Seals & pipelines 454

Seals and pipelines are used in transportation of hydraulic fluids in a plane.


They include:-

  1. DEF STAN 91 – 48 – mineral fluid for high pressures and low temperatures. it is coloured red.
  2. SKYDROL – It is a phosphate ester based synthetic hydraulic fluid . it is coloured purple.


They are used to prevent leakage of fluid. They are either :-

  1. o-ring
  2. u-ring
  3. r-ring
  4. wiper ring
  5. square seal

u-ring and r – ring are used where there is only one direction of pressure while o-ring and square seals are used where pressure is made in all directions.Wiper ring is installed in actuators to prevent grind on piston rod from contaminating seals


They should have :-

  1. Good lubricating properties
  2. In-compressible
  3. Good viscosity
  4. High boiling point & low freezing point
  5. High flash point & non flammable
  6. Chemically inert & non corrosive
  7. Resistant to eruption
  8. Free from sludging & foaming
  9. Reasonably priced
  10. Readily available


  1. Active system – This is where a pump applies pressure
  2. Open center system – This operates at one system at a time
  3. Closed system – This is where multiple services can be operated at the same time.


  1. Hand operated – mostly double acting
  2. Engine driven (EDP)
  3. Electric motor driven
  4. Pneumatically operated (ATM)
  5. Driven from ram air turbine (RAT)
  6. Hydraulically operated
  7. Using a power transfer unit (P.T.U)


  1. Constant delivery ( fixed volume type)
  2. Constant Pressure ( variable volume type)

Cavitation – Suction at the pump inlet can cause low temperature boiling of the fluid
Accumulator– Helps dampen out pressure fluctuations

The hydraulic fluid used must be able to transmit pressure needed, lubricate  the hydraulic system and act as a coolant in the system

N.B – In smaller aircraft in case of hydraulic failure you can override the system and fly a plane without assist of hydraulics however a larger plane has three to four hydraulic systems that renders it impossible for all systems to fail at once but you cannot fly a large aircraft without hydraulics.


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