Hydraulic systems perform a number of functions on modern aircraft. They can be used to move and actuate the landing gear, brakes and flaps. On larger aircraft they serve even an even greater purpose-as flight controls, thrust reversers and spoilers. While the earliest aircraft relied on manual controls to perform these functions, hydraulics allow for safer, more efficient operation. Hydraulic systems are used in because they can produce a very high force with very little hydraulic fluid. The most common use for hydraulics in aircraft is with power-assisted brakes.
Hydraulic pumps create the fluid flow that pressurizes the system's fluid. This pressurized fluid is then routed to motors and actuators that go on to operate a variety of mechanical parts. Because the fluid is nearly incompressible, hydraulic systems are incredibly reliable which means increased safety. They are sturdier and capable of transmitting higher pressures than pneumatic counterparts.
For in-flight systems, hydraulics are typically run by engine-driven pumps, operated by the jet engine's rotation. On the other hand, in emergency situations, pilots rely on hand-operated hydraulic systems. For example, these hand-operated hydraulic systems can be used to extend the landing gear in the instance the plane loses its normal hydraulic pressure.
A typical aircraft hydraulic system consists of many components all with individual job sets. Such parts include a reservoir to hold the hydraulic fluid, a pump to pressurize the system, an actuator to control amount the force. Other auxiliary parts to keep the system running properly include a filter to keep the fluid clean, selector valves to control the direction of flow, and relief valve to relieve excess pressure. A common modern jet's hydraulic system is pressurized at an unbelievable force, ranging from 3,000 pounds per square inch and upwards.
In an aircraft, the hydraulic fluid is pushed through the system, to an actuator or servo cylinder. A piston located inside the cylinder transforms the fluid power into the force that is needed to move the aircraft system controls. There are two types of cylinders, single-acting and double-acting. Pressure can be applied to one or both sides of the cylinder depending on the type.
The selector valve simply provides the controls for the direction of the fluid. For example, this hydraulic ability is used during in the extension and retraction of the landing gear during the flight. In this case, the relief valve will provide an outlet for the system in case there is an excess of fluid pressure within the system. While these are only a few simple examples of how hydraulic systems are utilized, each aircraft has individual hydraulic needs depending on the purpose of the aircraft.