Hydraulics & Pneumatics


Pilot Handbook of Aeronautical Knowledge, Basic Hydraulic System
Figure 1: Pilot Handbook of Aeronautical Knowledge,
Basic Hydraulic System


  • A basic hydraulic system consists of: [Figure 1]
    • Reservoir
    • Pump (either hand, electric, or engine driven)
    • Filters to keep the fluid clean
    • Selector Valves
    • Relief Valve
    • Accumulators
    • Actuators
  • Hydraulic Reservoir:

    • A mineral-based hydraulic fluid is the most widely used type for small aircraft
    • This type of hydraulic fluid, a kerosene-like petroleum product, has good lubricating properties, as well as additives to inhibit foaming and prevent the formation of corrosion
    • It is chemically stable, has very little viscosity change with temperature, and is dyed for identification
    • Since several types of hydraulic fluids are commonly used, an aircraft must be serviced with the type specified by the manufacturer
  • Hydraulic Pumps:

    • The hydraulic fluid is pumped through the system to an actuator or servo
  • Hydraulic Servos:

    • A servo is a cylinder with a piston inside that turns fluid power into work and creates the power needed to move an aircraft system or flight control
    • Servos can be either single-acting or double-acting, based on the needs of the system
    • This means that the fluid can be applied to one or both sides of the servo, depending on the servo type
    • A single-acting servo provides power in one direction
  • Hydraulic Selector Valves:

    • The selector valve allows the fluid direction to be controlled
    • This is necessary for operations such as the extension and retraction of landing gear during which the fluid must work in two different directions
  • Hydraulic Relief Valves:

    • The relief valve provides an outlet for the system in the event of excessive fluid pressure in the system
    • Forces exert equal pressure on system
    • Input smaller than output increases force
    • Pumps provide system pressure
      • Variable
      • Constant: pressure regulators control pressure
    • Pressure gauges provide a way to monitor the system
    • Relief valves return fluid to the reservoir
    • Check valves used for 1 way flow
  • Hydraulic Accumulators:

    • Accumulators provide shock absorption for 1 time use


  • Each system incorporates different components to meet the individual needs of different aircraft