Smaller aircraft have relatively low flight control surface loads, allowing the pilot to operate the flight controls directly by hand. Early hydraulic systems were first used only for aircraft brake systems. As aircraft became larger and faster, however, the aerodynamic loads on the control surfaces increased to the point where manual operation was no longer practical.
To overcome these increasing control forces, hydraulic power boost systems were introduced. In these systems, the pilot still actuates the flight controls through cables or push rods, but hydraulic pressure assists in moving the control surfaces. This greatly reduces the effort required by the pilot while still maintaining a direct mechanical connection between the cockpit controls and the aircraft surfaces.
Many modern aircraft use a hydraulic power supply system combined with fly-by-wire flight controls. In a fly-by-wire system, the pilot's control inputs are transmitted electronically to hydraulic servos, eliminating the need for cables or push rods. These systems provide improved control response, reduced weight, and greater flexibility in aircraft design.
Small hydraulic power packs are the latest evolution of the hydraulic system. A hydraulic power pack is a compact unit consisting of an electric pump, filters, reservoir, valves, and a pressure relief valve. [Figure 1] Because all of these components are integrated into one assembly, there is no need for a centralized hydraulic power supply system or long stretches of hydraulic tubing. This significantly reduces system weight and simplifies installation.
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| Figure 1. Hydraulic power pack |
Hydraulic power packs may be driven either by an engine gearbox or by an electric motor. The integration of essential valves, filters, sensors, and transducers reduces weight, virtually eliminates the possibility of external leakage, and simplifies troubleshooting. Some power pack systems also include an integrated actuator. These units can directly operate stabilizer trim systems, landing gear, or flight control surfaces without relying on a centralized hydraulic system.
Some aircraft manufacturers are reducing the number of traditional hydraulic systems in favor of electrically controlled systems. The Boeing 787 is the first aircraft designed with more electrical systems than hydraulic systems, further reducing the need for large centralized hydraulic installations.
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| Figure 2. Large commercial aircraft hydraulic system |
Figure 2 illustrates a typical hydraulic system used on a large commercial aircraft. The following topics describe the major hydraulic system components and explain their construction, operation, and maintenance in greater detail.
Hydraulic System Components
Major hydraulic system components used in aircraft power systems and their functions.
1. Reservoirs
Reservoir construction, fluid storage, pressurization methods, and servicing requirements.
View Topic →2. Filters
Micron-type filters, maintenance practices, bypass valves, and differential pressure indicators.
View Topic →3. Pumps
Hand pumps, power-driven pumps, and the operation of gear, gerotor, piston, and vane pumps.
View Topic →4. Valves
Flow control, pressure control, and shuttle valves used to direct and regulate hydraulic fluid.
View Topic →5. Accumulators
Accumulator construction, operation, and the different types used in aircraft systems.
View Topic →6. Heat Exchangers
Hydraulic heat exchangers used to control fluid temperature and improve system reliability.
View Topic →7. Actuators
Linear actuators, rotary actuators, and hydraulic motors that convert fluid pressure into motion.
View Topic →8. Power Transfer Unit (PTU)
Operation of the PTU and its role in transferring hydraulic power between systems.
View Topic →9. Ram Air Turbine (RAT) and HMDG
Emergency hydraulic and electrical power provided by ram air turbines and hydraulic motor-driven generators.
View Topic →10. Seals
V-rings, U-rings, O-rings, backup rings, gaskets, and seal materials used in hydraulic systems.
View Topic →