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
    • 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