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Hydraulic fluids…

  • A
    are highly flammable.
  • B
    are irritating to eyes and skin.
  • C
    are highly viscous.
  • D
    can be used in the lubrication system.

HYDRAULIC FLUIDS
Hydraulic fluids make possible the transmission of pressure and energy. They also act as a lubricating medium, thereby reducing the friction between moving parts and carrying away some of the heat. Hydraulic fluids should be handled with care as they are considered to be a skin and eye irritant. In aviation, mainly mineral or synthetic based oil are used:

  • DTD 585: a refined mineral based oil. Colour - red. Used with synthetic rubber seals.
  • SKYDROL: a phosphate ester based oil (synthetic). Colour - purple or green, depending on type. Used with synthetic rubber seals. Is fire resistant and less prone to cavitation because of its higher boiling point.

Due to the difference in composition, petroleum-based and phosphate ester-based fluids will not mix; neither are the seals for any one fluid usable with or tolerant of any of the other fluids. Should an aircraft hydraulic system be serviced with the wrong type fluid, immediately drain and flush the system and maintain the seals according to the manufacturer’s specifications.


Ideal properties of a hydraulic fluid:

  • Low compressibility: to ensure instantaneous operation;
  • Good lubrication and anti-corrision properties: hydraulic fluid acts as a lubricant for the pumps, actuators and motors within the system;
  • High flash point and low flammability: fluid ignition should not occur at the normal operating temperatures of the surrounding components;
  • Low volatility: be resistant to evaporation;
  • Adequate viscosity: the fluid used must flow easily at very low temperatures but must also maintain adequate viscosity at high temperatures. The ideal hydraulic fluid will have a very low freezing point and a very high boiling point.
  • Thermal stability.

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