Chapter 28: 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).