Hydraulic Systems

28.1 Principles of Hydraulics

• Hydraulic systems use a confined, incompressible fluid to transmit force and power high-
• demand systems like landing gear, flight controls, and brakes.
•  Pascal's Law: Pressure applied to a confined fluid is transmitted equally in all directions.
• (Pressure = Force / Area).
•  Force Multiplication: By applying a small force to a small piston, a very large force can be
• generated at a large piston, as the pressure throughout the fluid is the same. However, the
• small piston must travel a much greater distance.

28.2 Hydraulic Fluid

• Aircraft hydraulic fluids must have specific properties, including being nearly
• incompressible, having good lubricating qualities, a high flash point, and a wide operating
• temperature range. The two main types are:
•  Mineral-Based (e.g., DEF STAN 91-48): Red in color. Used with neoprene synthetic rubber
• seals.
•  Phosphate-Ester Based (e.g., Skydrol): Purple in color. Fire-resistant but is corrosive and
• harmful to skin. Used with butyl rubber seals. The two types are not interchangeable.

28.3 Hydraulic System Components

•  Reservoir: Stores the system's hydraulic fluid. It is often pressurized with bleed air to
• provide a positive fluid supply to the pumps and prevent foaming at high altitude.
•  Pumps: Provide the flow of fluid. They can be engine-driven, electric motor-driven, or
• powered by a Ram Air Turbine (RAT) in an emergency.
• Constant Delivery Pumps: Deliver a fixed volume of fluid per revolution.
• Constant Pressure (Variable Volume) Pumps: The most common type on large aircraft.
• They automatically vary their output volume to maintain a constant system pressure,
• eliminating the need for an automatic cutout valve.
•  Accumulator: A sphere or cylinder containing a diaphragm or piston, with hydraulic fluid on
• ne side and compressed nitrogen gas on the other. It stores hydraulic fluid under pressure to
• dampen pressure fluctuations, provide an emergency source of pressure, and handle initial
• peak demands.
•  Actuators (Jacks): Convert hydraulic pressure into linear mechanical motion to operate
• components like landing gear.
•  Valves: Control the flow and pressure of the fluid (e.g., Selector Valves, Check Valves,
• Relief Valves, Sequence Valves, Shuttle Valves).