, and in those systems provisions are made to fuse or block a line if a serious leak occurs. There are two basic types of hydraulic fuse. The first shuts off the flow if a sufficient pressure drop occurs across the fuse. The second type doesn't work on pressure drop at all — it shuts off the flow after a given amount of fluid has passed through the line. Normal operation of the protected unit doesn't require enough flow to let the piston drift completely over and seal the line. But if there's a leak, sufficient fluid flows that the piston moves over and blocks the line. Wheel brakes are invariably protected by fuse units. Finally, instrumentation. The cockpit needs indication of system condition and functioning. Light aircraft use some form of warning lamp, indicating the operation of the electric pump motor, in addition to undercarriage and flap warning lights or indicators. Larger aircraft — well, that's where we're heading next.